crankshaft stud issue

I'm attempting to install 3/8 studs into an early 750 crank but have hit a snag.

1) the nuts for the upper studs (11 and 1 o'clock) are too large for the recess in the 750 crank. The way I see it I have only 2 options: enlarge the recess in the crank cheeks, or put a chamfer on the nuts.

2) I drilled slightly undersize, and then reamed the 1st hole. Manual says the studs should be a tap fit, but mine are a snug sliding fit.

I haven't proceeded any further, and would appreciate any input from those who have performed this upgrade.

Cheers

I assume you are talking about just using the MK3 studs and nuts. I used to convert 750 cranks to 3/8" fasteners back in the '70s and '80s, using NAS fasteners, and encountered the same problem. I solved it by turning down the OD of the nuts to fit the spot facing on the crank cheeks. You can see the modified nuts in this picture, in the lower right. I see no reason you couldn't do the same with the nuts you are using. For what it's worth, this particular crank is still in service in a 920 race bike and has survived many years of hard racing.



Ken

lcrken said:

I assume you are talking about just using the MK3 studs and nuts. I used to convert 750 cranks to 3/8" fasteners back in the '70s and '80s, using NAS fasteners, and encountered the same problem. I solved it by turning down the OD of the nuts to fit the spot facing on the crank cheeks. You can see the modified nuts in this picture, in the lower right. I see no reason you couldn't do the same with the nuts you are using. For what it's worth, this particular crank is still in service in a 920 race bike and has survived many years of hard racing.

Ken

Click to expand...

Thanks, that's just the input I was looking for and I got similar advice from Mr. Comstock. I certainly wasn't keen on the idea of enlarging the recess. I think I'll put just enough of a chamfer on the bottom of the 4 problematic nuts to clear the shoulder of the recess