Tolerance Stack Up

 Tempting as it might be to save this title for a commentary on modern day culture, I decided to use it for this piece with a more traditional understanding of the term.  As it pertains to motorcycle mechanics, the term is commonly understood to refer to the manufacturing tolerances of parts that are used together and how those plus/minus tolerances can affect overall clearances and fit.

When dealing with just two parts, it’s a very simple concept to understand.  Say a shaft is designed to have an outer diameter of 1.000” with a manufacturing tolerance of +/- .0005”, and the bushing it rides in is designed to have an inner diameter of 1.002” inner diameter with a manufacturing tolerance of +/- .0005”. That means if the both parts are spot on spec, the combo will have .002” clearance.  If the shaft is at the low end of the tolerance (.9995”) and the bushing is at the high end (1.0025) you will end up with .003” clearance. On the other hand, if the reverse is true (1.0005” on the shaft and 1.0015” on the bushing, then all of sudden you only have .001” clearance. 

Obviously when dealing with multiple items that are part of the same assembly, it can get interesting in a hurry.  Consider the “squish” clearance (distance between the ledge on the cylinder head and the piston deck at TDC) on an Evo or Twin Cam.  Each part that contributes to that distance has its own manufacturing tolerance which affects the final assembled clearance.  Here are some of the measurements that come into play: The distance from the centerline of the crankcase main bearing to the surface where the cylinder mounts, the cylinder length, the gasket thicknesses, the center-to-center length of the connecting rods, the distance from the centerline of the piston pin to the upper surface of the top ring land, and even the centerline distance from the main shafts to the centerline of the crankpin (which also determines the stroke, BTW).  If all those tolerances stack up one way, the squish distance would be smaller than if they stack up the other direction. And that is why those who build performance engines take the time to measure and adjust that final distance to make the squish clearance as efficient as practical.

But what brought this issue to mind and caused me to write this piece, is much more mundane, and in fact a bit humorous.  The other day I was putting the finishing touches on a cone Shovelhead lower end.  I had already installed the new oil pump, checking that it turned smoothly. I had selected the pinon gear which provided just the right amount of lash between it and the cam gear to provide quiet operation (not too loose and not too tight). I had test fit both the cam and breather gear for proper end play before the final installation of the cam cover with Loctite and a torque wrench applied to the screws. Satisfied with my work for one day, I would get back to it in the morning.

But the next morning, when I went to the next step, that of installing the tappet blocks and lifters, much to my surprise, the crank would no longer turn! Where in the world had I messed up?  The cam still had end play so the next thing that came to mind was that perhaps the cam cover gasket had compressed more than expected, taking up the breather gear end play that I thought that I had established.  Off came the cam cover again. Nope, …still locked up.  Now this would have really been a puzzler except that right about then I remembered that I did one the last thing after installing the cam cover the previous evening.  I installed the timing hole plug. 

The brand-new S&S crankcases came with a nice zinc plated timing plug that would easily pass for an oem part. But the customer had a nice new shiny chrome plated Colony brand timing plug, so of course that’s the one I installed.  Sure enough, the chrome plug was just enough longer that it contacted the flywheel and kept it from turning.  But before we condemn Colony, let’s think again about this whole tolerance stack up issue. This particular motor has a set of original Harley flywheels, …you know the kind with the wide and heavy left side wheel. So, the fact is, the Colony timing plug would not have touched thinner S&S flywheels when used in S&S cases, and neither would it have touched stock flywheels in stock crankcases. And of course, the S&S timing plug doesn’t touch the stock flywheels when installed in the S&S case. But the combination of parts from two different sources with their differing tolerances, turned out to be just enough to make them incompatible.

THE PARTS IN QUESTION

 

And that brings to mind a similar situation I ran into about 30 years ago, also while building a Shovelhead engine.  The parts list included Delkron Shovelhead cases and a set of the generic aftermarket lifter blocks that nearly all the major parts distributers sold (and still do to this day). One of the lifter blocks, whether front or rear I do not recall, would not physically fit into the crankcase.  In an effort to determine where the fault was, I took a stock lifter block and it dropped right in to the Delkron case.  But then, before counting the aftermarket lifter block out, I tried it in a stock Harley case.  Sure enough it dropped right in.  Try as I might though, there was no way that aftermarket lifter block would go into the aftermarket Delkron case without one or the other having some metal removed!

