Model 20 piston ?

Do you have a copy of "Radco"..."The vintage motorcycle workshop"?  It's out of print now  but has a section on substitute pistons. Of course lots of things have been lost since it appeared in 1986. But if you know bore, crown height (with dome), pin diameter and length then the issue is finding a catalogue to identify a possibility. Perhaps a car or another motorbike. If the only one you have found is too tall, can you accept higher compression? Fuel is far better today than in 1931. You say 1/8" too tall...If it will safely move the length of the bore and keep away from the valves, it should be fine.

If your CR is 6.2, a piston 1/8" taller would give close to 7.2. You could always add a 1/8" plate under the barrel.

... be worth looking through any catalogues you could find on line eg here: https://www.enginepartsuk.net/catalogues for something similar or near enough. Unfortunately most don't give gudgeon pin diameters but a custom made bush would often sort that out.

I agree about the Radco book which has helped me out many times.

Available online as a PDF https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjPkKnnirvyAhUL36QKHaqKA3QQFnoECAQQAQ&url=http%3A%2F%2Fcentralcoastclassicmc.com%2Fdocuments%2FThe-Vintage-Motorcyclists-Workshop.pdf&usg=AOvVaw33UG_LfwXIHzOZi9ICmww8

Or on Ebay for £19 https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2060353.m570.l1313&_nkw=vintage+motorcyclists+workshop&_sacat=0

As David suggested a compression plate could be made and fitted below the barrel. I think that would be preferable to skimming the top of the piston. Maybe too late for you but I have found John Harding to be a font of piston knowledge, including suggestions for alternatives for otherwise unobtainable items. He advertises in OBM.

Hi Ian, 

   Your piston is the same as the 1932 to '39 CS1 OHC engine one. This has the Ernie Searle inspired moat shaped crown, which doesn't make much make much difference to performance. Another option would be an International piston, although this has a slipper skirt and fits the narrower Inter con-rod little end, so if you wanted to fit one you might have to remove material from each side of the con-rod. Both these pistons have the correct height you need. If your piston is already plus .020" you will need at least a plus .040" to get a good finish on your barrel if you rebore it.

Hi Ian,

   If you are going to fit a new reproduction piston, check what compression it will be in your engine. The maximum I would want to go would be 8 to 1 and preferable a bit less, but as long as the height is right, a flat top will piston will be just as good.

Thought the Model 20 was a 500 ?

Hi Alan,

   Yes, twin port 490cc. Feel a little red faced, so have edited previous post.