I will be set up in the Progress Building alongside the antique bike show. Stop in and say hello!
Thursday, June 13, 2013
Viking Chapter AMCA Meet this Weekend
If you are anywhere near Minnesota this weekend, come out to the Viking Chapter of the Antique Motorcycle Club of America's annual National Meet. 8 AM to 8 PM on Friday June 14th and 8 AM to 6 PM on Saturday June 15th at the Minnesota State Fairgrounds in St. Paul. Friday is traditionally the biggest day for the swap meet, though there will likely be some great last minute deals to be found on Saturday.
Friday, April 19, 2013
Proof!
For those of you old enough to remember, Johnny Horton famously sang, "when its springtime in Alaska, its 40 below." I could not help but hum a few bars of that song as I took a break on the deck in my back yard here in beautiful downtown Minnesota on April 18th.
I was not there long before I was joined by an old friend. From the frown on his face you can see that he too was a little taken aback by the snowfall so late in the year. The fact that he had already switched over to a straw hat, clearly shows that like so many others, he had assumed spring was here. As with most friends, it was good to have him stop by for a chat, but we hope that he doesn't wear out his welcome.
Clearly, more proof of Global Warming!
Friday, April 12, 2013
Another Age Old Problem & Another Cheap Tool To Fix It
Spring is in the air. The birds are chirping, the breeze is finally warm, and you can hardly wait to get your Harley out for its maiden voyage of the season after its long winter rest. You went and bought a new battery in anticipation of this day, since you once again left it go all winter without any preventive maintenance. But what's the price of a new battery when you will soon be back in the saddle, enjoying the wind in your face?
You kick the engine through once ....twice, ....(what is that gurgling noise?). On the sixth kick, Old Reliable catches, and with some quick finessing of the throttle, comes fully awake. But, now what? Suddenly you realize that your garage floor is rapidly being covered by a pool of oil. You quickly shut the engine down. Dead silence, except that the gurgling noise which nearly caught your attention before is still emanating from the bowels of your beloved steed.
A seeping oil pump check ball has struck again.
It is pretty much inevitable that if you own Harleys for long enough, you will become accustomed to this scenario, though you may never get to the point of actually remembering to plan for it in advance via a pan to catch the oil.
What has happened, of course, is that the spring and check ball in your oil pump has failed in its duty to keep oil from gravity feeding from the tank, through the pump, and into the lower end of the engine while the bike was at rest. With the return side of the pump unable to scavenge the excess oil quickly enough, it takes the path of least resistance and exits through the breather. The longer the bike is at rest, the more likely this will happen. I would go so far as to say that for it to happen over the course of a whole winter may not even be a sign of anything amiss to the point of worrying about, anything except how to clean up the mess that is.
On the other hand, if you get a similar result after giving your ride a two week vacation, then it may be time to address the situation. The Motor Company used to tell us that the fix was to remove the check ball and spring and then take the bike for a good ride so that the oil flow could "flush" out anything that may have gotten between the ball and its seat. That's fine as far as it goes, but if this is a re-occurring condition, you may have to do more. A new ball and spring may be in order, especially if they are original equipment - I don't put a lot of faith in springs in their old age. But, assuming you have tried these simple fixes with no success, now what?
As you might expect, any shop dealing with rebuilds on older Harley motors has to address this issue. The older the engines you rebuild, the more often it will be an issue. For years and years, I have lapped the check ball seats in oil pumps as a matter of course during a rebuild. What I did was take a new check ball and braze it onto the end of a screwdriver, dip the ball in lapping compound and lap away. One problem though. The heat from brazing took the hardness out of the check ball, causing it to "wear" away from the lapping rather quickly, especially on cast iron pumps. Frankly, it has been one of those little annoyances that has bothered me for years (like each time I need to replace the ball).
Bug, a fellow long time HD mechanic mentioned to me that he uses JB Weld to attach the ball to a screwdriver and it works fine. And here I though JB Weld was only good for gluing broken crankshafts back together (do they still use testimonials like that in their advertising?). But just as I was prepping my cheap screwdriver to glue, rather than braze, a check ball to it, I had a thought. As any good mechanic with hoarding tendencies, I have a whole bunch of stock Evo pushrods gathering dust on a shelf in my shop. The ball end on them is 3/8" just like the check ball. Hmmm....
