Sherco 2005 250cc Piston Size and crank distortion;

[mink_man]

Hello, I have had problems with piston slap since i got my bike a few weeks ago and decided to sort it now. I know many people have to do this to there 2 stroke Sherco's sometime and im looking for someone with wise words of experience to tell me what size piston to get. I have measured the bore today and it is a very even 72.80mm.The remaining bore is suprisingly very smooth and in good condition.

I was think of going for the B piston as it still has its original actually mesuring around 72.64 now :/

 

Second; 

I was worried about the crank as it seemed to loop when i looked at the fly wheel spinning in the case so i took the crank out and took it to the machine shop today he said it was 7mm out which is quite a bit. Has anyone had this before? Why does it happen and when he straightens it will it stay straight? 

Thanks!

[reggie]

Sometimes a big hit on the flywheel cover can cause the crank to come out of true. 

 

[mink_man]

56 minutes ago, reggie said:

Sometimes a big hit on the flywheel cover can cause the crank to come out of true. 

 

Now you mention it,there is a hefty scuff on the plastic flywheel cover 

[tony27]

Are you sure you don' have a 290 rather than 250, pretty sure the 250 is 70mm bore. Complete 290 top end is a straight swap

Where are you measuring piston size? Standard clearance is .03/.04mm measured just above the rear cutout

[mink_man]

5 minutes ago, tony27 said:

Are you sure you don' have a 290 rather than 250, pretty sure the 250 is 70mm bore. Complete 290 top end is a straight swap

Where are you measuring piston size? Standard clearance is .03/.04mm measured just above the rear cutout

Hello Tony, no i just double checked the Standard A Piston for the 290 are 75.95mm. 

Definitley measuring the piston correctly too, it was a pretty bad slap. 

[copemech]

10 hours ago, mink_man said:

Hello, I have had problems with piston slap since i got my bike a few weeks ago and decided to sort it now. I know many people have to do this to there 2 stroke Sherco's sometime and im looking for someone with wise words of experience to tell me what size piston to get. I have measured the bore today and it is a very even 72.80mm.The remaining bore is suprisingly very smooth and in good condition.

I was think of going for the B piston as it still has its original actually mesuring around 72.64 now :/

 

Second; 

I was worried about the crank as it seemed to loop when i looked at the fly wheel spinning in the case so i took the crank out and took it to the machine shop today he said it was 7mm out which is quite a bit. Has anyone had this before? Why does it happen and when he straightens it will it stay straight? 

Thanks!

Your piston slap noise could possibly be related to the issue with the crankshaft. I know that is hard to fathom and I cannot explain why, but I have seen this on a NEW bike with no piston wear. One sudden engine stoppage was all it took. Had several of us stumped for a bit. I am talking folks with decades of experience.

If you are worried about a recurrence, tac weld the ends of the pin to crank.

As far as the piston goes, either slap a B into it and hope for the best or just send a new piston with the cylinder for a re-plate, because if the first method does not work out then you can just send the new one in and you have lost nothing.

Trust me, sometimes you cannot see or measure these noises, as it requires some Very specialized tooling and gear. Pays to start FRESH!

[mink_man]

2 hours ago, copemech said:

Your piston slap noise could possibly be related to the issue with the crankshaft. I know that is hard to fathom and I cannot explain why, but I have seen this on a NEW bike with no piston wear. One sudden engine stoppage was all it took. Had several of us stumped for a bit. I am talking folks with decades of experience.

If you are worried about a recurrence, tac weld the ends of the pin to crank.

As far as the piston goes, either slap a B into it and hope for the best or just send a new piston with the cylinder for a re-plate, because if the first method does not work out then you can just send the new one in and you have lost nothing.

Trust me, sometimes you cannot see or measure these noises, as it requires some Very specialized tooling and gear. Pays to start FRESH!

Thanks for the detailed answer @copemech,much appreiciated! have you used the tac weld method ?

[copemech]

17 hours ago, mink_man said:

Thanks for the detailed answer @copemech,much appreiciated! have you used the tac weld method ?

I was not personally the one that welded the one referenced above, however I would say any good Mig welder would work.

[mink_man]

Updated on this; crank has been trued perfectly by a local engineer and the new piston and gaskets have arrived. Built most of it yesterday and putting the engine back in today! Pictures coming.. 

Thanks for the help lads!