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Con-rod update (harold)

Harold had asked me to post the final resolution of my wrist pin bushing experience. I was re-doing a bmw motorbike engine and found that the wrist pin bushings had become quite loose in the end of the cranks.

I had planned on pressing in new ones and boring to size, and was wondering if my inexpensive boring head could do the job.

Well as it turns out there is a better way. Ed Korn of Cycleworks in Madison WI

rented me an interesting tool to do the job.

Basically the bushings are pressed into the rod end, in my case they were a three or four thou interference fit.

Then one presses a 20.05 mm hard steel ball through the undersized bushing bore. This extrudes the material and provides a great surface finish for the wrist pin to run on.

The steel ball is 0.05 mm oversize from the wrist pin, but the elasticity of the system is such that it requires several go-throughs to achieve the thou or so clearance required for the pin.

The entire process *could* be done with the motor still in the bike, because the tool is small and can be put on the end of the rod as it protrudes from the motor.

Jim

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Reply to
jim rozen
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what keeps the path of the ball straight?

Reply to
bridger

It is a ball. It ball could twist any way you could think of but it will still follow the hole you are stuffing it through.

Reply to
Roger Shoaf

That system is no stranger to me, although I've never tried it, and I certainly didn't think of it. If you recall, about three years ago we had a discussion about boring Oilite bushings on RCM. It was that guy in Australia, Tom Martin, who had talked about pushing balls through bushings to size them, the preferred method according to what he had to say and what little I read about the process. I was concerned that burnishing the bushings would close the pores, making them less effective, but that is apparently not the case.

I think the one great advantage in your situation is the fact that you've achieved a surface that is already burnished, so the pin is highly unlikely to do much to the bushing in the way of effecting its diameter. I would expect a greater life span over boring unless you'd have set the bushings up quite snug.

Glad to hear you got the deal resolved, Jim. I trust you don't have any alignment problems, piston to cylinder? I can see how a slightly cocked bushing could lead to some trouble that the ball couldn't address.

Harold

Reply to
Harold & Susan Vordos

And of course here the bushings are plain bronze so that is not a concern at all.

I was really pretty pleased with the finish. Quite amazed actually. There were a few high spots where the press-up maybe had a bit of unevenness but I marked the fit and scraped them back down before one last burnish.

Don't think so. These are pretty low output engines so things are pretty relaxed. The rods don't seem to be twisted or tweaked in any fashion, and the wear on the pistons is pretty even. So I was loath to disassemble the pressed-up crank because it had been giving excellent service in all regards.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

Reply to
jim rozen

Reply to
Machineman

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