So, out came the torch one more time, but this time only to heat the shaft of the screwdriver enough so that it would pull out of the handle. Then a quick trip to the lathe to bore the hole in the handle to the proper diameter, and for good measure to put a little taper on the recently cut-in-two pushrod, and oh boy howdy ....another cheap special tool. Best of all, not only does it make for quick replacement even if the ball does wear rapidly (which I don't think it will), but finally a good use for some of those worthless parts that were cluttering up my shelves!
You kick the engine through once ....twice, ....(what is that gurgling noise?). On the sixth kick, Old Reliable catches, and with some quick finessing of the throttle, comes fully awake. But, now what? Suddenly you realize that your garage floor is rapidly being covered by a pool of oil. You quickly shut the engine down. Dead silence, except that the gurgling noise which nearly caught your attention before is still emanating from the bowels of your beloved steed.
A seeping oil pump check ball has struck again.
It is pretty much inevitable that if you own Harleys for long enough, you will become accustomed to this scenario, though you may never get to the point of actually remembering to plan for it in advance via a pan to catch the oil.
What has happened, of course, is that the spring and check ball in your oil pump has failed in its duty to keep oil from gravity feeding from the tank, through the pump, and into the lower end of the engine while the bike was at rest. With the return side of the pump unable to scavenge the excess oil quickly enough, it takes the path of least resistance and exits through the breather. The longer the bike is at rest, the more likely this will happen. I would go so far as to say that for it to happen over the course of a whole winter may not even be a sign of anything amiss to the point of worrying about, anything except how to clean up the mess that is.
On the other hand, if you get a similar result after giving your ride a two week vacation, then it may be time to address the situation. The Motor Company used to tell us that the fix was to remove the check ball and spring and then take the bike for a good ride so that the oil flow could "flush" out anything that may have gotten between the ball and its seat. That's fine as far as it goes, but if this is a re-occurring condition, you may have to do more. A new ball and spring may be in order, especially if they are original equipment - I don't put a lot of faith in springs in their old age. But, assuming you have tried these simple fixes with no success, now what?
As you might expect, any shop dealing with rebuilds on older Harley motors has to address this issue. The older the engines you rebuild, the more often it will be an issue. For years and years, I have lapped the check ball seats in oil pumps as a matter of course during a rebuild. What I did was take a new check ball and braze it onto the end of a screwdriver, dip the ball in lapping compound and lap away. One problem though. The heat from brazing took the hardness out of the check ball, causing it to "wear" away from the lapping rather quickly, especially on cast iron pumps. Frankly, it has been one of those little annoyances that has bothered me for years (like each time I need to replace the ball).
Bug, a fellow long time HD mechanic mentioned to me that he uses JB Weld to attach the ball to a screwdriver and it works fine. And here I though JB Weld was only good for gluing broken crankshafts back together (do they still use testimonials like that in their advertising?). But just as I was prepping my cheap screwdriver to glue, rather than braze, a check ball to it, I had a thought. As any good mechanic with hoarding tendencies, I have a whole bunch of stock Evo pushrods gathering dust on a shelf in my shop. The ball end on them is 3/8" just like the check ball. Hmmm....
So, out came the torch one more time, but this time only to heat the shaft of the screwdriver enough so that it would pull out of the handle. Then a quick trip to the lathe to bore the hole in the handle to the proper diameter, and for good measure to put a little taper on the recently cut-in-two pushrod, and oh boy howdy ....another cheap special tool. Best of all, not only does it make for quick replacement even if the ball does wear rapidly (which I don't think it will), but finally a good use for some of those worthless parts that were cluttering up my shelves!
Sunday, March 31, 2013
HE IS RISEN!
Now upon the first day of the week, very early in the morning, they came unto the sepulchre, bringing the spices which they had prepared, and certain others with them. And they found the stone rolled away from the sepulchre. And they entered in, and found not the body of the Lord Jesus. And it came to pass, as they were much perplexed thereabout, behold, two men stood by them in shining garments: And as they were afraid, and bowed down their faces to the earth, they said unto them, Why seek ye the living among the dead? He is not here, but is risen: remember how he spake unto you when he was yet in Galilee, Saying, The Son of man must be delivered into the hands of sinful men, and be crucified, and the third day rise again. (Luke 24:1-7)
HE IS RISEN!
