1974 Mk2A Project, what might have caused this?

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N0rt0nelectr@

N0rt0nelectr@

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I had to pull the head on my newest project because of a blown head gasket. When I got the head I could see that there was a leak from the combustion chamber into the push rod tunnel. What I didn't expect to find was a divot in the top of the left piston. It doesn't look to be from the valve hitting the piston.
Sent the pictures off to Jim Comstock for his opinion but I thought I would share them with you for your ideas and thoughts.

John in Texas
 

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If you're referring to the mark at 1 o'clock in the picture (next to the center-rear stud hole), it looks like a hex nut that landed on edge. Any matching divot in the head?

Nathan
 
Nater_Potater said:
If you're referring to the mark at 1 o'clock in the picture (next to the center-rear stud hole), it looks like a hex nut that landed on edge. Any matching divot in the head?

Nathan
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I thought about that and cleaned the combustion chamber and there are no other corresponding marks in the head and the valves are just fine.
I would have thought that if there was something that managed to get into the cylinder there would be more marks and the valves would be nicked also.
Jim said not to worry so I am putting it back together.
Should have it running again tomorrow. Do the final adjustments and take her out for a spin. Hoping for no more surprises.

John in Texas
 
Concentric carb. slides get sloppy and fracture , getting slurped in , then instantly spit out the exhaust. Check Slides.
 
Torontonian said:
Concentric carb. slides get sloppy and fracture , getting slurped in , then instantly spit out the exhaust. Check Slides.
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Never thought of that, good call. I will check the slides I a little while and let you all know what I find.
John in Texas
 
I once had a 6mm washer from the air filter assembly of a car get into cylinder as i was driving along. You could hear it rattling inside the motor even though it was liquid cooled an inside the engine bay (unlike a Commando) with a loud exhaust fitted (like a Commando). The top ring of piston was trapped in groove and piston and heat were a big mess. So if you just have a single mark you are very lucky.

Could it have made when fitting or removing the head by any chance?
 
toppy said:
I once had a 6mm washer from the air filter assembly of a car get into cylinder as i was driving along. You could hear it rattling inside the motor even though it was liquid cooled an inside the engine bay (unlike a Commando) with a loud exhaust fitted (like a Commando). The top ring of piston was trapped in groove and piston and heat were a big mess. So if you just have a single mark you are very lucky.

Could it have made when fitting or removing the head by any chance?
Click to expand...

I got the bike last September from an old friend and as far as I know hasn't been started in 20 years or more. Didn't find any signs of a broken slide but that doesn't mean a thing as that could have been replaced. Need to finish putting it back together and start it up again, without any more oil leaks!
Report to follow.
John in Texas
 
I f a bit breaks of the carb slide, it usually stuffs the spark plug and leaves very little other marks. It has happened twice to me because of lazy spark plugs on a big ride. first sign of the lazy spark plugs is popping back in the exhaust like a lean mixture.[ at least I thought it was a slight leanness in my carburation when I first got the symptom. ]
Dereck
 
The apparent fact that there is no matching mark(s) on the cylinder head and it appears that the divot has carbon baked into it strongly suggests this happened prior to assembly. Was this head ever off before - if not then it may have happened at the factory. The depth suggests it was something harder than aluminum and the fact that there are no other witness marks (head or piston) tells me this is not from something that passed through the engine after assembly. Usually, you will see a few witness marks to several to many bounces of the bit if there was something loose.

Could someone have dropped the head while offering it up to the cylinder barrels upon assembly and a cylinder head stud kissed the piston - really does not look like something from a stud. My guess is something (tool, part, or some other tough item) was dropped or hammered on the piston prior to assembly.
 
Hard to say what went on before I bought the bike. Did happen at the factory, could be but there is no way of telling.
Only time will tell.
John in Texas
 
Hopefully time will not tell. I would ignore this divit and ride on if everything else looks good to go. :)
 
Torontonian said:
Hopefully time will not tell. I would ignore this divit and ride on if everything else looks good to go. :)
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Well that's the game plan.
I have replaced al the wheel bearings, rebuilt the forks, replaced the brakes , rebuilt the master cylinder, new tires. Not going to take any chances on surprises. See where that gets me.

John in Texas
 
the mark is away from the squish area, so was probably on the piston before assembly. is there an aircleaner on the bike. it is rare for a bit of the carby to fall off and get swallowed.
 
acotrel said:
the mark is away from the squish area, so was probably on the piston before assembly. is there an aircleaner on the bike. it is rare for a bit of the carby to fall off and get swallowed.
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When I got it there was a stock Mk2 A aircleaner on it. The carburetor slides are intact so who knows. Poor quality control, who knows.

John in Texas
 
I am going to take it a step further and say unless someone else was in the engine before to machine or otherwise face up the piston crown, that divot was most likely in the piston crown before the manufacturer finished machining the crown. Maybe a casting flaw.

My reasoning is there does not appear to be any hint of a raised land around the divot. Your scraping the piston crown should have left witness marks around the divot where the metal would be upset, sitting above the crown surface, just like around the impact site of a meteor.
 
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