Maybe those two examples don’t quite fit the technical definition of a tolerance stack up, but they give some idea of the adventures that await those who source their parts from multiple manufacturers in their efforts to keep old Harleys on the road.

Just One More in a Long Line of Tech Tips

 

Here is a tech tip that I recently ran across which is so cool I just had to share it.

I’ve been changing tires for about 50 years now, first as a teenager for the “beater” cars that I drove and later on motorcycles.  Early on, I would use a bumper jack to break the bead and large screwdrivers to remove the tires from the rims. Later on, working in motorcycle shops, professional equipment such as bead breakers and tire irons made the work easier and more productive.  I’ve even had the opportunity to use a few different tire machines over the years, though I tend to be just as happy using hand tools.

One thing that remained constant through the years though, is the problem that would often come up when a tubeless tire seemed to be just too narrow for the rim, making it difficult to get the combination to start taking air.  One trick that would sometimes work would be to wrap a tie-down strap around the circumference of the tire and tighten it to spread tire’s bead apart.  On a combination with a particularly stiff and/or short sidewall tire and an unusually wide gap between tire and rim, that would be marginally effective.  Inevitably, at that point someone would raise the age-old suggestion of using starter fluid and a match to create a “small” explosion that would “blow” the beads of the tire into position. Well, despite the suggestion coming up often, and the general agreement that everyone “heard it works”, I never worked with anyone who had actually tried it. As someone who prefers to keep his rapid expansion of flammable mixtures inside of a combustion chamber, I never did attempt it myself, neither have I witnessed it in person.

That said, I had occasion to consider the options once again recently.  I had just finished mounting a pair of new tires for our “vintage” motorhome and the very stiff sidewalls were hopelessly far from contacting the rim.  Ratcheting a strap tightly around it did not move the tire beads an iota closer to the rim.  Finally I decided to do an internet search with the hope that someone, somewhere had come up with a good solution.  The one I found (and used successfully!) is ingenious, cheap, and safe.  And perhaps best of all, there is a video which saves any more explanation on my part.  Prepare to give Brian Jordan a round of applause.

 

What’s in Your Bead Blast Cabinet?

 

It’s been a long time (too long) without a tech tip, so here is a good one. 

My first introduction to “bead blasting” as it’s most commonly called, was during the fall of 1979 in the Motorcycle Mechanics class at Hutchison Vo-Tech.  And what a wonder it was to everyone in the class.  Parts that were dirty, carbon encrusted and with burnt-on oil deposits suddenly looked like brand new parts!  Of course, every motorcycle shop I worked in from there on out had a bead blast cabinet.  A great tool, and an even better one when used properly.  Can you believe it took nearly 40 years for me to discover the proper use of that tool? 

One of what I considered to be an idiosyncrasy of the bead blaster, turns out to be just a symptom of that misuse, or should I call it misguided use.  But recently, I found the answer to the question that I had failed to ask, or rather the observation that I should have put in the form of a question. That observation goes something like this: when you put fresh clean glass beads in the cabinet, you get a bright clean finished product, especially when the part you are working with is aluminum.  But when you load those nice clean beads, it always seemed that they just didn’t want to clean all that carbon and baked on grease off of the part as well.  Then after running a goodly number of parts through the cabinet, it would seem to remove the deposits more quickly, but as the cleaning improved, the finish went from bright to a dull color that seemed to also pick up any greasy fingerprint that came within spitting distance of the part.  I, and I am sure many others, got in the habit of trying to make sure relatively clean beads were in the cabinet whenever a nice final finish on the part was important.  I had often considered purchasing a second blast cabinet so that one would always have fresh clean beads for that purpose, but suffered along with changing the beads as needed instead.

So, where did I finally learn the proper use of a blast cabinet, you ask?  Probably the last place in the world one would think you would find technical information.  Ebay. Yes, … Ebay.  It probably began with a general internet search for glass beads because the source I had been using suddenly and without warning tripled its shipping charge to get a 50 pound bag  to me from the other side of the city. As is so often the case, a search for nearly anything will come up with a result from Ebay stating “buy fill in the blank on Ebay". But this time it was a winner. 

It turns out that Tacoma Company not only sells their abrasives on Ebay, unlike other suppliers I have dealt with, they provide information on the proper application of their wares. Not saying other suppliers don’t have that information, but sometimes you don’t even know that you should be asking a question, let alone what that question might be. 