HE IS RISEN!
Wednesday, March 20, 2013
Aftermarket Knuckle Heads
This is not the sort of post that I enjoy writing. I would much prefer to write a glowing report on a good product. On the other hand, these things are not cheap, so I do believe it is reasonable to sound a warning so that buyers can make an informed purchase.
Reproduction Knuckle heads from V-Twin Manufacturing. The "issues" I will list range from minor annoyances to full blown problems.
First let's look at the annoying things. The black paint on the heads is thin to the point of being translucent in many places, except of course in the areas that have runs. As it turns out the runs are just as annoying as the see though if you opt to blast it off for a full refinish since the runs do a pretty good job of resisting removal with glass beads.
Speaking of paint, the heads were obviously painted before machining operations were performed, leaving large areas of bare cast iron. Notably missing paint are the large spark plug "wells" and the tops of the rocker box supports. Perhaps for this reason the machined surfaces were not deburred leaving sharp edges, many of them sharp enough to cut you quite easily when handing. As it turns out, that can be VERY annoying.
The set I purchased was the version without rocker boxes and shafts, so they came without the upper rocker cover tins installed, but gaskets for them were included. Upon inspection, they found a new home in a trash can, since they were so dried out and brittle that clearly any attempt at installation would have been an exercise in futility.
The spring cups, or lower covers as they may also be called, have a nice Parkerized finish, and seem to be fairly good stampings (better than those from the same vendor a number of years earlier). The brazed in oil return lines, however, have a copper plating which does not "take" the Parkerizing. That may be a good thing in some ways, since the color may discourage some from trying to pass them off as originals.
The last item on my list does not really fit into either the annoyance or the problem column, and so possibly not worthy of mention, but I won't let that stop me. Both exhaust valves had been treated to an approximate 45 degree cut on the combustion chamber side of the O.D. I have seen the claim in print that this enhances flow. It does not. My guess is that this is an old wives tale started because someone saw a set of high performance ported heads with this modification and assumed it was for better flow. In actuality, this extra angle is a last resort option for valve to valve clearance during overlap. If one had a finished set of ported and flowed heads with less than the minimum required valve to valve at TDC, one might cut an angle on the margin of the exhaust valve only (because the resulting flow loss would less costly there). Many years ago I tested this on my flow bench, and the result was so profoundly bad that it made an impression. The margin thickness on a valve can have a large effect on its flow characteristics.
That pretty well wraps up this product review. As I stated at the beginning, I wish I had better things to say about these heads, but there is this: I think we can all be thankful that V-Twin put these heads into production. The supply of repairable original Knuck castings is fast dwindling. I believe they would be a better value if they were available as bare castings, but still, better something to work with than nothing at all.
Reproduction Knuckle heads from V-Twin Manufacturing. The "issues" I will list range from minor annoyances to full blown problems.
First let's look at the annoying things. The black paint on the heads is thin to the point of being translucent in many places, except of course in the areas that have runs. As it turns out the runs are just as annoying as the see though if you opt to blast it off for a full refinish since the runs do a pretty good job of resisting removal with glass beads.
Speaking of paint, the heads were obviously painted before machining operations were performed, leaving large areas of bare cast iron. Notably missing paint are the large spark plug "wells" and the tops of the rocker box supports. Perhaps for this reason the machined surfaces were not deburred leaving sharp edges, many of them sharp enough to cut you quite easily when handing. As it turns out, that can be VERY annoying.
The set I purchased was the version without rocker boxes and shafts, so they came without the upper rocker cover tins installed, but gaskets for them were included. Upon inspection, they found a new home in a trash can, since they were so dried out and brittle that clearly any attempt at installation would have been an exercise in futility.
The spring cups, or lower covers as they may also be called, have a nice Parkerized finish, and seem to be fairly good stampings (better than those from the same vendor a number of years earlier). The brazed in oil return lines, however, have a copper plating which does not "take" the Parkerizing. That may be a good thing in some ways, since the color may discourage some from trying to pass them off as originals.
On to what I consider a little beyond annoyances. The valve spring are green. So, what do you have against St. Patty's day, you might ask. Well, green is not my favorite color, but this goes a little past interior decorating choices. The valve springs are painted green. In fact they are thickly painted green, possibly with a brush from the look. Thick to the point of chipping off. I can't say just what chunks of this green paint would do to an engine, or where it would ultimately wind up, but its certainly not something I would want to take a chance on.
