CMB has tightened up their tolerances on the crank journals recently, they used to be about 1/2 thou sloppy (our VAC 91's), they're now about 1 tenth under size (perfect), instead of the bearings dropping on you now have to push them on with a "light press fit". The loose fit wasn't a problem for the front bearing as the inner race is clamped between the crank step and the flywheel collet but the big bearing's inner race could spin on the crank and overheat, expand and cause bearing failure. I've had inner races turn blue and if not caught in time it Brinells or the balls start to chip. Ceramic balls help but recently I've discovered another issue.
When your running in very cold temps I don't think the bearings are loose enough for the "new" fits CMB are giving us. I've recently been running my SAW boat at temps in the low 30's and had a bearing Brinell. Even thou I fit them to the high side of C3 (4-10 tenths for 10-18mm bore) the cold would squeeze the case down and there wouldn't be enough room. For fun I put a case in the freezer overnight and the rear bearing pocket went from 0.0006" to 0.0018" undersize, it shrank over a thou! I remember Jim Allen was a big fan of C4, I don't know what his press fits were but I can see why. The rear bearings on our little motors are kinda unique in that both races havta be secured, guys have tried other ways over the years (clamping nuts, grinding the inner race right into the crank etc.) but for us I think just extra clearance might be needed. I'm pretty sure David Spitzer ran into this also at the Cinci record trial, it was low 30's each morning and he smoked a good set of bearings in his CMB 80.
Is this what's happening with the new K21 as well? Either the bearings don't have enough internal clearance for the case/crank fits, or the crank is too short between the journals compared to the case?
ps: I'm thinking about having the cranks on my VAC91's flash chromed, a couple tenths per side would be just about right...
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