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Hole in piston
#21
Hi Colin,

apologies if my mention of the head gasket led you to believe I was questioning it - to be clear, I was not.

Is it one of the black coated ones? As I said, I found them to be very durable.
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#22
We all await the outcome with great interest. I hope the tale  does not peter out as so many other puzzles have.
( I think we are still waiting the outcome of the mystery drive clonk video of a few years ago!)

In such events it is often a puzzle whether the heating both preceded and followed mechanical failure, or just followed due to the leakage paths. I encountered a similar mysterious case with another make, of modest performance, car not being very hard driven and with nothing noticed by the driver.
 
I can understand weak mixtures causing trouble on a race car flat out and reliant on rich charge for cooling but not sure of the mechanism at moderate output. There is nothing more weak than a diesel and these survive at colossal outputs! With other makes I have driven extensive miles on somewhat  weak mixtures to the detriment of valves, seats,  but not pistons. Perhaps the slow burning causes a burning mixture to be compressed? I have had pistons on others crack across the crown such that blow through was notable, but no melting of piston.
 
For those that know far less than Alan the detonation failure sequence may be of interest. It is reckoned that normally an insulating layer a few molecules thick survives through each explosion .(Despite the colossal flame temp the film of oil on upper cylinder survives and often oil use is nil). But the shock waves from detonation (pinging) strip the insulating  layer away and a damaging amount of heat is conducted to the piston crown and elsewhere.  Once a leak path occurs the flame ripping through makes matters worse.
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#23
A B5hs plug (equivalent to Champion L86C) is 'correct' in as far as it ought to go alright on most Austin Sevens.

'Correct' plug heat range though is determined by a combination of factors which include (you guessed it) compression ratio, ignition advance and mixture strength. In an engine running high compression, excessive ignition advance and/ or weak mixture a colder plug may be needed to protect against detonation (simply put, the 'standard' plug may overheat and ignite the mixture before the plug sparks).

An exhaust valve stuck open (even poorly adjusted tappets can cause this) will sit in the gas flow and overheat, which likewise can pre-ignite the mixture.
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#24
A most unlikely theory on the subject, but you never know stranger things have happened, having read this post it occured to me, that with a cast iron manifold and aluminium pistons you have the recipe for thermite. If a bit of rust from the manifold came into contact with a bit of aluminium oxide on the piston top, this would need a temperature of some 1650 C to ignite it, that is only a fraction of the 2500 C in a combustion chamber and it would produce a reaction of 4000 c that would easily melt a hole in a piston.
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#25
Yes but aluminium melts at 660degC
Alan Fairless
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#26
(01-10-2018, 02:59 AM)Colin Cheesman Wrote: I have had a closer look, the piston appears like it has had some extreme heat and disintegrated, not sure how this has happened as the other three are in good condition. The melted piston has left some aluminium on the bore, i am hoping it will clean up. We will pull the engine out and dismantle later this week, plan is new piston and rings all round with a good hone.  Would like to know everyone’s thoughts as to what may have caused this to happen.
We do not have much history on the car, it has sat unused for around 10 years or even more. We have only had it running for about two hours.

Colin,

You say the car was unused for 10 years - was it stripped and checked before running ? There appears to be rusty water on the block around the valves. 
Personally I would never run an 'old' engine without a thorough check. Stuck valves would be a good place to start.

We did stock the black/copper gaskets a few years ago- all sold now. Out of interest we have had a problem with some New Zealand supplied gaskets.

Was it Number four ?

Cheers, Tony.
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#27
What issues did you experience Tony?
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#28
Yes Tony, number 4, hindsight is a great thing.

               

Hi all, update on engine, please see photos. The back of the block was full of debris, guessing due to sitting dry for so long and the rust and other debris flaking off and all going to the back of the block, causing the overheating on number 4. Would like people’s thoughts on this.
The number 4 big end has heat damage and will need white metaling. 
A complete rebuild is about to start. I think a good engine flush may have stopped this happening?

Colin
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#29
The plot thickens. Still difficult to know what started generation of all the heat in the first place. Possibly a part seizure which has persisted long enough generate a lot of heat and  to overload the big end.  I wonder if the engine had been recently rebuilt tight or piston walls built out with corrosion? Is the piston the less foregiving solid skirt type? 
Whether the big end or piston seized first, the conrod has had a severe working.
Not relevant, but are the top rings different thickness'?
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#30
Without seeing how blocked the jacket was it's hard to know if it may have been cause or simply a contributory factor. I'd still check all the above. No surprise the big end has gone trying to heave that piston up and down the bore...
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