Hi Ralph,

Have you considered getting a cone precision machined up in steel to bring the gear doen to 101 size? After machining to size a keyway slot would have to be added. The gear you have looks too good for the junk box !!

The cone would have to be loctited to the gear  and/or possibly brazed at each end for safety, although this operation could soften the teeth .

I looked around for nearly a year for a second hand primary gear for my bike, but gave up looking in the end and kept with the 90% ok original. I too have a chief gear here.

I should have added that a deeper key would be needed if a sleeve was made.

I wonder what George would have done faced with the same circumstances ?

Mike.

Carl-Erik, don’t forget that when the power is shut off, and the engine goes from drive to overrun, the crankshaft goes from driving to driven.  The sideload caused by the helical gears will reverse direction in this case, maybe more significant to the wrist pin walking you describe.

I can’t recall whether it’s in the R&O or not, or even if it’s applicable to 101 motors and excuse me if I’m covering well-trodden ground, but I have cured this issue on other motors (Russian flathead motors suffer from this) by removing the legs from wire type wrist pin clips, and grinding a 45° angle on the outer circumferences of the wrist pin.  Wrist pin lateral movement then holds the clip into its groove rather than forcing it out.

Stuart Morgan wrote:

when the power is shut off, and the engine goes from drive to overrun, the crankshaft goes from driving to driven. 

Well that is relatively once in a while you power on and off if compared to the revolutions a engine makes, so the constant pounding from loaded to unloaded in every combustion pulsation is more wearing. It has to be rods vibrating that makes this kind of damage in my opinion. The damage when the lock wears the groove wide does not come from a intermittent big push but from gradual wear and a constant vibrating force upon it.

I had C clips in my pistons, no tangs at all, chamfered pin ends with fairly small play between the pin end and clip. I had to scrap the pistons because the pins had pushed on the clips so much that the grooves on both sides was twice the width from the start (and I am not using the crankshaft or the gear anymore until I have tools to check those).

The engine was blueprinted partly by me but also by a trusted machinist, crankcase cylinder surface planed, races honed and checked twice. All shafts and bearings new, cylinders new but checked and shaved for straightness, piston play and ring gap ok, all play as recommended, rods new races and bushings, straightened to 1/1000mm straight and rechecked. And of course the crankshaft was straight to the Tee. I did the assembly all within or better than specifications (sorry but stupidly enough I don't have all measures written down)

The only thing not checked by me was the work the machinist did due to my lack of jigs and tools, and the crankpin relation to crankshaft and drive gear concentricity and contact path. I am out of ideas. Maybe I am looking at the wrong end, maybe it is the cam side that is the culprit but I don't belive that. Some engines encounter the same problem but some does never have this kind of problem. Maybe the symptom can be concealed or hidden with a larger play at the pin bushing, but that doesn't solve the enigma.