piston stuffed again

[lowbrowbsa]

well not a good day well sum good bsa ran fine to mummby then on the way home she stoped left piston again on the intake side  no worning got to hot on left side well ill pray to speed gods for sum new pistons again mybe the head needs a little porting on the left side of the manifold ????????? heres sum pics of the damage

[bsa-bill]

Looks pretty clean in there, how far was the run?
Just wondering if it's a bit lean

[lowbrowbsa]

run was about 50 ks but i set it a bit rich to run in pistons got me stuffed maybe spark broke down

[A10Boy]

Bad news indeed.

I would check the cam ring to make sure the timing is correct. If you time the righthand cylinder like most of us do and the cam ring is not opening at 180 degrees you will have  inaccurate timing on the LH pot which can cause overheating and failures like this.

HTH

Andy

[lowbrowbsa]

thank a10 boy this never crossed my mind but makes sence ill rember for the next rebuild 

[bsa-bill]

maybe spark broke down

Don't know that this would cause overheating though, more likely to stop or run cooler due to incomplete burning, A10 boy might have a point though

there are various opinions as to the reason for the hole in the lefthand conrod, the article I read (Bsaooc handbook IIRC) reckoned the hole was there to increase splash to the left hand cylinder to address overheating problems there, I know there are differences of opinion on this but wonder if you recall if the hole was there and pointing to the centre of the engine.

Just a random thought

[cotterpinkid]

Hi Lowbrow,

Definitely check the timing / cam ring on the mag. When I rebuilt the A10 PL it ran hot on the LH pot. I was going down the track of induction bias, but quite rightly people said check the timing. I purchased a bolt on timing disc and borrowed a strobe and found that the LH pot was firing five degress earlier (retarded) than the RH (having timed the RH). When I compared the new cam ring with an original one provided by mate, you could see the difference in the cam lobes. I have now junked the new one and permanantly borrowed my mates and both cylinders now fire at exactly the same position.

Brian

[chaterlea25]

Hi Lowbrow,
Shame what happened again
Looking at your photos it appears thet the exhaust valve on that side is very recessed??
This could cause hot running, because of the restricted gas flow??
(along with all the other things mentioned !!)

Have you checked the valve guides for valve/guide play? maybe its drawing air?? (leanness)
Also check that the guides are tight in the head !! quite common to find the guides come loose in the ally heads
Heat the head up to 100deg then give the guides a light tap with a removal drift, if they move easily at 100deg then they are def going to move when the engine is running

I would recommend short runs, after you rebuild again, stop and keep checking the engine and plugs

HTH
John O R

[beezermacc]

Oh Bu88er!

There's quite a bit of stuff about cam rings on this thread. Basically, retarded timing causes overheating. My mate's A10 seized both his exhaust valves due to retarded timing. The problem is more often the cam ring housing rather than the cam itself though there are some ropey cams about. If your points gap measures the same each side there's still no guarantee the timing is correct. To be absolutely sure you are best using a timing disc and a timing light across your points.

[RichardL]

Just to rule this out,  I'll ask the dumb question.  Considering where the damage is,  right next to the relief in the piston for the intake valve,  is it possible that the intake valve is just barely kissing the piston?  Looking at the valve,  it doesn't appear so, but just asking

Richard L.

[kiwipom]

hi guys/Lowbrow, the top piston ring looks awfully close to the surface? compared with this one, what do you think?cheers,Bob

[bsa-bill]

sort of along Richards line of thinking - does the left hand inlet valve look off centre in its seat, probably just the way the light hits it.
Bob you might have just spotted something there

[BSA_54A10]

I'll go along with the Kiwipom.
That ring is way , way , way too close to the top of the piston.
Don't know where you go them from but you should take them back and insert them in a place where the vendor will get the least amount of nutritional benefit.
If they were flat on top I would have said that they were compressor pistons.

[A10Boy]

I suggested checking the cam ring because there was an article by Hughie Hancox recently in one of the classic bike mags, Clasic Bike Guide I think it was. He was telling the tale of when he worked at Triumph and a customer had a T110 that kept holeing LH pistons. After three rebuilds someone thought to check the timing and the LH pot was way out, they fitted a new cam ring and that was the end of the problems.

Its good to learn from the experts  

[Triton Thrasher]

run was about 50 ks but i set it a bit rich to run in pistons got me stuffed maybe spark broke down

How do you know it was rich?  I don't see much sign of richness in your photos.

Definitely check timing on both cylinders.  Definitely follow the carburettor tuning drill, involving all the carb circuits, especially needle position and main jet, starting form definitely too rich (so rich it doesn't run properly) and adjusting weaker until correct.

Looks like your piston was eaten away by detonation.  Weak mixture is a very common cause. Also too much ignition advance. Also labouring in too high a gear (sometimes a problem after fitting some silly big gearbox sprocket). You weren't going up a long hill or into a headwind in top gear, by any chance?  Too high compression also causes detonation, but your pistons don't look like hi-comp ones.