So the answers to some of the questions I had not known to ask are as follows:

1. Glass Beads are round and are meant to “peen” the surface, not to clean it. 

2. Glass Abrasives are sharp and do a great job of cleaning but leave a dull finish that dirt will impregnate easily.

3. Glass beads can and should be used at a much lower air pressure which keeps them from shattering and turning into abrasives.

Armed with this information, a couple years ago I added a second blasting cabinet to my shop.  Now the first stop for heads and other parts needing the carbon, baked on grease and grim or paint removed is into my old industrial quality blasting cabinet containing glass abrasives to clean then quickly and easily. Then the clean part goes into an economy blasting cabinet at a lower air pressure for a bright finish that does not pick up dirt easily.  An added bonus is that the glass beads now last many, many times longer since they are not being broken up by the high air pressure.  Who knew?

Now, if you have a supplier of the proper abrasives down the street from you, obviously you can purchase the correct product for your application cheaper if you don’t have to deal with shipping costs.  As for me though, I will continue to purchase mine from Tacoma Company since the information they provided has not only saved me gobs of time, it has helped me to present a better looking final product.

 

Shipments arrive with a courteous Thank You note

 

 

... Along with some words of wisdom

 

And even a pocket size copy of the U.S. Constitution

 

 

My kind of people!
 

A Head Scratcher

Nobody likes one of those “head-scratcher” comebacks, though any honest mechanic will probably have a few tales that they are willing to share.  Of course, when the comeback winds up being no fault of your own, then the telling comes a little easier, accompanied as it is with a sigh of relief.

But to shift into reverse for a moment, in case any readers are uninitiated, a “comeback”, in the mechanical repair game, refers to having a customer returning with the same (or closely related) problem which was recently “fixed.”  A head scratcher, on the other hand is something that is confusing, mysterious or hard to understand.

One of the first useful things most “techs” (as they are called these days) learns is that when attempting to find a problem with a bike, the first place to look is the last part of the bike that was worked on.  That rule of thumb is valid whether said previous work was done by the owner, another shop, or the ace mechanic you consider yourself to be.

The tale of this particular head-scratcher is presented here, not only for its educational value in general, but also in the hope that it might save someone else from chasing an elusive problem.

Earlier this year I had occasion to do a stock valve job on a set of late model Twin Cam heads. The heads were carried in, by a customer who we will call John. We’ll call him John for two reasons.  One is that “John”, being a very common name will grant the customer a certain degree of anonymity, and secondly because that is his name.  While not a professional, John is also not a stranger to R&R (Removing and Replacing) top ends. The reason the heads were brought in was basically as preventative maintenance due to about 70,000 miles on the clock.  The top end was off to install some big-bore cylinders and pistons, so John decided that it would be worthwhile to be sure the valve job was up to snuff at the same time.  The job appeared to be about as straight forward as they come; the exhaust guides were bell mouthed past service wear limits so I replaced them, sized them and cut all four seats.  John also brought in a new set of stock valve springs and guide seals purchased from the local Harley dealer where he had acquired the cylinder and piston kit. He requested that I install them as part of the valve job.  I wrapped the job up in short order and life was good.

Then, a few weeks later, John returned with the rear head.  As an aside, let me tell you how much I appreciate this type of customer.  Most of us who have been working on bikes for a few decades have had to deal with the customer who returns with the attitude that you have screwed up his bike and he’s not going to put up with it (after all, his bike was perfect before you fixed it)!  John was the opposite of that.  He explained what had taken place, described the symptoms and calmly asked what I thought might be causing it.  

The symptoms were that once the engine reached full operating temperature, it would begin smoking profusely from the rear head.  John reported that his first thought was that he may have inadvertently installed a piston ring upside down, which only goes to show that the guy is no hack, since that also would have been my first suggestion to check.  However, the rings were in correctly, and there was no sign of oil leaking in the head gasket area.  Yes indeed, it did seem as though a close re-examination of the head was in order.

Which I did promptly.  But here is where it gets to be puzzling.  The valve seals were still in place.  The valve to guide clearances were good: right where I had measured them when freshening the valve job.  I put lacquer thinner in the spring pockets to see if I could detect any porosity: none detected.  I even went so far as to give each guide a solid rap to ascertain that they were not loose in the head: they were not.  

