Wednesday, August 15, 2012

EL Bonnie Part 3: Don't Try This at Home

As I left off with the 61 cubic inch EL Bonnie motor, after a detour to make the cam cover usable, I was finally ready to blueprint the breather timing.  Mocking up the lower end reminded me that I would have to cut the flywheel diameter down to clear the crankcase oil scraper (pre 1940 flywheels were slightly smaller diameter than those used since). Some judicious work on the lathe brought the scraper to flywheel clearance to a snug .006".

With the mocked up crank assembly in the cases, next up was to get the degree wheel mounted and accurately indicating TDC.  If you have never done this, there is slightly more to it than just bringing the piston to the top of the cylinder and lining up the TDC mark of the degree wheel to a pointer.  That gets you in the ballpark, but certainly not close enough to base any timing events from.  Since there was no head installed, here is an easy way to get the degree wheel "degreed in:
  1. With the piston at the top, mount a dial indicator and zero the needle
  2. Install the degree wheel on the sprocket shaft and align a pointer with the TDC mark
  3. Pick an arbitrary number on the dial indicator (example - .050) 
  4. Rotate the crank counterclockwise until the needle on the dial indicator matches the number you have chosen.  Note the exact reading on the degree wheel at that point (example - 9-1/2 degrees after TDC)
  5. Rotate the crank the opposite direction, past TDC until you reach the same dial indicator reading you chose in step 3 above, and again note the reading on the degree wheel
  6. If the two reading from steps 4 & 5 match exactly (unlikely), you are done.  If they don't match, then loosen the degree wheel and move it to split the difference.  For example if your reading from step 4 was 9-1/2 degrees after TDC and the reading from step 5 was 10-1/2 degrees before TDC, then move the degree wheel so that its pointer is on 10 degrees. 
  7. Repeat steps 4 & 5 until both give exactly the same number of degrees before and after TDC
(note:  Blogger, in their infinite wisdom, autmatically converted the numbered bullet points I have in the draft form to the goofy looking flowers that you see - guess you will just have to count them)

Once the degree wheel was properly installed, the opening and closing specs for the breather gear was checked, looking through the lifter block holes.  Rather than go through the process here, I suggest you download the excellent instructions available from S & S Cycle here.

Since a degree wheel was installed, it only made sense to check out the cams, especially given the discrepancy found in cam opening and closing specs due to mis-machined lifter blocks.  The lifter blocks seemed to be OK, but the exercise did pretty well make the cam choice for me.  The KN420 cam from S & S checked out good.  The old Sifton, not so much.  One intake lobe showed a 12 degree discrepancy on the opening, 5 degrees on the closing (for a total of 17 degrees less duration) and .020" less lift than it should have, all despite showing no sign that it had ever been run.

Of course since the motor was mocked up as far as it was, it also made sense to clay the pistons to get a sense of how much valve to piston clearance there was.  The answer to that question was a lot.  In fact, more than a lot; make it a huge amount.  That was a bit worrisome, given the fact that I was already apprehensive about the compression ratio the engine would wind up with.  The custom built pistons claimed a 10:1 compression ratio but the large diameter intake valves were sure to need a little more radial clearance in the valve pockets.  Given my "druthers" I'd have liked to see 12:1 for a starting point.

Time to check the piston dome volume, another fairly straight forward procedure which involves putting some grease on  the rings, installing the piston into the cylinder to a measured depth, capping the cylinder with a Plexiglas plate and a burret to measure the amount of oil it takes to fill the void.  Then by mathematically calculating the number of cc's that would be in the cylinder without the piston dome (using piston depth and bore) and subtracting the measured cc's with the piston dome in the cylinder, the actual dome volume is revealed.

Entering all the figures into my Engine Analyzer program, which takes into account head volume, piston deck height, head gasket thickness, head gasket bore and adjusting for fire ring and fire ring volume, I came up with a very disappointing 8.25:1 compression ratio.  Worse yet, there was no room to shave the heads due to the proximity of the 2.060" intake valve to the fire ring.  Now what?  I was really beginning to worry that this little 61" motor would be badly over-cammed at this compression ratio. 

I decided to call Jim Leineweber of the cam company that bears his name.  While he was not wild about the low compression ratio, he said he currently had nothing on the shelf that would work any better.  Jim did boost my confidence by affirming the clearances that I planned for the various engine parts, but then I always have been one of those who do well on tests.  And it really did seem much like an exam, as he asked what each clearance I planned to use, and then gave each of my answers a "OK- that's good - a lot of guys set that too tight."  One key piece of information that I was not sure of, Jim provided.  Set the ignition timing to 42 degrees BTDC as a starting point.  That tidbit probably made the whole conversation worthwhile; though any conversation with a living legend goes down as worthwhile in my book.

But, memorable conversation or not, it really did not solve my compression ratio problem. In fact it really narrowed it down to the "too deep" valve pockets on the pistons...

Now we come to the "don't try this at home" part of the title of this post.  It was far too late to consider having another set of custom pistons built, even if funds had been available for such a thing.  But out of the recesses of my mind, I managed to dredge up something that might be of value.  Some years ago Mike Roland mentioned to me that he had once welded up the domes on a set of pistons for a drag racer who was on a tight budget.  Hmmm.  Seemed that a consultation with the best welder I know might be in order.  John from PMFR informed me that as long as the pistons were not alloyed with silicone, I should be able to weld on them with no problem.  He further reassured me that I would be able to tell the moment I struck an arc - a silicone alloy piston would immediately result in massive amounts of black soot. 

Still, what would be worse?  Going to Bonneville with a motor down on compression, or missing another year because I screwed up the pistons?  It was about this time that I spent a Sunday afternoon watching "The World's Fastest Indian" again; just for a little inspiration.  The more I pondered old Burt Munro casting his own pistons in the little shack he lived and worked in, the more I knew I had to give it a shot.  Besides, the ceramic coatings I planned to use on the piston domes should give a little extra protection.

That's not to say I wasn't still plenty nervous about it.  What if I managed to distort the ring lands?  What if I screwed up in some other unforeseen manner?  And what might that unforeseen screw up be?  Obviously no point in pursuing that line of thought.  If I could come up with a possible screw up, it would not be unforeseen.

To make a long story short, the pistons welded very nicely.  In fact a steadier hand on my part would have resulted in a nice looking job.  But, despite the lack of beauty in my welding, I was confident that it was structurally sound - least as structurally sound as one can be when welding a piece that normal people would not consider mistreating in such a way.  So, back to the mill to re-cut the valve pockets to a minimum depth, and then mock up with clay once again.  With only a minimum of frustration I achieved valve to piston clearances of an acceptable distance.


Machined after Welding

Back to the same old drill of cc'ing the newly larger piston domes and entering the results into the computer.  To be honest, by that point in time I would have been happy to see 9:1, so I was quite pleased to find we were up to 9.8:1!  Next up: balancing the flywheels.

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