Darryl.Richman's picture

Today Team Boxer Rebellion took the ailing R52 to Big Twin BMW in Boise. Fred Wiley helped us a great deal when he put us in contact with Lamb Cylinder (208-322-0348).

Brent Lamb immediately took an interest in our problem. As mentioned in the last entry, we felt the way forward was to modify the tops of the valve stems in our new cylinders to accept the collets from the old valves. But Brent argued for a different approach. Instead, he bored our original cylinders to accept the pistons from our new cylinders. This way, all the valve gear continued to function.

He put about 5 thousands of an inch clearance between the new pistons and the original cylinders, because our new pistons are actually very old school. Pistons are not, these days, round. They are slightly oval. The piston pin defines the narrow axis. The reason for this is that the piston skirts are relatively thin and don't expand much with temperature, but the heavy piston pin supports expand much more. So the narrow axis grows more with heat, and the difference in axis is intended to allow the piston to heat up to become nearly round. (Pistons are also slightly tapered, narrow at the top and wider at the bottom, because there is more metal at the top and so they expand more there under heat.)

But before WWII, most pistons were made round (and straight). To accommodate the growth the of the piston diameter in one direction, piston skirts were sliced, and the growth was absorbed by this slice. Our new pistons are made this way, and adding a bit more room in the bore is a safety measure.

We reassembled the bike and fired it up. It sounded quiet, even and did not ring or knock. Unfortunately, as it was just warming up a bit, it seized. As soon as it cooled a bit, the engine freed up again. So it all had to come apart.

At first Brent opined that perhaps the slice, which we had consistently oriented down, should have been up, because the heat from the exhaust gas would be concentrated up, near the exhaust valve. But after putting micrometer to the top, bottom, pin axis and across the pin axis, he said that the pistons were junk and were built wrong. He found that the pin axis, which should be the same as across the pin, if not smaller, was in fact larger, and that the piston was also larger in diameter at the top than at the bottom.

But we were really in a hole at this point. These are our only new pistons and none of the used pistons would do. (Besides, our cylinders were now all too large to work with anything else.) After some discussion, Brent bored an additional 3 thousands from the cylinders. We were very concerned that this would make the pistons slap and essentially put the cylinders into an already worn out dimension.

It was already after 5pm when we finally got the bike reassembled a second time. (I have to tell you that there's a lot of work to getting the cylinders off on this bike. The heads must be removed, the carburetor and its induction tube, the clutch cable, the entire exhaust and the floorboards, the shifter gate, and more. And on reassembly, all this must go back, as well as getting the right torque on the head nuts, cylinder base nuts, putting gasket "goop" on the base, compressing the rings on the pistons so they can be slid into the cylinders, and a valve adjustment.)

We fired it up -- first kick, just like the last time -- and let it idle to warm up. It was still quiet and even, and it didn't seize. When it was warmed for a few minutes, I took it on a brief ride. It had good power and acted well. Our fingers have cramps from being crossed so long.

The bike went into the van and we dashed for Burns, OR. Tomorrow, I will try to get another short ride in, long enough to fully warm the motor and the oil, but very gentle. Then we will change the oil and retorque the head nuts, before the day's run to Klammath Falls, OR. I will take it very easy on the bike and try to stop at frequent intervals to allow the cylinder bores and pistons to mate and break in.

Comments

Good luck!

liebenstein7594's picture

Daryl - best of luck! you are fighting hard - great spirit. Glad you met people who are willing to help and have expertise. Hope it works and see you coming into San Francisco.
I try to make my way up on the weekend.
Holger

WooHoo!

Voni's picture

Go, Team. Never say never. Wish we could be there.
You guys are fighting one Hell of a fight!

I am slack-jawed at your

David Brick's picture

I am slack-jawed at your (plural - you've got help!) persistence and resourcefulness and creativity in meeting these challenges. Onward!

Pistons

Jim Hansen's picture

I recall reading a statement, I think it was by Duane Ausherman, that some of the older BMWs (R60/2) would seize due to the top of the pistons (above the top ring land) being too large. If your pistons continue to seize, maybe this part of the piston could be filed down a little, if the alternative is to junk the piston. Another wild idea of desperation, try adding some two-stroke oil to the gasoline.

Rooting for you to the finish line

Brandall Wong's picture

Darryl,
What will and determination you possess. You're a winner in my book wherever you place.
See you at the finish line.
Brandall

It's a setback...

Mr. Jim's picture

...Not the finish.

Now heal and be well...

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