hole in piston

[lowbrowbsa]

 first let me start by saying hello to all
i finished building my super rocket (bobber style) and today as i was running it in lost power
got it home ihad no compression in left pot so off comes the head and as i thought hole in piston
once i get the hang of this i will put a pic up .
so that was my day

[lowbrowbsa]

ok so why did this happen

[manosound]

It almost looks like it was never running on the left side,  or that the damage happened almost immediately.  Eight to thirty folks here who are more expert than I might chime in with their own experience,  but I am thinking manufacturing error in the aftermarket 10:1 pistons.  Obviously,  any flaw or weaknees in those is going to be exagerated versus lower compression pistons.  Also,  it looks as if the hole was blown out from the backside,  though that is probably not the case.  Wondering if a closer view of the head would show any mark where the peak of the piston collided with the bridge between the valves.  Finally,  it also looks like the right piston  may have weak spots in approximately the same geography.  With 10:1,  I would think you need to use almost 100 octane fuel to avoid serious preigniton,  which might be a choice as culprit.

Richard L.

[manosound]

Oh,  one more thing, welcome to the forum.  Glad you could join us.

[groily]

Welcome lowbrow - shame to join on such a lousy note though. That's a nasty one.
As to 'why' - I'm sure Richard's onto something re the pistons and the compression ratio/pre-ign, but would add I don't like the look of that head gasket, unless it's just the picture. Looks blown across the front of the lh cylinder to me and maybe between pushrod tunnel and bore (plus at the front) on the other side too. If the lh pot had been leaking  . . . could be a  cause of the damage even if the ignition timing and mixture were right or near enough, and the plugs were suitable for the beast.  How few miles did it actually run, was that literally it's first run out?
Lots of stuff (and help) on here about all these things - but I guess that's the only little bit of good news.

[manosound]

If you don't already know this, when you do repalce that head gasket, be sure it is fully annealed dead soft. I've run some informal tests (shown somewhere on here on the forum) which show that a new head gasket from, say, SRM, is not as soft as one annealed with my propane torch, even though they say the new gasket is soft.

Richard L

[lowbrowbsa]

thanks for the info the this was the bike second run
i am runing 98 octane  it was a sudden thing eng was sweet for about 20ks then lack of power so i shut it down
got the ute and took it home pulled out the plugs right side fine Lh not so fine 
would you recomend what pistons and should a hone be on the cards
this is the first a10 650 i have built any imput will be much helpfull 

[manosound]

I'm chimin' in here just so you don't think I'm ignoring your question about pistons, but others who have experience with the various compressions are better suited to answer. For the record, mine are 9:1.

Richard L.

[MG]

Hi lowbrow!

Sorry to hear 'bout your piston failure, nevertheless welcome to the forum!

From personal experience, I can highly recommend Wiseco forged pistons, available from Cake Street Classics (Roger Shirman, probably spelt wrong, sorry Rog  ). Those are 9:1, not particularly cheap, but well worth the money!
Since it has only been the second run, the barrels will probably be okay. If you can still see the honing pattern, they will be good to use as are.

Your pistons look very HC indeed, I think more than 10:1.
It doesn't exactly look like the usual pinging/pinking meltdown, there are no signs of molten material on the piston crown, around the hole (hard to see on the pic though). Did you notice any pinging?
Once you have the piston out, can you check what the hole looks like on the inside? Is the hole funnel shaped, i.e. bigger on the inside of the piston, especially along the last 2/3 of its length?

The head gasket definitely looks blown, an air leak past the head/barrel joint definitely isn't helpful for a proper air/fuel mixture, as groily has indicated already. Proper annealing of the gasket which has been mentioned already is as important as correct tightening of the head bolts, both in sequence and torque figures. The joint faces ahve to be dead flat, if the engine is built from pieces that didn't belong together originally skimming both is pretty much unavoidable (guess why I know). Have you checked both surfaces with a straight edge or have they been skimmed?

Cheers, Markus

[A10Boy]

Hi Lowbrow, shame about your problems.

My eyesight is not what it once was, but to me, it looks like a valve has been clattering the top of the piston. Are there any witness marks on the valves? What kind of RPM were you up to?

I'm not a fan of HC pistons. Fuel in the UK is rubbish nowadays and lower comp pistons are more suited.- of course others will have a different opinion........................... 

