Crank Main Roller Bearing Fit

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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Martin.S
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Crank Main Roller Bearing Fit

Post by Martin.S »

Hello, The drive side main Roller Bearing is I believe a LRJ 7/8 or CRL 7 but do we know what the spec of these bearings should be - C2 C3?
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Martin.S
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Re: Crank Main Roller Bearing Fit

Post by Martin.S »

Did a little more reading - C rated bearings have a different clearance between the inner and outer races - std being in between C2 and C3.
C2 is a little tighter than std - would that be OK for the Cranks Main Roller bearing?
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clanger9
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Re: Crank Main Roller Bearing Fit

Post by clanger9 »

I wouldn't go any tighter than C3. The oil in these engines isn't especially clean...
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Martin.S
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Re: Crank Main Roller Bearing Fit

Post by Martin.S »

But a C3 is looser than a standard bearing
Mick D
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Re: Crank Main Roller Bearing Fit

Post by Mick D »

Hi

The clearance should be indicated on the edge of the bearing outer race as a suffix 'C digit', how is your current bearing marked? The absence of a 'C digit' suffix indicates it is a standard clearance bearing.

In the absence of any reference to a specific clearance I'd opt for a standard clearance bearing.

Regards Mick
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Martin.S
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Re: Crank Main Roller Bearing Fit

Post by Martin.S »

I haven't got an old bearing - just empty crankcases.
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Ozmadman
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Re: Crank Main Roller Bearing Fit

Post by Ozmadman »

Martin.S wrote:I haven't got an old bearing - just empty crankcases.
Buy one from club spares or AMC classic spares by AMC part number if you are not sure and that will be the correct spec
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clanger9
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Re: Crank Main Roller Bearing Fit

Post by clanger9 »

Martin.S wrote:But a C3 is looser than a standard bearing
If it's slightly looser than standard, you can be certain it won't seize. If it's tighter, you can't be sure.
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Mick D
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Re: Crank Main Roller Bearing Fit

Post by Mick D »

clanger9 wrote:If it's slightly looser than standard, you can be certain it won't seize. If it's tighter, you can't be sure.
Hi

In a roller bearing there is an increased tendency for the elements to skid rather than roll as the clearance and rotational speed increases - it's very unusual in my experience for a roller bearing to seize.

I stand by my recommendation of a standard clearance.

Regards Mick
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clanger9
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Re: Crank Main Roller Bearing Fit

Post by clanger9 »

Mick D wrote:In a roller bearing there is an increased tendency for the elements to skid rather than roll as the clearance and rotational speed increases - it's very unusual in my experience for a roller bearing to seize.
Agreed. Seizure is unlikely.

I'd usually associate C2 bearings with high precision, high speed applications. I was thinking more about possible damage to the bearing from the grotty dirty oil/high temperature environment in the crankcase. A C3 bearing should be well below its maximum RPM in this application (always check with the manufacturer, etc).

How tight were the clearance tolerances on "standard" bearings anyway?
1989 Moto Morini Dart 350
1993 Ducati 750SS
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