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Be cautious if you do need to fit a shim Colin, some of the ones I have encountered have not been cut as accurately as they could have been, as a result the pistons make contact at the bottom of the stroke. Easily fixed, but if not noticed on assembly could mean another rebuild!
Black Art Enthusiast
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If any piston is going to overheat then it's going to be no. 4, does seem a bit odd though that the others show no sign of deterioration.
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01-10-2018, 10:15 AM
(This post was last modified: 01-10-2018, 10:22 AM by Ian McGowan.)
A possibility that occurs to me (because I had personal experience of this some years ago) is that a little end pinch bolt may have come loose, or broken in thread and come out smashing the front of the piston and everything else followed from that. Just a thought.
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I think you can discount my theory Colin, I took and old piston today and skimmed a ridiculous 80 tho off it, I then hit it with a ball pein hammer in the position yours was holed, it took quite some force to break through. The area of the hole would be the thinnest part of most pistons I have in my scrap box, (I have quite literally dozens!) and after machining 80 tho off the crown was still 100 tho thick. I am now feeling that Ian McGowan may be on the money, will be most interesting to learn what the actual cause turn's out to be.
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02-10-2018, 08:10 AM
(This post was last modified: 02-10-2018, 08:12 AM by Ruairidh Dunford.)
Do I spot one of the Australian head gaskets - one side coated in a black sticky compound, the other copper? I found these to be extremely durable but they stopped selling them in this country as many wouldn't use them due to the 'differing from original' design. Are they still available in Oz?
They look like JP pistons as well...
The only time I saw a piston melt was on Frank McDonald's Speedy engined AEW - it holed near Inverness (I think) on a very fast JOGLE attempt. The car had extraordinarily high compression.
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It looks to me as if that piston has melted rather than broken. So, there are 3 or 4 possibilities. Either the mixture is very weak - so look for a sticky valve or an obstruction to the airflow, or maybe a head or manifold gasket which has an airleak. Or maybe an inappropriate spark plug or the ignition is way over advanced- and as it’s one cylinder I’d be thinking about a bent or worn distributor.
Alan Fairless
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My monies on 10 years of corrosion and a bit of seizing resulting in fatigue failure. The top looks corroded.
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02-10-2018, 12:16 PM
(This post was last modified: 02-10-2018, 12:17 PM by Chris KC.)
(02-10-2018, 09:34 AM)Alan Wrote: It looks to me as if that piston has melted rather than broken. So, there are 3 or 4 possibilities. Either the mixture is very weak - so look for a sticky valve or an obstruction to the airflow, or maybe a head or manifold gasket which has an airleak. Or maybe an inappropriate spark plug or the ignition is way over advanced- and as it’s one cylinder I’d be thinking about a bent or worn distributor.
Precisely. It's hard to be 100% sure looking at a photo but a melted crown is caused by pre-ignition.
Mixture too weak, ign. too far advanced, too much compression, wrong spark plug range, sticking valve (I'd look for exhaust not closing properly), obstructed cooling / airflow issue, gasket leak, fuel octane too low (contaminated with diesel, perhaps).
I'm told excessive oil in the combustion chamber can also cause pre-ignition - so failed or badly worn piston rings too. Though I'm guessing they are pretty knackered now either way.
I like the theory of bent / worn distributor but if so can't understand why mine is still running!
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The white flecks on top of the piston aren’t corrosion, they are aluminium spatter from the burning piston crown. It is absolutely typical of a failure of this kind.
Alan Fairless
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Thanks all for your replies and suggestions, the engine should be pulled down by end of week, I will take some more pictures and post.
The spark plugs were correct and all the same, B5hs, I don’t believe the head gasket was the problem, it looked good. The exhaust valve appeared to have something under it, I believed it to be aluminium from the piston, could have been something else. Not sure about the manifold gasket, will need to check this. I will check the distributor for wear and bent shaft, great tip.
When the engine was running it was very smooth, even at 45mph, no noise, also idled quietly and smoothly.
Thanks again and I will let you know when the engine is apart.
Colin
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