The heads come with plumber style intake nipples installed. In fact, they are installed with a "stock style" rivet to keep them from turning. And when I say "stock style rivet" it is because the rivet is of the 1/4" diameter oversize normally reserved for a damaged hole on a used and abused head. Too bad about that too. Once the oversize rivets were removed, the nipples could be removed easily enough, meaning they were not installed very tight to begin with. That added to the fact that the nipples had been installed with no type of sealer, leads one to conclude that the possibility of intake air leaks would be somewhere between likely and inevitable.
Next up is the alignment of the spring cups. Keep in mind that these are held in place by being sandwiched between the valve guide and the head, so they need to be in the right position before the guide is installed all the way. I keep a spare set of rocker boxes (knuckles) with shafts (no arms) among my special tools for just this purpose. That way, I can insure that the spring cups align properly with the holes for the rocker shafts and that the other end of the cover fits into the hole in the rocker box. In the case of this set of heads, not only did the rocker box end of the spring cups not line up with the knuckles, there was a good sized gap between the cup and ear on the head where the rocker shaft passes through it. Any attempts to correct this without driving the valve guides partially out would result in bending the lower spring cups, possibly with further damage.
 |
| Hard to tell form the poor photo, but this shows daylight between the spring cup and the rocker support ear on head |
Going a little deeper yet, I found that the I.D. of the valve guides to be abnormally rough; not necessarily a good thing for longevity. But, as it turns out, that would not be a problem after all, since the valve to guide clearance was too tight, meaning that by the time you honed the guides for more clearance, they would likely have a smooth interior. Measuring the valve to guide clearance with a dial ball gauge showed .0022 to .003" on the intakes and .0034 to .0055 on the exhausts. However, the fool proof final check for guides (at least in my book) is checking with a plug gauge (also known as a go/no go gauge). This takes into account a valve guide bore that is not straight, something the ball gauge will not tell you. In this case, a plug gauge .001" larger than the valve stem diameter would make it through one intake and and .002" larger on the other intake. On the exhaust side a plug gauge .002 larger would go through one and .0025 larger on the other. Stock clearances for a Knuckle are .004 to .006" clearance on both intake and exhaust. I cannot imagine that these heads would have lived at these clearances.
The next item is only a problem if you plan to use these on a 61 inch motor. The counter bore for the fire ring in the head is a slight interference fit on an OEM 61 inch Knuck cylinder. That doesn't affect you if you have a 74 inch since there is no fire ring on the larger cylinder.
That pretty well wraps up this product review. As I stated at the beginning, I wish I had better things to say about these heads, but there is this: I think we can all be thankful that V-Twin put these heads into production. The supply of repairable original Knuck castings is fast dwindling. I believe they would be a better value if they were available as bare castings, but still, better something to work with than nothing at all.
Friday, March 1, 2013
Ready to be Offended?
Despite finding myself in the midst of crazy busy season in the motorcycle engine rebuilding world, I do take a few moments each day to peruse the web while eating my sandwich. One of my favorite stops is the Pyromaniacs blog. This week Dan had an absolutely great post that I felt I should share with my readers, especially since I can't seem to find the time to write anything worthwhile of my own.
The most offensive verse in the Bible
by Dan Phillips
In the Sunday School class at CBC we're doing a series called Marriage, the Bible and You. In the second lesson of the series, I brought up the subject of secular talk shows and how they like to try to beat up on Christians of any size, shape, and significance about whatever topic they think is most embarrassing and controversial. Of course, at the moment it's "gay" "marriage," or the topic of homosexuality at all.
In the course of the lesson, I remarked that I think — from the comfortable quiet safety of my study — that I'd take a different approach.
