Small end play

[johnnyXS]

the more this discussion continues the clearer it seems to me that there is a lot of misconception about this whole issue and a clear and comprehensive reference guide is needed.

Perhaps there might be room for an expansion on the subject and perhaps a compatibility table in the third Sticky manual

[HxPaul]

I think WT's idea is a very good one,if the gap in the piston where the small end bearing fits is as large as the OP says,a 16x20x22 bearing with a piston shim at either end will be a good compromise.This way the small end eye of the conrod never touches the shims but it still floats on the bearing.I haven't been on google to see who sells the shims,but I know SR sell them,there 95p each and they call them piston spacer washers.

[a-teamlambretta]

i had to shim my recent vega engine rebuild . new casa crank, checked big end float as there was no shims and made a note of this. rod replaced with one from a motocross, big end float same as before. piston used is from another motocross bike , gap between the shoulders on the piston was 16.75mm little end of rod is 14mm , nearest bearing to fill the gap was 14.70 ish . so i had to shim with some thrust washers as the gudgeon pin was 12mm dia so had to use something else to shim other than proper shims . the rod also was wider than the standard and was close to the webs . with a combination of miss matching parts you have to compromise. cheers simon

[JETEX]

Not a fan of piston shims myself, only something else to break up wreck your engine. I've read through all these posts and it seems pretty clear to me that if you have a 'narrow' con-rod eye, then your small end needle roller cage is going to slide out side to side within the piston. You can alleviate this by fitting a wider small end bearing as has been suggested. The gudgeon pin would still be supported along it's full length by the bearing. Does that make sense, or am I reading it wrong ?

[Steve J]

That makes perfect sense - provided there's clearance in the piston to fit a wider (say 22mm wide) needle roller, I can't see a problem. It's worth remembering that the standard small end width in the conrod is nominally 20mm, and a 20mm wide needle roller is also standard. In fact, the rollers are a shade under 16mm long, meaning that at any point in time, the load is effectively carried by only 80% of the width of the bearing surface in the conrod. It also means that the small end needle roller bearing could move approximately 2mm in either direction from a nominal central position before the very ends of the rollers would become unsupported by the small end eye in the conrod. Fitting the wider bearing, which presumably has rollers approximately 18mm long, would help to spread the load slightly more evenly across the width of the conrod eye - provided it remained within 1mm of its nominal central position within the conrod eye.
It seems to me that the primary reason to fit shims to the small end would be in an application where the conrod is designed to have a large amount of sideways clearance at the big end, and the shims serve to centralise the conrod relative to the piston (e.g. original Innocenti SX cranks) - hopefully keeping the conrod aligned in the centre of the big end. In an application where a small sideways clearance at the big end serves to centralise the conrod, it's better to use a wider needle bearing assembly at the small end, and avoid using shims, IMHO.

[a-teamlambretta]

Just to fuel the fire , my shiming compromise stud up well today over 11000 rpm now the motor is set up better. How long it or the shims will last who knows

[dscscotty]

11000 rpm, i'd be a bit worried about one or two other items letting go as well!

[a-teamlambretta]

11000 rpm, i'd be a bit worried about one or two other items letting go as well!

i was very worried when i was holding a fan near the motor as no cowels were fitted , flywheel straight into my boat race if it had parted company lol

[Nbowe]

Thanks for all the interest.just to let you know I've fitted a 22mm bearing and I'm gonna run without the shims for now and see how it goes. It,s gonna be on the test bench for a while and not in the scoot so no worries there. Cheers all

[Steve J]

You should be OK with that - provided the bearing fits within the piston with a bit of clearance, there won't be a problem.

With a crank which has minimal sideways clearance at the big end, effectively controlling the sideways movement of the conrod (regardless of whether it's shimmed or not), the risk is that fitting shims at the small end with a 20mm wide needle roller bearing effectively also attempts to centre the conrod from the piston. If the centreline of the piston (and therefore the cylinder) aligns exactly with the centreline of the big end, you might be OK. On the other hand, if the two centrelines are slightly misaligned (as may happen due to a build up of tolerances, or using an aftermarket crank with wider webs), there's always the chance that the combination may attempt to push the conrod sideways, placing unwanted side loadings at both ends of the conrod. This is the situation to avoid at all costs.

[corrado]

This comment is purely to communicate & hopefully clarify my understanding of the clearances in generic Lambretta designed/based [insert component name], not to dispute anything that anybody has contributed. Please see it only as my opinion through experience & it may even overlap something already said, possibly, for which we should consider ourselves fortunate in ‘singing from the same hymn sheet’

It is only, like all other opinions, merely an attempt to help.....

Sorry fellas didn't mean to start an argument.

These quotes should automatically appear when you press the reply button .... with your comments in between.

[Wee Mark]

Hello all
I have had both types of MEC CRANKS in my motor, the first type(full circle, silver webs & silver conrod) did NOT need shims, the 2nd type(black webs/Cooper colour conrod did. I built the motor without shims and ran it round the block a few times when I got but into the garage and was check the motor over I could tell on turning the flywheel there was a wrong-sounding rattling...mmmm I got my stethoscope(leak test kit spare) and found the rattle was in the cylinder,so stripped it down & checked...everything looked ok but I was NOT happy with sideways slop between small end and piston anyway phoned a very well known Leeds dealer who had some quality Piston shims(recommended in Stickys book) and fitted those....rattle gone!!!.......that weekend very quick ride up Oxford Roadrunners Rally 200+ miles round trip...perfect....went great and she got caned.....fit them in my experience they are needed
Cheers, see you in Adria!!!
Mark

[Warkton Tornado No.1]

Post by Wee Mark » Mon Jan 09, 2017 12:29 am

Hello all
I have had both types of MEC CRANKS in my motor, the first type(full circle, silver webs & silver conrod) did NOT need shims, the 2nd type(black webs/Cooper colour conrod did. I built the motor without shims and ran it round the block a few times when I got but into the garage and was check the motor over I could tell on turning the flywheel there was a wrong-sounding rattling...mmmm I got my stethoscope(leak test kit spare) and found the rattle was in the cylinder,so stripped it down & checked...everything looked ok but I was NOT happy with sideways slop between small end and piston anyway phoned a very well known Leeds dealer who had some quality Piston shims(recommended in Stickys book) and fitted those....rattle gone!!!.......that weekend very quick ride up Oxford Roadrunners Rally 200+ miles round trip...perfect....went great and she got canned.....fit them in my experience they are needed
Cheers, see you in Adria!!!
Mark

If you read through this whole Post, the issue turns out to be ONLY the control of the needle roller bearing floating about on the gudgeon pin as the space between the bosses is too great.

I absolutely agree with you, Wee Mark, that shims should be fitted on this occasion, but that is because I’m not getting confused (any longer!) whereas other, well-intentioned contributors believe the problem is about the control of the con-rod side play.

It isn’t!