Scooterotica

MEC "race" cranks have shims, and MEC std replacement cranks don't . . . simples. Until you have different rods fitted, then it's take the crank builders advice. Drunkmonkeys post is really helpful . . . go with the crank builder. Personally I hate the idea of piston shims so I'd always use a crank with low rod clearance.

I run cranks cranks with and with out shims, when you strip a crank and see the wear on shims and then think of the possible failures and subsequent hassles that can be caused by them, just as piston shims.

Wear will take place to the big end area of rod and the crank webs where the rod runs but the cost of new Yam rod and cleaning up the face of webs is negligible in the long run.

You can smooth/radius the edges of the oil cut outs to the sides of the rod.

very interesting post, enjoying it very much! The whole thing about shims I think isn't covered as much as it should be

Special X wrote:Personally I hate the idea of piston shims so I'd always use a crank with low rod clearance.

how big/small is low rod clearance?

A crank that a "grown up" tells me is ok without piston shims. Should measure really but never have. MEC EUR with big end shims is what I would normally use.

everyone is too scared to commit to a measurement in case everyone else contradicts them

cezeta wrote:everyone is too scared to commit to a measurement in case everyone else contradicts them

Or someone has a catastrophic crank failure running the suggested clearance.

i`m going in the garage to strip a gp in the next hour - will measure all the cranks i have

60mm mec with jap(rd 110 rod) and 0.5mm shims (jon betts altered the webs on this one previously)

58mm sil with jap (rd115) rod without shims

vespa mec with italian pa rod with shims

all 3 cranks were built by the same person - a well respected long standing local motorbike engineer

will report back later

0.5mm to 0.7mm is an industry standard seen in race cranks on high performance mx/road race motors and what ive seen give good life on many race engines, ive seen failures on motors with 0.4mm or lower where shims have picked up but despite this have had many new expensive race lamby cranks with as little as 0.2mm
the bearing can and does tolerate these larger 0.5-0.7 clearances without problem and the situation with piston shimming shouldnt exist as the small end should be wide enough in the first place to suit the piston in use giving full contact to the rod at all times

special x......you know im only teasing

so it looks like there is an ideal world of cranks being designed and running with big end shims ( I usualy use an alpha crank or a mec race crank if its a standardish tune 225 and they both have shims), from what i have seen they are alloy/tin of some kind? so are softer than the webs and rod so will reduce heat from friction of the two and "self lube" like a brass bush would.

plus they keep the con rod from drifting side ways? so if there is no bearing issue with this side drift is there any other negative results from a mm of side drift ie balance ect?

im just trying to get to understand the ins and outs of it as im now curious (although i do have every intention of regurgitating everything in the pub and taking full credit for it all )