When Piers or Larry or Tavis or Rosie or Ellen or The View or whoever tried probing me about homosexuality, or wifely submission, or any other area where God has spoken (to the world's consternation), I think I'd decline the worm altogether. I think instead, I'd say something like,
"You know, TaPierRosEllRy, when you ask me about X, you're obviously picking a topic that is deeply offensive to non-Christians — but it's far from the most offensive thing I believe. You're just nibbling at the edge of one of the relatively minor leaves on the Tree of Offense. Let me do you a favor, and just take you right down to the root. Let me take you to the most offensive thing I believe ... (click here to continue reading)
Thursday, February 7, 2013
The Grey Goose 2013
As promised, here is a little more prospective on the hole in the rear piston phenomena which may be prominent on the Salt Flats. As you may recall, just two posts ago I told the sad tale of the season ending piston failure in the Grey Goose; Joe Taylor's 1939 Knucklehead Bonneville racer. To review what we already know:
To clarify, for those not up to date on Harleys old wasted spark ignition system, it works like this. On all but the latest offerings from Milwaukee, the timer, whether it be a battery/points ignition or a magneto, turns at half the speed of the engine. This timer has two lobes (or notches for later electronic ignitions) which open the points, initiating the spark. One lobe is set to open the points at the correct time for the front cylinder and the other at the correct time for the rear cylinder. In the case of the Goose, that time was at 42 degrees BTDC (before top dead center) on the compression stroke.But since these lobes both open the same single set of points, each cylinder gets a spark from each lobe, once at 42 BTDC on the compression stroke, and once on the exhaust stroke.
Now you need to remember that the two cylinders on a Harley form a 45 degrees angle (thus the term 45 degree V-Twin). That means that at any given point in time, the front piston will be at a point 45 degrees behind the rear piston in crankshaft rotation (I know that sounds backwards, but its not). For instance, when the rear piston is at TDC on the compression stroke, the front piston will be 45 degrees away from reaching TDC, but it will be on the exhaust stroke. This 45 degree offset is what makes things interesting.
Obviously if the rear piston is the one that normally takes the hit at Bonneville, there must be something that differs front cylinder to rear which is the culprit. And if it seems to be exclusive to wasted spark ignitions, then that would be a good place to look.
First lets look at the front cylinder. After plotting the SS Cycle KN420 camshaft that the Goose employs, we find this: At 42 degrees BTDC on the compression stroke the rear cylinder spark plug fires, and the wasted spark is produced at that same instant in the front cylinder. However, the front cylinder is not at 42 BTDC on the exhaust stroke, but rather 87 degrees BTDC (remember it trails the rear by 45 degrees). At this point in time the front intake valve is still on its seat, in fact still about 6 degrees before it comes to the opening ramp on the cam. The exhaust valve is just starting to close, but still near full lift. OK - no problem. That spark in the front cylinder with the exhaust valve open and intake valve closed won't do much.
Now for the rear cylinder. When the front cylinder spark plug fires at 42 BTDC on the compression stroke, the rear cylinder is in a much different position. Because of the the 45 degree offset, the rear piston is at 3 ATDC (after top dead center); technically not even still on the exhaust stroke, but rather beginning its descent on the intake stroke. The exhaust valve is closing, but still .135" off its seat. The intake valve on the other hand has already started to open to the tune of .185" off its seat. Valve overlap is the common term. Now that gives one something to think about doesn't it?
So her is some other pertinent info gleaned from PipeMax that may shed some light on the issue at hand.
Assume the Goose's engine were running 5500 RPM. At the point where the rear cylinder receives its "wasted spark" (3 degrees ATDC in overlap), the piston has already started down on the intake stroke, accelerating to 325.5 feet per minute providing a "piston demand" of 10.1 cubic feet per minute of intake air flow. Raise the RPM to the target 7500 and that becomes a piston speed of 443.8 feet per minute with a piston demand of 13.8 cfm. All that is at a mere 3 degrees after top dead center.
If you put stock in David Vizard's theories (and I do), you may recall that he puts a large amount of emphasis on the overlap portion of the cam timing. One of his conclusions is that the low pressure area caused by exhaust outflow results in the single strongest action initiating intake flow during this overlap period (hopefully I have paraphrased him properly). So what effect does it have when you throw a spark into the middle of that overlap period? A spark which incidentally does NOT occur during overlap on the front cylinder. Hmmm.
So here are a few thoughts. Obviously the rear cylinder is subjected to a spark during overlap which the front cylinder does not. Since that cylinder is in overlap, there will be a fuel air mixture present to burn. Now, that fuel air mixture is not compressed, but certainly it can burn none the less. And what naturally comes along with burning fuel and air? That's right - heat; heat that the front cylinder is not subjected to. I have no way of knowing or even estimating how much extra heat the rear cylinder gets this way, but the evidence would suggest that it may be just enough extra heat to melt a piston dome.
