Hi all, picked up a pretty good pair of YM20 cases (YM20 356). There were two flywheels with them, no crank pin and rod. What I'm a bit confused about, is there are no numbers on the flywheels at all. I've checked Ian's very good specification list, and no "numberless" are listed. Stroke is M20, and they look like M20 flywheels. But no numbers?

email (option): oldjunk@btinternet.com

I have some M21 flywheels equally devoid of any numbers, so its nice to know someone else has been scratching their head :-).

I would say these flywheels are probably cast iron examples...Though that is only an 'educated guess' based on the large number that have been through my hands and some of the variations I have seen...

Both the M20 and M21 had flywheels made from either material at different times (and for different users)...

Small differences in the surface finish of the unmachined parts can give it away if you have more than one set or are very familiar with them...The cast iron wheels generally have a coarser surface finish...Ian

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

Thanks Ian, I thought that may be the case. But I also remembered something from the dim and distant past. I put one of the flywheels, held by the shaft, in the vice and flicked it. I then did that with one of my others (from 1942 ish) The 1942 one went "Ding" with a nice ring to it, the later one went "Dink" and sounded flat. Pretty conclusive, and thanks to my old metalwork teacher, from about 1970!

email (option): oldjunk@btinternet.com