What holds the crank bush in place?

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CaptainSparrow
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What holds the crank bush in place?

Post by CaptainSparrow »

The top crank bush on my 102 moved (rotated) and blocked the oil hole causing it to over heat. i pressed out the bush and fit it back onto to the crank shaft with grinding paste and a lot of twisting to get a good fit. the tolerance between the bush and the crank is good, freely rotates but no sideways movement.

i heated up the crank case and the bush just dropped in, let it cool and it seemed secured. however after a 20min run the revs were down and soon the motor stopped with a seized up top bush yet again.

so before i strip the whole motor again i need a plan to keep the bush in place, i cant afford to buy parts so im stuck with what i have.

some options: drill and tap for a grub screw. drill and pin. hand file a key way. silver solder... is there something obvious im missing here?
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Collector Inspector
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Re: What holds the crank bush in place?

Post by Collector Inspector »

Um...what oil mix are you using?

Is the flywheel proper tight and not out of balance at all?

and

Is everything Gull?

Never had this problem myself so over to the team...

BnC
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Oyster 49
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Re: What holds the crank bush in place?

Post by Oyster 49 »

Why are you using grinding paste?

You should be fitting the bush to the crankcase by heating the case and freezing the bush. Then if the bush is too tight for the crank to fit then it should be line reamed to size.

The bush is designed to be fitted to the crankcase and the crankshaft should rotate within the bush. If the bush is rotating in the crankcase, then I'd remove and clean everything, then hold the bush in the crankcase using a suitable compound such as loctite.
Horsley-Anarak
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Re: What holds the crank bush in place?

Post by Horsley-Anarak »

Oyster 49 wrote: Wed Jul 29, 2020 11:35 am Why are you using grinding paste?

You should be fitting the bush to the crankcase by heating the case and freezing the bush. Then if the bush is too tight for the crank to fit then it should be line reamed to size.

The bush is designed to be fitted to the crankcase and the crankshaft should rotate within the bush. If the bush is rotating in the crankcase, then I'd remove and clean everything, then hold the bush in the crankcase using a suitable compound such as loctite.
like he said
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Charles uk
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Re: What holds the crank bush in place?

Post by Charles uk »

Perhaps we should be asking ourselves, why the bush got hot & spun in the first place.

At what temperature Loctite breaks down.

Contact the man you got the carbs from & ask if he's got any crankcases.
Make it idiot proof and someone will make a better idiot.
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Collector Inspector
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Re: What holds the crank bush in place?

Post by Collector Inspector »

Exactly Charles.

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CaptainSparrow
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Re: What holds the crank bush in place?

Post by CaptainSparrow »

When working on a 70 year old motor getting new parts is difficult and expensive. yes replacing bad parts often fixes the problem but its not always a viable solution.

Loctite breaks down with heat and as such is never going to work.

The grinding paste was used to fit the crank shaft to the bush as the fit was extremely tight, i dont have access to a machine shop unfortunately so it was a low tech solution which worked very well.

Im going to braze the bush in place with silver and see how that goes.
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Rig pig
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Re: What holds the crank bush in place?

Post by Rig pig »

Grinding paste is horrible stuff to use in that application, you will struggle to get it all out of the Bush and any left behind will carry on grinding.
I'd check the fit in the crank case and make or buy an oversized bush.
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JERSEYMAN
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Re: What holds the crank bush in place?

Post by JERSEYMAN »

If the crank is tightening on the bearing and spinning in the case the cases would need replacing or a proper engineering repair to fit/make up a new bearing and line bored/reamed. The question is why is the crank/bearing getting hot? Are you using the correct oil mix? Does the crank spin freely In the built up cases when cold? are the cases matched pairs? There should be no sideways play and very little resistance when spinning the crank. These motors/bottom ends don’t run at all hot.
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Collector Inspector
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Re: What holds the crank bush in place?

Post by Collector Inspector »

I have a feeling Mr Sparrow that, a set of new cases will be required for peace of mind.

Also another crankshaft as well. You have been busy with that I understand.

All good then and

Pics?

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