When would you check a used crank for true?

[Storkfoot]

I have stripped my GT186, post Adria, for a rebuild as it has done 7000 to 10,000 miles with little more than new rings and reeds.

On getting the crank out, I see it has been catching on the retaining plate (an original one that I now see is slightly warped), and so, I am going to get the crank checked for true (or, is it balance?).

Anyway, I was just wondering when people get their cranks checked out. Is it, only when you have a problem? Every 5000 miles? Never ?

Here is the crank - Indian GP200 webs, RD400 conrod.

[holty]

one of my pals siezed his engine up coming home from scarborough this year, when it came to rebuid time he was worried that his crank had twisted so we took the crank to rayspeed, it was the idea to get a new bearing fitted to the big end, ray checked it out for true and said it was well within tolerance and would be fine, he didnt charge us anything, but we did buy a new piston and carb when we where there, a very good service i think.

[dscscotty]

On rebuilding an engine cranks should be checked for true, you'd be surprised how many new ones are out and need fettling prior to fitting, if like in your case the crank is out of the motor get it checked, they all generally have a little run out, and you'd be surprised at how many still run fine when not in tru.

[Storkfoot]

Thanks, lads. Seems very sensible advice. If you have crank out, get it done for peace of mind.

This crank had a decent Yamaha big end bearing and has driven a very smooth engine. But, you never know.

I was going to get the retaining plate skimmed but I'll probably buy a new steel one.

[Fast n Furious]

Over thick halite washers, combined with overzealous tightening of the retaining screws are generally responsible for seal plate distortion especially with the aluminium ones.
An alternative to the halite washer is to use an o-ring. The right diameter and thickness o-ring will provide a seal between the bearing's chamfered outer edge and the back of the seal plate washer face edge. Because Viton rubber has a lot more compressible elasticity than a mineral gasket the screws can be tightened all the way down so there is metal to metal contact between the rear of the plate and the crankcase counterbore without distorting the plate.
I done it this way for years and never had a failure.

[Storkfoot]

Just to finish this off, I took the crank in to Grampians in Crosby. I was accused of riding the engine very hard from a visual check of the small end. He also noted that there are better big end bearings than the RD400 one that was in there. Given that the small end was showing signs of wear, I decided that new bearings all round was the way to go. So, there we are. I never actually found out whether the crank was out of true.

I did ask whether the conrod was still good to use. His response was "It's a Lambretta you're running it in, it'll be fine. "

[Fast n Furious]

I use RD 400 bigend bearings without any issue. The silver ones used in the RD 500 have a higher spec.
I would never use new roller bearings with a used conrod! It's false economy.
To get a decent lifespan from a bearing assembly, all the surfaces should be replaced at the same time. So in this case:- Gudgeon pin, Small end bearing, Conrod, Shim washers, Bigend bearing and bigend pin.

[Storkfoot]

I can't argue with that, F&F. But everything is a balance of risk when you're trying to keep 3 scooters on the road without a bottomless bank account. That said, I did make the decision that I should not leave it that long again before a rebuild. At that point, I shall replace the conrod