Small end shims.... Indian pistons

[johnny650]

Bit of a dilemma and I need some help.
I am currently fitting a 200cc small block cylinder kit to a series 3 crankcase .

I have fitted an aftermarket crank of unknown origins and I'm now not sure of how I should shim the small end . I have read through Stickys appendix but am still non the wiser.

Stupidly I do not have a picture of the crank to show but it has no flats and no web balance holes .

The crank pin is 13mm wide and small end is 20mm wide with 13mm wide crank webs.
The big end has no big end shims and float is 0.40mm

The piston has bevels where the needle bearing fits as can be seen in the image and small end float is approx 3.0mm

So how should I shim this and what would be the permissable small end float.
lambretta small end shims by John, on Flickr

[Warkton Tornado No.1]

The big end has no big end shims and float is 0.40mm

The piston has bevels where the needle bearing fits as can be seen in the image and small end float is approx 3.0mm

If that is the case, then the big end float, being the lower figure, is controlling how much the con rod moves from side to side. The con-rod @ the little end could not come into contact with the piston bosses unless there was a severe ‘lack of symmetry’

If you are asking as to whether 0.4 mm side float is excessive, that is difficult to give a finite answer. Personally, I 'could live with it' in most circumstances, but I'd keep an eye on the bottom end for signs of overheating/wear which will be exasperated by heavy loads or anything within the bottom end that is out of line. If that all sounds negative, I suspect that everything will be fine & I'm attempting to paint the whole picture, warts & all.....

However, from my understanding of the information (& the images would seem to confirm it) there would appear to be the need for a wider small end bearing so that the con-rod little end bearing surface can be sure to always run on needle rollers, rather than ‘fresh air’

Alternatively, you could, as you appear to be suggesting, run piston shims but I would advise against that if a wider small end bearing can be sourced.

There are a number of small end bearings available (including from other manufacturers such as Yamaha) but it’s worth shopping around.

I hope that helps....

[johnny650]

thanks for responding .
the more I read about this the more this problem seems to be a common source of different opinion ...bit like 'which is the best gear oil '?

there are quite a few threads on various forums but none seem to arrive at a definitive answer.

I finally ended up where I should have started in the first place really...at MB Scooter site .https://www.mbscooters.co.uk/info/setti ... s+163.html
Mark has this to say

'If you are still using an original style crankshaft with no shims at the big end then it is very important that you get the small end / piston combination correct. Measure the piston where the small end fits, if it measures 20 – 20.5mm then it uses a standard width small end bearing at 20mm, with NO side shims. If the measurement measures 22 – 22.5mm then this leaves room for 2 x 1mm shims to be fitted either side of a standard 20mm bearing.
Either way the piston / bearing and or shims should move freely and have approximately .015’’ gap. If not lightly file the inner edge of the piston to loosen it all up.'

I've ordered up some 1.0 mm 0.5mm and 0.1mm 16 x 22mm shims and I'll experiment a little but I think as there is 0.4mm float at the big end i shouldn't need too much float at the small end.

[Warkton Tornado No.1]

I've never experienced a problem with shims but neither have I tested them on pistons with cut-away bosses as depicted in your image. That would be my main area of concern as any shims are not kept parallel & could cause flutter/side loading/fatigue.

BTW, UK bearing stockists sell the wider little end bearings as do RLC (23 mm)

[johnny650]

There shouldn't be any issues with shim flutter as the recommended gap is only 0.015mm . I presume the cutaways are to increase lubrication of the gudgeon pin.
i would be more concerned with using a 3mm gap and allowing the piston to float on a 22.5mm extended needle roller bearing due to the natural rocking motion of the piston .
Why is nothing ever simple ?
As the cylinder kit is to fit a small block with Li crank I would expect the piston to be suitable for the stock series 1,2,3 crank and conrod