Keeping in mind that the engine in question did not have any symptoms prior to pulling the top end for the big bore kit, there was only one other possibility concerning the head itself that I could think of. If the rear head had porosity between the spring pocket and the port one would have expected that to show up pretty early in the life of the bike (and remember this one had about 70K smoke free miles in its history).  However, for quite some time now, Harley heads have been powder coated before they are machined in the manufacturing process.   Because of this practice, the inside of the ports (and to a lesser extent the spring pockets) leave the factory with a nice thick durable coating.  Of course, when cleaning up the heads for a valve job, media blasting removes the carbon and along with it the powder coat.  What do you suppose the chances are that by removing the carbon from the port, I had also removed the coating which had been sealing porosity, and in this case porosity which only opened up at operating temperature? 

If the scenario of a porosity which only opened up at temperature sounds bizarre, I might have agreed had I not seen it in the past. Way back in the early 1980s, the dealership I was employed by had a new Superglide, still in its 90-day warranty, return with similar symptoms. Under the tutelage of the dealership owner/ head mechanic (and when is the last time you heard of anyone holding that dual title?) I put the head on a burner of the shop’s electric stove with oil in the spring pocket.  Sure enough, once the head got enough heat into it, the oil started dripping through into the port at a brisk pace.  Rare?  You bet; but if I ran across one 35 years ago, why not another one now? 

Since the architecture of the Twin Cam head did not lend itself to holding oil in the spring pocket while heating on a stove, instead I chose to coat the interior of the spring pockets and oil drain passages with a product called “Gasoila Hard Set” to seal any potential porosity from the top side.  This had the added advantage of sealing off any possibility of oil getting between the head and the outside of the valve guide.  Still, it was a bit of a shot in the dark since I was unable to actually pinpoint any problem.

That of course still left the rest of the top end as the potential culprit.  Though John had checked to be sure he had installed the piston rings in the proper orientation, he took my advice and replaced the new rings based on my theory that it was better to spend a bit more money than to chance spending a bunch more time later.  Of course if the end result was success, then we might never know for sure whether the heads or the rings were at fault, but that was something we were both willing to live with.

So that is what the John did.  He once again installed the top end, with new rings & gaskets along with the (hopefully) repaired heads.  And all was well with the world, …at least until the engine once again reached operating temperature! Whoa!  Or maybe even Woe!  What in the world could possibly be going on?  At this point he made a trip to the dealership and described the conundrum to the service techs, but none could offer any ideas which had not already been addressed.  Now what?

Well, after a few more test rides to be sure it wasn’t just some left behind oil in the exhaust pipe causing the smoke, John received his first solid clue.  Pulling the “smokin’-hot” bike into his garage (well, …”smokin’-warm” to be more accurate),  he started to unscrew the oil reservoir cap only to be greeted by a sound just as if he had removed the valve stem from one of the wheels.  Suddenly everything pointed to the breather system.  And just as suddenly John thought of the new breather assemblies which came with the kit, the ones he almost didn’t change out but at the last minute decided he might as well install since he had them. You know, the stamped steel ones that come pre-assembled. Yeah, those.

Sure enough.  As it turned out, the umbrella valves in both of them were installed upside down, effectively sealing the engine and turning the crankcase into an air compressor. I must admit to being a bit impressed by how good a job Harley has done with sealing these Twin Cam engines, because evidently there was not a hint of oil weeping from any joint, seal or gasket.

 

The whole situation reminded me a bit of a stroker Shovelhead motor that I built for a customer back in the late ‘80s or early ‘90s.  Harley had recently started to install a little squiggly plastic “oil separator” into the breather pipe passage between the cam cover and breather trap.  When I built the Shovel motor I decided to try one since some oil carryover was not entirely unusual on strokers. Perhaps Harley’s latest technology would help, I reasoned. 

The results could have been a disaster.  The motor “wept” oil from the base gaskets, the tappet block gaskets, the pushrod tubes, and nearly every other place you can imagine.  The customer was not impressed.  When he brought it back, I immediately decided that the new-fangled oil separator was too restrictive, and removed it.  The only question I had was whether the oil leaks, once started, would go away when the added restriction was removed.  I thought it worth a shot, though the customer was convinced that it would not, and perhaps suspected that I was trying to dodge my responsibility to stand behind my work.  But he doubtfully took the bike, and sure enough, a couple weeks later came back to say that the engine had completely dried up – no oil leaks or weeping anywhere.  I felt like I had dodged a bullet and never put another one of those separators in a motor and removed them from any motor I found one in.  Evidently Harley came to a similar conclusion, because they did not make it into many engines.

So, I guess the moral of the story is beware of your Harley's breather system or it might turn into a real head scratcher.