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Even More Crankshaft Woe?

Submitted by matthew_hassock on Tue, 16/04/2024
Forums
Commando

Keen eyed readers will notice that I have posted here before on problems with my 1972 750 Commando crankshaft. The one which came with the bike was ruined beyond repair (QV). I then bought a second-hand crankshaft (in pieces) which I returned because a) the bolt holes didn't align and b) it was (0.018") bent.

Having given up on a decent SH crank I bought a brand new CNC machined steel crankshaft supplied unbalanced and sent it to my friendly engineer with conrods, pistons etc. for balancing,

The engineer has said that due to the holes in the flywheel the crank is already at 52% and will require weight added to get it to the 55% (dry) balance that he would consider a minimum. Additionally the drive and timing shafts are not concentric with the main bearing journals with a 0.010" (+/-0.005) runout. 

I spoke to the supplier and he said the balance would be fine - "install it as it is" and that the runout would be unnoticable when installed.

My engineer thinks that installation in this state would be disastrous.

I'm between a rock and a hard place. What should I do? Install or return? and what alternatives exist?  

Submitted by stan_thomas on Tue, 16/04/2024

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I can get better on the 60 year-old Colchester Student in my shed. 10-thou is totally unacceptable. One piece cranks, i.e. Nourish, usually have a screw in plug that can be altered to give the required balance factor. Crankshaft balance factor for a Commando is 52%. If you are putting it in a Featherbed or Seeley frame then maybe 80%. Andover Norton will sell you a new one-piece - good but not cheap. Otherwise, keep looking. Remember that any crank pre-Mk3 will be fine (or better) so 750 or 850 up to 1974.

Submitted by ian_soady on Tue, 16/04/2024

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... the CNC crankshaft? As Stan says, .010" is huge. If I couldn't mnage a couple of thou on my cheap Chinese mini lathe I'd give up.

Submitted by norman_lorton on Tue, 16/04/2024

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Matthew Stan and Ian have given you a correct view IMHO. I would say to return the crank and demand a refund. You have an 'engineers' measurements (in writing?) and not yours to rely upon in any dispute, if you have to go to trading standards or the courts. Norm

Submitted by paul_griffiths on Tue, 16/04/2024

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For a small fee you can obtain a form from Citizens Advice with a view to take legal action against this rogue trader. In most cases this action is all that is needed to bring these charlatans to behave in a customer friendly way. I have used this route a few times over the years with excellent results. Once these people get the idea that you are talking seriously about your grievance and its going to cost them an unknown figure of money it will focus them into giving you your refund PLUS all of the  outlay that you have had to fork out  to bring this issue to your satisfaction. These Double dealers need to be shown that they cant get away with these sharp practices.when a Norton owner uses thier services. I wouldn't hestate to drag this guy to the small claims court knowing that it is not going to break the bank in doing so.  

 


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