G80CS crankshaft endfloat - Advice please

[catalina500]

Hi im a newcomer to AMC and the club. Im just in the process of stripping my very late G80CS engine number 556X so one of the very last. I have the engine on a stand, all the primary side, rotor/crank sprocket removed and was just working on the timing side removing the Norton type oil pump. As i pulled the pump from its studs I `noted quite a significant axial/end float movement on the crank, a mm or two (oh no being my instinctive reaction LOL) does this suggest i will find something very nasty or missing when i split the crank cases or is the crank simply pulled/held axially against the drive side main bearing when the rotor nuts are tightened up - the latter would surprise me, but pleasantly on this occasion. Your advice/experience appreciated

[g80csp11]

Excessive clearance with the primary side off is not a problem , only when the primary side torqued up will the shaft be pulled one way in the crankcase . the clearance you feel now just ensures there is allowance for thermal expansion

[catalina500]

Thank you for an early response, that gives me a great deal of comfort and will ensure i sleep tonight. i must confess the bike sounded mechanically nice and quite, for an all alloy motor, before i started the strip-down.

[Joker_Bones]

or is the crank simply pulled/held axially against the drive side main bearing when the rotor nuts are tightened up - the latter would surprise me,

This totally puzzled me when I assembled an alternator engine.
It was a '59 350 road engine and I don't know if it is the same arrangement of bearings etc. in a late CS engine.

This is what I couldn't understand...

The two drive side bearings are fitted with the washer/spacer free to move between them so as "not to side load the bearings"
When the engine is assembled...
Internally the drive side axle bears on the inner race of the inner bearing.
Externally a spacer is fitted up against the inner race of the outer bearing. The drive sprocket bears on this spacer, then the rotor spacer then the rotor.
So when the rotor nut is tightened the inner races of the two bearings will be side loaded together?

[mdt-son]

JB, I don't have these parts in front of me, but if the spacer between the bearings has reduced thickness at the inner part, the inner races won't touch as the crankshaft is pulled towards the drive side when rotor nut is tightened. In the running condition, no side load will be transferred to the outer ball bearing.
Why this was so important to the AMC designers, I don't understand. The inner ball bearing _is_ subjected to axial loads, which it is capable of absorbing, under normal conditions. Furthermore, axial loads may act on the outer bearing if the primary drive isn't aligned perfectly.

-Knut

[mdt-son]

Excessive clearance with the primary side off is not a problem , only when the primary side torqued up will the shaft be pulled one way in the crankcase . the clearance you feel now just ensures there is allowance for thermal expansion

It's elementary structural mechanics really. An axle loaded partially by an axial load needs one axial restraint to remain a statically determinate support system. In the present case it's the inner drive side ball bearing. The timing side bush and oil pump drive of pre-64 engines allowed the crankshaft to slide somewhat in and out, as the engine heated up. Sliding clearance allowed the same on post-64 engines, i.e., the timing side inner race has to have a loose fit on the shaft.

- Knut