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Cracked Crank

Submitted by pete_young on Mon, 13/01/2025
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Heavy Twins

So having decided from an earlier thread that my 88 crank is as strong a crank as I'll find I bought a dye penetrant test kit to have a look at it. Initially I focussed on the big end journals and found no issues. I was going to send it off for regrinding tomorrow but thought as a last check I'd NDT the main bearing journals. Lo and behold there's what looks to me to be a crack going about a third of the way around the drive side journal.
Im totally new to playing with cranks so would appreciate views on this (see pic) and suggestions what to do next i.e. is there an economical repair method or is it scrap.
thanks,
Pete
 

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Submitted by pete_young on Sun, 19/01/2025

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Having taken a closer look at the crank and bearing (with better glasses on!) I can see that at some point something hard/metallic has been caught between the main bearing and journal and has gouged both surfaces due to the crankshaft spinning in the inner race of the bearing. 
It's not clear whether or not the feature in the dye test is a crack or deep scratch (or a crack developed from a sharp, deep scratch).

I've measured the ID of the bearing at 30.0mm and the OD of the shaft at 29.975mm. The bearing feels loose on the shaft.

i intend to do this once and do it right. Do I have any alternatives to replacing the crank? 

I wouldn't want to use loctite for this.

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Submitted by terry_maslin on Sun, 19/01/2025

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It might be worth using a long strip of fine emery wrapped around the shaft, polish it up nice and clean then do the crack test again, costs nothing, could prove good or bad news !!

Submitted by david_cooper on Sun, 19/01/2025

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There are other more sophisticated crack measuring procedures, some of which can measure crack depth. Bearing in mind cost and inconvenience of replacement, it might be worth taking it to a professional steelwork inspection company.

Submitted by george_farenden on Sun, 19/01/2025

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This can be overcome quite easily by getting the bearing inner copper plated. 
Its not a bodge as it sounds either, and has been/is still done in many industrial applications today. You just need a plater who knows what they are doing.
Basically, the whole bearing is plated, then what is wanted is covered in wax, followed by 'stripping' the plating from where it isn't wanted. No harm will come to the bearing.
That said, .025mm (0.001") is within loctite bearing fit territory should you prefer. 

Hope the 'crack' is just a scratch!

Regards,
George 

Submitted by andrew_mackenzie on Sun, 19/01/2025

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Pete,

I had a similar experience with a loose drive side main bearing on a T120 crankshaft.  I was fortunate to be able to get the crank land plasma-sprayed and ground back to size in a RAF workshop - at no cost too!  The repair worked perfectly. A quick web search showed there are companies around who plasma spray, so an option to consider perhaps?

Andy

Submitted by william_dunn1 on Sun, 19/01/2025

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Just a suggestion, the critical question to me is it a crack or just a slight scratch? If it's a crack then it's scrap, if it's a scratch then it's repairable.Unbolt the two journals from the flywheel, take the two journals  to a known crank specialist and have them "ring it", tap it and see if it rings, if it does it's not a crack, if it doesn't ring it is  a crack. Just one more suggestion, good luck with it.

Submitted by pete_young on Sun, 19/01/2025

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Thanks for the suggestions folks.
so I've put the crank between centres and polished the scratched area lightly but not enough to take the scratches/gouges out. 
I've then redone the test but being a little more careful with cleanliness and part temperature so as to not wash the dye around/out with solvent from the developer. This has made a significant difference and I think is a more valid test as the result now just shows the multiple scratches on the journal and not one dark line where the solvent has washed the dye to a low point. 
I'm now reasonably confident that the journal isnt cracked.
this just leaves the loose fitting bearing. Interesting to hear the ideas on this. I'm reluctant to reduce the size of the bearing as this would need the crank grinding to get the bearing past the oil seal portion on the crank. Unless it's shrunk on perhaps.
I'll have a chat this week with the engine builder who's sorting my barrels and see what his thoughts are.
 

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Submitted by robert_tuck on Sun, 19/01/2025

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I have bearings that are oversize to compensate for worn cases . Perhaps there are bearings that are undersize for the shafts or metric that can be ground to fit?.A 650 does not need the special bearings like the 750/850 does . 

Submitted by michael_rettie on Sun, 19/01/2025

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......will be definitive, I believe.  See local automotive engine builder.

Submitted by pete_young on Tue, 21/01/2025

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Had a chat with the engine builder who thinks loctite is the way to go. I trust his judgement more than mine so that's sorted. He's also looked at the dye penetrant test pic and said it's not cracked as not enough dye has bled through to the developer which means it must be from a surface scratch. So it sounds like I'm back in business (again). This crank has has me going round in circles so it'll be great to move on to a new head scratcher.
thanks all,
pete

 



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