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[john.658]

What ya reckon to this,






It's the crank from a 250cc 2 stroke Sherco trials bike. Doing a top end rebuild and came across this chip on the crank. There are a couple of marks on the conrod also. Must have been damage from the previous owner, i've had the bike for a few years now and this is the first attention the engine has needed, except oil and cluth plates. Will the chip unbalance the crank and put strain on the big ends? They seem fine just now and the bike runs ok.
Cheers. 

[roy]

if it was me ....why its apart id just stick a new one in just to be safe .
you don't really want to do all that work and then do it all again later on        thats the if onlys ....and you don't want them

[john.658]

You're probably right Roy, getting a hold of a decent crank without buying new might be tricky though. I'll check the price of a new crank, might not be worth repairing, could probably pick up a spare engine. Cheers. 

[pat]

Crank appears to be a counterweight rather than the crank itself & looks like it's been like that for a while, so may not be a problem as such - do you notice much vibration?  The marks on the rod will potentially be stress weak spots, so worth getting it replaced. 

I'd talk to a reputable stroker shop like PJME - http://www.pjme.co.uk/acatalog/Crank_Shaft_Rebuilding.html 

[john.658]

+ bhp for the link Pat, nice one. I'll get on to them in the New Year. 

The only vibration was from the top end, hence the rebuild, bottom end seems ok. Gets a bit noisy if oil is not changed regularly so every second outing it's changed. Bike isn't used for competing so I'll go with the cheapest option. Big end bearings are cheap though and it is an easy enough job on a wee 2 stroke so I might change them anyway.

Cheers. 

[ZX10R-RACER]

its for rasing compression , it`ll be fine just use it as is.

[john.658]

its for rasing compression , it`ll be fine just use it as is.

Nice one, 

Thanks for the advice, that's put my mind at ease.

Cheers and happy new year 

[roy]

its for rasing compression , it`ll be fine just use it as is.

hi Liam sorry but i think thats the wrong advice to give  if moor of it brakes up it will fuck the motor.
and parts moving that fast could come out and hurt him .

its a damaged part and should be replaced.

[ZX10R-RACER]

its only a plastic type material, yes the right thing is to replace but i`v had loads of two strokes and had this a few times , it`ll just get passed through the exhaust, it`ll be fine.

[pat]

its for rasing compression , it`ll be fine just use it as is.

Raising compression ? 

[ZX10R-RACER]

yes its a 2 stroke engine...

[pat]

yes its a 2 stroke engine...

So how does sticking a lump of plastic on a crank web raise compression ? 

[davemfox]

I'm with Pat on this Liam - please explain

Dave

[john.658]

Is something to do with maintaining the momentum of the crank/piston back to TDC, as it is a single cylinder and no other firing piston to propell it round?  Just a guess. 

[ZX10R-RACER]

gasses pass through the crank on a 2 stroke engine,

hope this helps

http://www.animatedengines.com/twostroke.shtml