Crankshaft bearing woes

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
sno335
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Location: Long Buckby

Re: Crankshaft bearing woes

Post by sno335 »

Thanks Steve,sorry if I sounded critical of you,that was not intended. I just found it all a bit confusing.That's a good observation about the shaft as the spline is perfect,unlikely I agree to be original.
So I'll rub the shaft down.
Regards John
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Pharisee
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Re: Crankshaft bearing woes

Post by Pharisee »

When I built up my basket case '54 G3/LS I had to put new bearings onto a new drive side axle on the crankshaft. This proved to be remarkably easy and stress free... hooray! The two new ball races were placed on a baking tray and put into the oven... 200°C. The crankshaft was supported on wooden blocks on the table outside. The first hot ball race was dropped over the drive side axle and down to the locating shoulder. The washer and spacer that go between the two bearings were dropped on next, followed by the second hot bearing. No persuasion needed, they just dropped into position easily. Time for the drive side crankcase half to go into the oven. By the time it was up to 200°C, the bearings on the crankshaft had cooled down and were fitted tightly, so it was simply a case of dropping the crankcase half over the bearings. It dropped on with a satisfying 'ding' and that was that... drive side main bearings installed.

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sno335
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Re: Crankshaft bearing woes

Post by sno335 »

Thanks,I guess it's just a matter of whether you follow the advice in the manual or not?
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Pharisee
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Re: Crankshaft bearing woes

Post by Pharisee »

sno335 wrote:Thanks,I guess it's just a matter of whether you follow the advice in the manual or not?
The guy that wrote the manual probably didn't have an oven, just a big hammer.
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mdt-son
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Re: Crankshaft bearing woes

Post by mdt-son »

Pharisee wrote: Sat Jul 11, 2020 9:25 am
sno335 wrote:Thanks,I guess it's just a matter of whether you follow the advice in the manual or not?
The guy that wrote the manual probably didn't have an oven, just a big hammer.
The guy that wote the manual knew that some some day you will need to dismantle your engine again. How do you manage that, when bearings have been fiited by an interference fit at the inner AND outer race? This is not how it should be done.
The manual is right. The interference fit has to be at the outer race. The inner race should provide a "snug push fit" at the shaft. If specified, C3 clearance is for the inner bearing at the drive side, as it will allow a tiny angular deformation of the crankshaft assembly.
Given the clearance stated, the spinning and flexing crankshaft will produce a radial load at the bearings which locks the inner races and enforces them to spin along, and there will be no risk of having the crankshaft rotating independently of the bearings.

There is a much greater danger of having the outer race spinning in the housing. This may happen if bearings were replaced without heating the case, or if bearings have been replaced numerous times. Abrasion and plastic deformation will weaken the interference fit. If this has happened, the outer race has to be locked in place by chemical or meachanical means.

- Knut
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Re: Crankshaft bearing woes

Post by 56G80S »

Just to say that I have had a similar experience to sno335. And Mick D provided the same advice as above, which I have, due to other matters, not yet applied but I am confident it will with patient work. Alpha Bearings replaced my axles and big end, just as they did back in the very late 1070's on the same engine.

I have the same issue with the timing side bush, which is also new, and the axle won't insert into it; it will be line reamed to match the axle and thanks to Ken de Groome for his advice.

Additionally, I had thought the drive side bearings were not spinning sufficiently freely but when revisiting this I think I am wrong. Spinning an almost assymetrical crankcase round the bearings is never going to be the same as spinning a wheel!

I hope sno335 gets sorted and reports back.

Johnny B
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