Here is something else to consider though. What happens when that wasted spark fires off and the fuel air mixture is richer than ideal? The BTUs are in the fuel, not the air, so I would assume that you would be releasing even more heat than with a correct mixture. Hmmm. Remember that I said that the Goose was not run too lean. It actually had the baseline jetting that it was dyno'ed with here in Minnesota. The reason I was pretty sure that the piston did not fail from a lean condition was that I believe that it was "pig rich" (as I like to call it). Could this be a case of a rich mixture giving the opposite results that one would expect?
But why does this happen at Bonneville, but not on the drag strip or on the street? Well, my guess would be that it has everything to do with length of time spent under a heavy load. Remember that aerodynamic drag becomes a huge factor at high speeds. There are plenty of horsepower/MPH calculators available on line. Plugging in some estimates (guess-timates?) for weight, frontal area and drag coefficient, we find that if it takes a mere 29 HP to hit 100 MPH, the same bike would need 55 HP to get to 125, and 77 horses to reach 140. And if that is not enough of a wake up call, if you want to raise the MPH from 140 to 150 you better be ready to call up an extra 17 HP to wring out that 10 MPH. Bottom line is that high speed puts a tremendous load on a motor, and the longer that load is present, the better the chance for heat build up.
Now we know that a drag motor will not see much time under full load and even less time at high speeds. If you hit 100 MPH in the 1/8 mile, then you will likely see the 1/4 mile finish line in another 4 seconds. And on the street? Despite thousands of bar room stories to the contrary, most street motors will never get more than a few seconds at full throttle and high speed before law enforcement rains on that parade.
So where do we go from here? Obviously a single fire ignition system is in order. The exact form that will take is still up in the air, with part of the team leaning toward keeping things as simple as possible, and part leaning toward as hi-tech as possible. The other obvious bit is that new pistons are needed (you didn't really think I would weld them back up, did you?). That part has been settled.
Four new pistons from Arias arrived this week, with the domes finished as per my sample (the undamaged front piston). They are down right beautiful, and hopefully the two spares will remain in the box as spares for a long, long time.
- In order to raise the compression ratio without having another set of custom pistons made, I welded the domes to fill in unneeded valve pocket clearance.
- Despite the welding and re-machining, the piston dome thickness remained at a reasonable .200".
- The engine was not run too lean, in fact just the opposite.
- Rear cylinder piston failure is common on Harleys with dual fire (aka wasted spark) ignitions at Bonneville.
The small holes were drilled after the fact to check dome thickness
Exhaust pocket had also begun to "sag"
To clarify, for those not up to date on Harleys old wasted spark ignition system, it works like this. On all but the latest offerings from Milwaukee, the timer, whether it be a battery/points ignition or a magneto, turns at half the speed of the engine. This timer has two lobes (or notches for later electronic ignitions) which open the points, initiating the spark. One lobe is set to open the points at the correct time for the front cylinder and the other at the correct time for the rear cylinder. In the case of the Goose, that time was at 42 degrees BTDC (before top dead center) on the compression stroke.But since these lobes both open the same single set of points, each cylinder gets a spark from each lobe, once at 42 BTDC on the compression stroke, and once on the exhaust stroke.
Now you need to remember that the two cylinders on a Harley form a 45 degrees angle (thus the term 45 degree V-Twin). That means that at any given point in time, the front piston will be at a point 45 degrees behind the rear piston in crankshaft rotation (I know that sounds backwards, but its not). For instance, when the rear piston is at TDC on the compression stroke, the front piston will be 45 degrees away from reaching TDC, but it will be on the exhaust stroke. This 45 degree offset is what makes things interesting.
Obviously if the rear piston is the one that normally takes the hit at Bonneville, there must be something that differs front cylinder to rear which is the culprit. And if it seems to be exclusive to wasted spark ignitions, then that would be a good place to look.
First lets look at the front cylinder. After plotting the SS Cycle KN420 camshaft that the Goose employs, we find this: At 42 degrees BTDC on the compression stroke the rear cylinder spark plug fires, and the wasted spark is produced at that same instant in the front cylinder. However, the front cylinder is not at 42 BTDC on the exhaust stroke, but rather 87 degrees BTDC (remember it trails the rear by 45 degrees). At this point in time the front intake valve is still on its seat, in fact still about 6 degrees before it comes to the opening ramp on the cam. The exhaust valve is just starting to close, but still near full lift. OK - no problem. That spark in the front cylinder with the exhaust valve open and intake valve closed won't do much.
