My old WM20 was fitted with an Indian (as in motorcycle) piston and despite many miles of thumping away happily that piston has now comprehensively retired from the up and down thing.

My friend who now owns said machine has an M21 piston and so he wondered if it could be fitted into the WM20. To that end is there, or can anybody tell us, comparative info on pistons. It seems that the main difference is in the stroke and so the piston top-gudgeon pin dimension must also alter (we're assuming bore, barrel height, ring assemblage are the same) to accommodate the extra stroke length - or is that not so?

If the M21 piston isn't suitable then where can we get a suitable M20 piston?

Thanks

email (option): rick.howell@talktalk.net

You are right Ricardo. The M20 and M21 pistons retain the same overall dimensions.....But the gudgeon pin to crown height is totally different. Ron

email (option): ronpier@talk21.com

The M21(112mm) has a stroke that is 18mm longer than the M20(94mm)...
However, it uses the same conrod and cylinder so to maintain the overall crank/rod/piston assembly height at TDC the gudgeon pin was positioned 9mm (354" approx.) nearer to the piston crown in the M21...

The bore(82mm) remained unaltered and the rings and pin were the same but the pistons themselves are not interchangeable between the two engines...

What is the current bore size?...I have found in the past that the Indians frequently go beyond +.060"...Ian

email (option): ian@wright52.plus.com

thanks gentlemen; I'll find out Ian.

email (option): rick.howell@talktalk.net

Hi Ricardo,
I got a +40 thou (NOS) with rings from Les at Russells only 2 months ago.
HTH

Scott

email (option): larkesrATgmail.com

my friend tells me that measuring with a digital vernier the barrel is standard

email (option): rick.howell@talktalk.net

Yes but a vernier won't be able to measure the bore ware/ovality/taper accurately. That will require the use of an internal micrometer. Ron

email (option): ronpier@talk21.com

This diagram illustrates how the bore should be measured to achieve an accurate picture of the wear, which will be uneven. It's best to take the cylinder to an engine shop if you don't have the correct bore gauges/micrometers....

A Vernier caliper is totally unsuitable for the job...Ian

email (option): ian@wright52.plus.com