Big ends and crankpins

TC · Post by TC » Sat Nov 03, 2007 6:02 pm

I got my KR with a blown big end. BDK have stripped the crank for me but the crankpin where the dead big end went has been scored and worn. Naturally it's a 1-piece web/crankpin so you can't just get a new crankpin, and spare cranks are like rocking horse sh!t, I wonder why!!!

I'm thinking of getting the crankpin ground back to clean it up then hard chromed back to size. I may be able to get this done at work, unfortunately as a foreigner I couldn't offer it out.

Still, I wondered, has anyone tried this before and is there any spec to aim at for the chrome (thickness etc?). Alternatively does anybody have a timing side inner flywheel/crankpin in good condition, or a n old salvageable crank they could sell me?

the-elf · Post by **the-elf** » Sat Nov 03, 2007 6:41 pm

Just checked and the spare blown crank I have also has a knacked big end on the flywheel side. The other two spares have blown big ends one on each side and I'm hoping to get them rebuilt my-self. Put an Ad in the wanted column and I'm sure someone will have one they can let you have.

Never thought about hard chroming them? that could work, what temperatures are involved as the crank is forged and heat treated so you want to make sure it won't knacker up the material's properties. From memory I don't think they are readily weldable otherwise you could get the bearing surfaces stelited and ground.

I know that new parts aren't available cause when I had mine rebuilt, I had to buy a second one and get one good one made from the pair, and I would guess that's the best way for you to go as well.

the-elf · Post by **the-elf** » Sat Nov 03, 2007 6:45 pm

Just checked and the spare blown crank I have also has a knacked big end on the flywheel side. The other two spares have blown big ends one on each side and I'm hoping to get them rebuilt my-self. Put an Ad in the wanted column and I'm sure someone will have one they can let you have.

Never thought about hard chroming them? that could work, what temperatures are involved as the crank is forged and heat treated so you want to make sure it won't knacker up the material's properties. From memory I don't think they are readily weldable otherwise you could get the bearing surfaces stelited and ground.

I know that new parts aren't available cause when I had mine rebuilt, I had to buy a second one and get one good one made from the pair, and I would guess that's the best way for you to go as well.

TC · Post by TC » Sat Nov 03, 2007 7:02 pm

Hard chrome is a wet process so it wouldn't destroy any heat treatment. Of course this may already be compromised by the heat generated during the failure!

I know we do similar repairs for aircraft parts at work so it can be done.... I'd just have to work out when!

Then again no doubt there would be an optimum thickness of chrome so it didn't flake off.

You wouldn't stellite something like this by the way, that's only for the likes of cam followers. And if you over-hardened the pin you could end up embrittling it. Nitride perhaps, but that's all starting to sound a bit expensive on a bike that's not supposed to be costing much (that's what I'm telling the wife anyhoo)