Originally Posted by koan58
According to TMS, he says that any T120 crank pre 1968 has main journals 0.0005" larger than post 68 cranks, and should run a C3 driveside roller, later cranks should run a C2 roller (I wonder if he meant a CN really, but I didn't think about it till after hanging up).
If I use the C3 when a CN may have been more "perfect", do you consider that it will result in any adverse consequences eg longevity, vibes etc?
I don't know of any change to mainshaft diameter.It's always been listed as 1.1247"-1.1250" on the drive-side,even for a T140 (well after '68).Trident was slightly smaller,1.1245"-1.1248".
CN roller was always fitted,until '76.
'76-onward used C2 (BUT SAME PART NUMBER !!).I think the difference was in the crankcase interference,not the shaft.The bearing ID is around 1.1247",so there was very little,if any,interference on the shaft.
Before fitting to crankcase or shaft,the bearing has a little extra internal clearance to allow for interference fits.It will tighten up with interference.
C2 has 0.0004"-0.008" clearance
CN has 0.0010"-0.0016"
C3 has 0.0018"-0.0022"
The biggest problem would be a bearing that doesn't have enough clearance when fitted,and won't turn freely.That's not good for bearing life,and likely to fail.
If it's too loose,you'll get some rattle/rumble when you back off at higher rpm.
If the bearing turns freely and has some small minute clearance,all is good.There will be more clearance when the crankcase is hot.
If it's tight or extra loose,you can use a bearing with more or less internal clearance.If the crankcase was an unusually tight fit on the bearing (possible,but unlikely),you may need a C3 roller.That would give you 0.0002"-0.0012" more clearance than a CN roller.