7 replies to this topic

[Juzz80]

On a previous topic I put that my engine was making a knocking noise, my mate has since had aloof at the engine and the crankshaft has some side to side movement, and also the big end bearing has some movement as well. Does anyone know what tolerances their should be in them or where the best place to find out is?
Cheers all

[paul w]

no play in the big end at all "up and down" there should be slight side to side movement.
sounds like a full rebuild.

[Juzz80]

Is that goin to mean a whole new crank and conrod?

[paul w]

no, the crank can be rebuilt. You will need for a bottom end rebuild;
2x crank seals
2x main bearings
1x rod kit
various gaskets.
depending on what the piston looks like and the state of the bore will determin if you need to do the top end as well, that would require;
1x piston kit (this will include rings and a gudgeon pin)
1x little end bearing.
1x base gasket.
Gear oil
Coolant
possible a recoat of the bore (nicosil)
parts approx £300 ish. (not including the recoat)
labour, it depends who does it, £100 - £200?
these costs are based on motors i have done recently for friends.
not great news, sorry.

[Juzz80]

Cool, any idea where to get the conrad kit from? The only price iv seen is £250 pounds for a set, is that about right?

[copemech]

As you have not seen fit to put up any measures on any of the findings, it is anyones guess here.

Splatshop.co.uk will send you parts. I just hope someone at the other end has a clue, as conrod kits are seldom neccessary. For all w know, you may just have clogged carb jets and pinging going on?

[mcman56]

As copemech states, I would be sure about the big end bearing before changing. There should be no up and down play. However, there can be quite a bit of side to side rocking/ tilting. Put an indicator at the small end and measure side to side rock. I don't have a spec for a Sherco but some older Japanese bikes allowed 0.08". If you can find a Montesa 315 manual, it may have a good spec for reference.

For main bearings, grab the flywheel and try to move it up and down. There should be no movement in that direction. There does need to be a little side to side movement.

See 2-3 of http://dl.dropbox.co...15R_02_2002.pdf
This shows big end tolerances.

Edited by mcman56, 28 January 2012 - 04:10 PM.

[rlracer]

Side to side play is a given. When rebuilding a sherco crank the bottom pin can only be pressed to the outer edge of each crank half. I have had two done. The second crank was tweaked from the get go, who ever gets that rebuilt crank will get one sweet crank. If you have to get a crank rebuilt find some one who knows what they are doing.