piston shims ~ needed ? or potential hazard ?

[byron]

I'm rebuilding a mate's SX150 top end after a nasty heat induced end to the piston.
it's still got the SX crank with no shims in the big end, and about 2.5mm of side float on the rod... so I was thinking I'll fit some piston shims, but then have heard horror stories of these breaking up and causing damage...
as far as I know, I haven't got any from previous engines so would be looking at buying the shims new.
the next top end is a just run in AF SR kit with Asso piston

[bristolmod]

SX150 was built with the small end shims in place.

If you are doing a standard rebuild to the engine, then I would make sure the shims are in place. Many people do leave them out, but if Innocenti added them on standard units, they must have done so for a reason.

Chris

[a.j]

Only thought they fitted them on 200's?

[bristolmod]

No. Fitted to both SX150 and SX200 on introduction. Both of these standard crankshafts have considerably more side play that their LI predecessors, due to the cranks being alligned from the small end.

Chris

[156 D]

...........whatever one may think or do.......the shims have to be fitted either at the crankshaft or the small end.....not both......one end governs the other .... if there is no room left at the crank then put them at the top end.......IMO if the engine is stripped down and there is room at the crank then put them there as this will add to the Primary Compression, however small...........

[byron]

thanks for the replies guys.
I'm going to fit some to the piston.
noticed that the one edge of the rod has worn against the webs, there were no shims in the piston I removed.

[rog60]

For Info: MBD did have some NOS Spanish ones which I have fitted on my Indian GP (piston had been machined to fit them but none fitted). Covered about 800 miles with them fitted and a recent inspection showed no appreciable wear on the shims.

[byron]

For Info: MBD did have some NOS Spanish ones which I have fitted on my Indian GP (piston had been machined to fit them but none fitted). Covered about 800 miles with them fitted and a recent inspection showed no appreciable wear on the shims.

thanks, that's good to know

[dapper]

Couldn't you fit a wider japanese small end bearing to take up the gap? Or is this a serious No No?

[davidblythe]

Couldn't you fit a wider japanese small end bearing to take up the gap? Or is this a serious No No?

you can but it still doesnt centralise the conrod if the crank end isnt shimmed and free floating