Regards

[manosound]

Gents,

Help me here, how could a leaky head gasket lead to a leaner, hotter-burning fuel mixture. Air drawn in on intake will be by the path of least resistance via the intake valve, leaving virtually no suction to draw past a mostly working gasket. On compression, I would think the gasket leak would tend to push out very little fuel, but some air, leading to richer, cooler explosions. If the black is not just by leakage, it might be by the same catastrophic explosion(s) that blew the piston, if indeed that is how it went.

Richard L.   

[MG]

Richard,

The piston on down stroke will indeed suck quite some air past the leaky head gasket once the leak is big enough. A not yet fully open or about-to-close inlet valve and an eventually closed/not fully open throttle slide offer serious flow resistance in the inlet tract.

There can be another issue when running the (air-cooled) engine with a leaky head gasket: The hot combustion gases that are blown out past the tiny opening between head/barrels and gasket at high velocity heat the surrounding material like an oxy-acetylene flame. Ridges and other small parts of it that are eventually protruding into or adjacent to the combustion chamber can reach temperatures high enough to keep them glowing and form ignition sources for detonation.

[manosound]

OK. Education accepted.

[MG]

Didactic objectives achieved for today, thanks. 

[Flatboy 1950]

The high temperature resulting from the lean mixture will also explain the clean piston crown & combustion chamber.
Remember using the oxy-acetylene torch to clean the silencer baffles of every Villiers engined bike on earth ??

[lowbrowbsa]

cheers guys mg the hole was bigger on the inside but no damage to anything else valvers are good heads good boars still got hone marks i think i just got un lucky with these pistons
ive been serching and i think ill go jp 9:1
on a positive note this is all part of owning the bsa its great to ride and working on it is  as much fun 
thanks all for your imput much apreaciate it 

[MG]

This is typical for hemi engines, they have a tendency to hole their pistons when operated at high compression, even if mixture and ignition timing are set correctly and there is no detonation occuring.
Certain disturbances in the combustion process heat up a critical point on the piston crown very quickly, so the piston material becomes plastic there. Inertia forces and the fast flowing gases excavate the soft material, and material strength in this point is therefore decreasing continuously. The combustion pressure then knocks the remaining 2/3 of the material out to the inside, leaving that funnel shaped hole.
This happens in very short time, with usually no other signs of overheating on the rest of the piston crown or the skirt or signs of a seizure.

So if your ignition timing and carb are set up correctly, and there is no air leak past the head gasket, I would bet my money on this damage mechanism. Still, I'd check the head and barrel joint faces and have both skimmed if necessary!

I'm running 8.5:1 JPs in the A7 SS and they have been working properly ever since, although the bike is usually ridden like it had just been nicked to keep up with more powerful 650s.  
So if you can rule out all other possible causes, I'm sure she'll be running fine with JPs at a reasonable 9:1.

Good luck!
Markus

[cyclobutch]

The only time I saw this was on a Daytona 500. Needed a full strip, I had alu particles everywhere inside the motor.

[manosound]

So,  Markus,  is the crown of the left piston fractured away or melted away.  It looked fractured to me but,  as you know me,  I can convince myself of anything.

Richard L.

[MG]

We are going to bore some people to death, Richard. 

But here goes:
Hard to tell from the picture. But it looks like it started to melt on the piston top/apex, and then finally the remaining material fractured away, around the weakest spot, where most material got dragged away, or around an existing imperfection (casting defect, crack, etc.).
Could have been a totally different story of course (shot head gasket, detonation, etc.), but I thought I'd bring it up, since it could be a realistic option for what caused that failure, once all other possible issues are ruled out.
There are some arguments against detonation meltdown however. There are no signs of overheating and it happened very fast. Otherwise you would expect the piston crown to cave in, it would look burnt, you might find a blued gudgeon pin, and with a pinging engine, it will take quite some time before you finally hole the piston, if at all. With a new engine chances are rather high that it will seize due to overheating anyway. And most obviously, there will be a clear pinging noise from the engine. Furthermore, the location of the hole in the piston would be exactly opposite of the component that caused the premature ignition (ie spark plugs, injectors, glowing ridges or carbon deposits, part of the head gasket protruding into the combustion chamber, etc.). I can't see anything like that in the head, near the area of failure.
Hard to give a definite answer based on some low res pics. Grey is all theory.