Top end issues

CLEMTOG said:
you are forgetting the centrifugal sling of a big end with collapsed or missing rollers Cam,when the crank pin get to TDC with extra clearance, and starts to decend the rod and piston will carry on by inertia and then get yanked down as the pin contacts the eye again.
CLEM
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Read my post again Clem. I said "when the crank is turned by hand" which is what oehlin was doing when it came to a stop both forward and backwards. You'd have to be pretty damn quick with the hand to get any inertial effect from the piston.
 
Found the cause of the piston stop.😭
A screw has apparently been sucked in via the carburetor (no air filter was fitted) and the remains stopped the piston to passed the TDC.
The damage to the piston is only a number of small jacks and no damage to the valves is visible.
1. Is it enough to polish up the piston top?
Do I need to change the top and bottom gasket (they have only been running for 15 minutes)?
3. When the marking on the cam gear is aligned with the marking on the top, the exhaust valve on the right cylinder is depressed. OK?
 

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Is the screw off the carb, I guess that answers 2 questions. What happened and should I run an air filter.
 
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I wouldn't use the piston, the damage is realy unknown, any sharp edges will create hot spots, there can be cracking that you cant see, if I was being realy tight I would lift the barrel and remove both pistons from the rods, then clean up the damaged one and take a very close look, I have strong suspicion that it may be a 1200 piston, the top looks very flat. Even though this will result in lower compression, the edges have a tendancty to touch the valves, the only way to check is to rebuild the engine with plasticine on top of the piston, oil the valves (to prevent sticking) then turn the motor one full turn, strip again, and inspect the plasticine, for thickness, 50 thou minimum is good starting point.
CLEM
 
I agree with what Clem says. Risky to re-use the piston but possible cleaned up if a very thorough inspection shows no structural damage. I would also check that the valves seal properly with kero in the manifold. I always re-use head and base gaskets, the head gasket on my race motor had been re-used so many times I lost count. From memory both the inlet and exhaust valves are open a bit at TDC overlap. The piston does not appear to have a flat squish band like on an SF piston. If your pic has the piston at TDC it is wrong for the motor.
Is there any sign of scoring in the liner?
 
That solves that mystery. I had a Mini Cooper that swallowed a screw. Got stuck under a valve. From where the screw came remained a mystery, best theory was that it somehow found its way into the air filter housing during a service.
 
I’d definitely have valves out for a check - my Montjuïc that are a nyloc nut bent the valve, head was tilted, might not show up on a flood test . Hopefully you’ll get away with fitting the right piston, in which case its not a bad outcome all considered.
 
Tippie said:
I agree with what Clem says. Risky to re-use the piston but possible cleaned up if a very thorough inspection shows no structural damage. I would also check that the valves seal properly with kero in the manifold. I always re-use head and base gaskets, the head gasket on my race motor had been re-used so many times I lost count. From memory both the inlet and exhaust valves are open a bit at TDC overlap. The piston does not appear to have a flat squish band like on an SF piston. If your pic has the piston at TDC it is wrong for the motor.
Is there any sign of scoring in the liner?
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Would you use gasket silicon when reusing gaskets Tippee ?
 
I would never re-use a composite head gasket, I do re-use most other gaskets, as I always grease both sides prior to instalation (just plain old HMP grease) and clean with solvent, and re-grease when using again, NOW my solid copper head gaskets for triples? that's another thing, use them again and again.
CLEM
 
CLEMTOG said:
I would never re-use a composite head gasket, I do re-use most other gaskets, as I always grease both sides prior to instalation (just plain old HMP grease) and clean with solvent, and re-grease when using again, NOW my solid copper head gaskets for triples? that's another thing, use them again and again.
CLEM
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Or you could fix the problem.
 
MarnixSFC said:
Looks like a piston for an elettronica SFC to me, totally unfit for use in an SF.

Marnix
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Exactly what I was thinking Marnix - either that or a piston that simply isn't right.

I'd reuse the gaskets because they are so new - and after reading what Tippie said I'd reuse the head gasket even if it was old!
 
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