This is an update from the handlebar vibration discussion, going from the suggestion that I check piston weight before dismantling the bottom end. Here are the initial results of piston weight (and photos, and other observations):

Piston weights are as follows:
3-rings: total of 33.8 grams
2 circlips: 1.3 grams
Gudgeon pin: 89.1 grams
Piston: 427.8 grams
Total: 552 grams, 19.47 oz

See photos if that is informative about piston type. So what does this piston weight tell the experts?

For what it’s worth, the bottom end SEEMS pretty tight, from what I can tell without dismantling it. The con rod appears in good shape, the small end bush looks great (it even has an oil hole in it). Bush/pin/piston fit are all perfect. I do not feel any vertical play/looseness between the rod and crank pin at all. There is however a small amount of lateral (left to right) play between the rod and crank pin, is that acceptable?

Any other thoughts on vibration and ways to decide whether a crankcase rebuild is worthwhile at this point?

email (option): cpdoberstein@gmail.com

I've not got any pistons to check the weight of them, but just looking at the underside of the piston, it looks like a pattern one and it looks a lot heavier construction than the BSA piston or a Hepolite one.

email (option): davmax@ntlworld.com

Just weighed a genuine BSA std piston and it weighs 426 grams without rings/pins so not much difference.Your piston looks like a pattern one though.I would check conrod with straight edge as a rough guide as piston looks like its had a hard time at top on side of crown.Dave

Yeah, I noticed the piston crown, that's why I photographed that side. Other side of crown is fine. Con rod seems straight, but I haven't dismantled the case yet so it's hard to say.

I do have a little bit of side-to-side play in the con rod at the crank pin though. Is that acceptable?

I'm not anxious to tear-down the crankcase and go through the rebuild, but since I'm already on my way there I suppose now is as good a time as any.

email (option): cpdoberstein@gmail.com

Hi, I used to run my wd m20 over long distances and use it for work every day, trips to the IOM and full throttle round the full course. My piston used to look like yours. If your rod was bent it would show at the bottom skirt at the other side. What this is is a build up of very hard carbon at the top of the cylinder at the valve chest area. This carbon is hard to remove. I have looked at my stock of WD bits and i have a piston just like yours still boxed in its wd box and packed in grease, it's identical to yours with the strange cast marks on the under side of the crown.
Side play on the rod is not a problem rock it side to side, find the centre and feel for up and down play, use both hands and feel for play with your fingers right at the bottom of the rod. All old british bikes vibrate some more than others. If your crank spins smoothly over by hand look elsewere. Your wheels could be way out of balance. Old guys used to rap lead strip round the spoke nipple to get the wheels balanced.
Have a look at the other side of the piston and see what you think, i cannot weigh my piston as there must be half a pound of hard grease on it but it does look heavier than a latter one i have.
let us all know as i am getting back into M20s again.

email (option): t.j.walker@btinternet.com

Hi Tim, I don't know if you read the previous thread on this, we were just going through all the possible reasons for his vibration, so we were starting at the simplest and easiest to check things that could be causing it. We've been through all the obvious things like tyres not seated and such like, but I think it is starting to look like the crank may be too much out of true (they will still spin freely in the case with 0.005" runout, but obviously they will vibrate a lot more) So if his piston is weighing in at about the same (we thought it may be a pattern type that was heavier) So I guess the next logical thing to go for is checking the runout on the crank, but this is probably a good idea as it gives him chance to check out the big end.

email (option): davmax@ntlworld.com

I reckon best bet is to strip out motor to checkout crank.When I bought my HM21 I was told it just had a new big end bearing fitted but this bike throbbed thru the footrests so badly I stripped out engine to find no up & down play at big end but could grip the conrod at the base and push it side to side BSA service sheets specify around the 12 thou mark.And if you held conrod at the small end eye this thing rattled around quite a bit!! I replaced the thrust washers on the flywheels and rebuilt a old pin I had with a set of Dave plumbs harley type rollers no more vibes thru footrests,Also have done a couple of more cranks using these rollers and have now made a jig to true up flywheels/mainshafts.I also hammer the guts out of my old WM20 but on a recent top end strip my piston dont look like craigs so somethings wrong somewhere. Dave P.S I,ll post a photo of jig when I have more time..

Thanks for the input guys, very helpful.

The lateral con rod play is as much as 1/8" measured at the small end eye. Seems strange to me that there is absolutely zero vertical play, given this lateral looseness. I've never disassembled the crank though so it's hard for me to envision the mechanics, even with the great descriptions from Dave and others on this M20 page. I can't even say if my crank has the bearing pairs, or newer harley rollers.

In any case, I agree that a tear-down and rebuild is likely the best solution. I'll try to do some flywheel run out and truing measurements while it's in the case just to see what I can. Selfishly though, I might put her back together for now and enjoy the summer, then tackle this when the rain returns... unless the vibrations worsen or other problems arise.

email (option): cpdoberstein@gmail.com

Yes Dave i have come into this thread without reading the whole thread, the last time i looked at this site was when you had to guess a part of a m20, that was a long time ago. you are correct if the wheels and tyres and obvious parts have been checked. It now looks like a crank check, also a check on the drive side spacer for length, as you will know if it has been run with the shocker nut loose this wears the spacer and oil flinger and pulls the crank over to the drive side but the side clearance on the piston to little end will take this out. But it's best to have the piston shoving the crank central.
If i can find some old pics of my m20 i will post them, it was one of those bikes you regret letting go, i actually swapped it for a grandfather clock, still got the clock that was around 1990.

email (option): t.j.walker@btinternet.com