Okay I giveup. If you read my earlier post you'd see that my bike keeps stopping when i am at medium hi revs going up a long slight hill. The valve tapping goes from a tap to a clanking and the bike stops. (no tapping when bike starts)

Thought it might be that I was running with no spacer on the carb. But put a 1/8th spacer and same thing happens at same spot. Leave bike alone for 5 minutes starts right up, tapping but no clanking and can drive as long as I am slow.

Could I be running out oil? Sure seems to be flowing in the oil sump. Any help is appreciated

email (option): jeff@infinitymedspa.com

Jeff I'm not sure if you answered the question that Dave (I think) asked in the original thread. Has a new piston been fitted? It does sound like the piston might be nipping up in the bore when hot.
It happens if the piston hasn't got the correct amount of skirt clearance. The piston will expand with heat and get to tight for the bore.
Basically an M20 bore should be around 5 thou bigger than the widest part of the piston skirt.
When the engine stops, can you kick it over immediately or is it tight?
I'm just trying to help diagnose your problem before we get round to the fix. Ron

email (option): ronpier@talk21.com

Certainly sounds like a piston 'nipping' to me...Ian

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

Hi Jeff
I'm coming in a bit late and have not read all the replies so i may double up here. The tapping turning into a clank does sound a bit serious but if the engine will restart ok then it may not be to bad. Rons question is a good one about the piston nipping up but this would lead to some serious damage faily quickly. How abou valve clearance's if the exhaust valve clearance is too tight , it may lead to the valve not closeing properly when hot. The clanking may be the valve hitting the head . I'm not sure of the clearance between the head and valve when fully open. Hope this of help but what would i know, i don't even have a bike that goes.
Cheers
Phil

email (option): wpgarage@xtra.co.nz

After the restoration of my WDM20, the slit skirt piston seized after some 30 - 50 km, and just on flat roads (remember, our highest mountain measures 320 m, and we have to share it with Germany and Belgium )
According to a metallurgist friend, the trouble was a "Friday afternoon - Monday morning" piston, not properly de-gassed, that would continuously and excessively expand and not shrink back to normal.
The problem was solved by bringing the clearance to the maximum allowed.
According to my friend, I could also have used the hot-cold treatment: put the piston on a hot plate, heat to 200-250C for 15 minutes, let cool down for 45 minutes, and repeat that cycle for a few days. After that, the piston would be dead tired, and think twice before expanding too much again....

email (option): viaconsu # planet dot nl

Jeff, I too have a theory on what may be happening, and at least you can easily test it without dismantling anything much.

I figure that if the exhaust valve lifter has too little clearance, the operating peg may just be touching the valve collar at normal engine temperatures (the light tapping noise) but would start banging into it (louder clanking noise) as the cylinder expands upwards at increased temperatures. This would prevent the exhaust valve from closing fully and cause the engine to lose power. The flaw in this logic is that I wouldn't expect the engine to stop suddenly, rather just run worse and worse, but every good theory has to have a flaw!

Anyway, you can remove the valve chamber cover and check the gap with a cold engine - I'm not sure if BSA mentioned a figure apart from the peg must be "well clear" of the collar - or you could check to see if there's any gap at all next time you get the problem.

Come to think of it, there's an even easier way. When the symptoms start, put finger pressure on the valve lifter handlebar lever, just enough to take up the cable slack, and if you feel the lever vibrating then you've found the fault.

Here's hoping for a simple fix.

ChrisJ

I don't think it is a new piston, at least I haven't replaced it. It turns easily over when hot. Clanking is coming from right side (I think) where the valve rockers are.

email (option): jeff@infinitymedspa.com

I have checked the clearances. It did it wne the exhaust was at the "stock" .012 and then also when I tightened it up to a loose .08 as Ian suggested. Can't figure out why it would completely shut down though.

email (option): jeff@infinitymedspa.com

Checked the cold clearance and there is quite a bit. Thanks for the idea though.

email (option): jeff@infinitymedspa.com

I think it's about time to start thinking about lifting off the cylinder. It's the only place it could be coming from unless the timing is so far advanced it'd kick you back over the handlebars even fully retarded! I have had so much trouble with pattern pistons, I do rebores and quite frankly the only ones on M20's that I haven't had any problems with are genuine BSA ones and Hepolites.

email (option): davmax@ntlworld.com