The 1971 RT1B I have been working on had been running, albeit not great, but now will not start.  I thought I knew the history on this bike but after getting the head off, I saw what is in the picture.  I was 100% confident it would be the original standard piston.  Help............what is this??????

Still std 80mm. No oversize numbers visible just numbers relating to std. Not sure what they mean, but think it's clearance related to 80mm. i'll take a guess it means 79.968mm or just under 79.970mm. So about 0.03mm or 1.2 thou under 80mm.
THAT'S ALL WRONG but i'll leave it there lol to be put right?.
Anyway--Std size so don't worry. Lack of performance i'm still stuck on exhaust. Also flywheel crank key if its been off recently.

I agree that's what the numbers mean...  stock bore is sized as advertised (+/-) and std piston is slightly smaller than advertised bore size to provide correct clearance. 

Mark and RT325, thank you both. That puts me on track. RT, I have checked the key and have had the exhaust off twice both don't see any issues. It was running fine before except at over 5000 RPM. I brought it in, parked it, and could not get it to start again. That's when I started questioning the compression. With 4 kicks with the compression release not activated, I get about 85 PSI. 10 kicks gets about 120 PSI. The piston shows a lot of burning on the exhaust side across the whole front of the piston from the rings down to about an inch below them.  Does this sound typical.  Its the original piston and rings and the bike has 4800 miles.

Top end is probably worn, those comp numbers seem low.

Unlikely compression causing power to drop of at 5000 & would show up as 'not wanting to idle' reliably in the forst place. So Oring on the jet setter is important but will show up rich like the choke is on or worse as the revs rise--or try to. So exhaust--i'm still stuck on exhaust--you said you took it off but what happened then. They block down in the back end deep in where the spark arrester is built in--if i'm thinking right!!. Also a performance killer is retarded timing from points heal wear to the stage of points barely opening at all. So just a few random ideas for a Friday from downunder.

Thanks, Dan.  I feel like you are right.  I talked to a machine shop today.  I may just have it re-bored and go from there.  At least then I know where I am.

Thanks for the thoughts RT.  I have blown compressed air through the exhaust and it seemed fine.  It is still off the bike so I am going to run a wire through it tomorrow just to make sure.  The points are new so wear on them should not be a problem.  If I introduce a few drops of gas into the throat of the carb all it will do is kick back.  This seems like early timing but retarding it did not help.  I'm running low on ideas................

Ok, kick back & new points has me immediately thinking its eaten the crank key while running & now the flywheel has moved so timing is way off--my guess without thinking too hard just yet.

Compressed air would still go through a restricted pipe until its totally plugged--not that they're ever totally bunged-- so keep that in mind but not whats stopping it starting at the moment.Here's the header pipe on my 100 twin, both pipes were similar. Pic won't load. Found my old link, so easier this way.
yamaha-enduros.com/index.php/forums/1972...aust-on-ls2-100-twin