Now for the rear cylinder. When the front cylinder spark plug fires at 42 BTDC on the compression stroke, the rear cylinder is in a much different position. Because of the the 45 degree offset, the rear piston is at 3 ATDC (after top dead center); technically not even still on the exhaust stroke, but rather beginning its descent on the intake stroke. The exhaust valve is closing, but still .135" off its seat. The intake valve on the other hand has already started to open to the tune of .185" off its seat. Valve overlap is the common term. Now that gives one something to think about doesn't it?
So her is some other pertinent info gleaned from PipeMax that may shed some light on the issue at hand.
Assume the Goose's engine were running 5500 RPM. At the point where the rear cylinder receives its "wasted spark" (3 degrees ATDC in overlap), the piston has already started down on the intake stroke, accelerating to 325.5 feet per minute providing a "piston demand" of 10.1 cubic feet per minute of intake air flow. Raise the RPM to the target 7500 and that becomes a piston speed of 443.8 feet per minute with a piston demand of 13.8 cfm. All that is at a mere 3 degrees after top dead center.
If you put stock in David Vizard's theories (and I do), you may recall that he puts a large amount of emphasis on the overlap portion of the cam timing. One of his conclusions is that the low pressure area caused by exhaust outflow results in the single strongest action initiating intake flow during this overlap period (hopefully I have paraphrased him properly). So what effect does it have when you throw a spark into the middle of that overlap period? A spark which incidentally does NOT occur during overlap on the front cylinder. Hmmm.
So here are a few thoughts. Obviously the rear cylinder is subjected to a spark during overlap which the front cylinder does not. Since that cylinder is in overlap, there will be a fuel air mixture present to burn. Now, that fuel air mixture is not compressed, but certainly it can burn none the less. And what naturally comes along with burning fuel and air? That's right - heat; heat that the front cylinder is not subjected to. I have no way of knowing or even estimating how much extra heat the rear cylinder gets this way, but the evidence would suggest that it may be just enough extra heat to melt a piston dome.
Here is something else to consider though. What happens when that wasted spark fires off and the fuel air mixture is richer than ideal? The BTUs are in the fuel, not the air, so I would assume that you would be releasing even more heat than with a correct mixture. Hmmm. Remember that I said that the Goose was not run too lean. It actually had the baseline jetting that it was dyno'ed with here in Minnesota. The reason I was pretty sure that the piston did not fail from a lean condition was that I believe that it was "pig rich" (as I like to call it). Could this be a case of a rich mixture giving the opposite results that one would expect?
But why does this happen at Bonneville, but not on the drag strip or on the street? Well, my guess would be that it has everything to do with length of time spent under a heavy load. Remember that aerodynamic drag becomes a huge factor at high speeds. There are plenty of horsepower/MPH calculators available on line. Plugging in some estimates (guess-timates?) for weight, frontal area and drag coefficient, we find that if it takes a mere 29 HP to hit 100 MPH, the same bike would need 55 HP to get to 125, and 77 horses to reach 140. And if that is not enough of a wake up call, if you want to raise the MPH from 140 to 150 you better be ready to call up an extra 17 HP to wring out that 10 MPH. Bottom line is that high speed puts a tremendous load on a motor, and the longer that load is present, the better the chance for heat build up.
Now we know that a drag motor will not see much time under full load and even less time at high speeds. If you hit 100 MPH in the 1/8 mile, then you will likely see the 1/4 mile finish line in another 4 seconds. And on the street? Despite thousands of bar room stories to the contrary, most street motors will never get more than a few seconds at full throttle and high speed before law enforcement rains on that parade.
So where do we go from here? Obviously a single fire ignition system is in order. The exact form that will take is still up in the air, with part of the team leaning toward keeping things as simple as possible, and part leaning toward as hi-tech as possible. The other obvious bit is that new pistons are needed (you didn't really think I would weld them back up, did you?). That part has been settled.
Four new pistons from Arias arrived this week, with the domes finished as per my sample (the undamaged front piston). They are down right beautiful, and hopefully the two spares will remain in the box as spares for a long, long time.
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