Piston broke( often spelled differently)

[LI-599]

I have a request for a .040" o/s piston for my '69 S4 Elan as a piece of one of mine has mysteriously vanished from near the exhaust valve indent( no sign of heat and no burnt valve).
The car has done less then 5000mls since total rebuild and has never been driven over 5000rpm and there are no other apparent faults so I am supposing it must have been a faulty bit of casting.
Yours in anticipation, Ian

[LI-599]

Adding to the above post, we were out enjoying a run through the wonderful West Gippsland countryside, cruising nicely at 60mph when the note changed slightly as if a plug was starting to foul, there were no other signs or sounds to indicate what had happened. The piece of piston must have broken into very small pieces as the complete piece(see pic in other post) would not fit through the 1/4" opening( Sprint Cams) and there is no damage to the valve or seat. I look forward to any comments.
Ian

[john.p.clegg]

Ian

most likely,I'll look in the shed later...

John

P.S. where are you?

[englishmaninwales]

Ian
This happened to me in my previous Elan S3 s/e 30 years ago. I got away with no other damage and I just replaced the offending piston. The cause was not obvious. Check the ring gaps and clearance in ring groove.

Rohan is not too far from you in Melbourne and I expect he'll be along soon with the location of a spare.

Malcolm

[LI-599]

Many thanks for the replies so far. I now have the piston and rod out and there is not a scratch on bore or piston skirts or rings and groves. The piston is .040" o/s made by AE.
Cheers
Ian

[512BB]

Ian,

I understand that you just detected a slight difference in the exhaust note, so why did you decide to remove the head if there were no problems with the motor?

Leslie

[john.p.clegg]

OK,see you're in Australia....will leave you in nearer more capable hands..

John

[LI-599]

Sorry for not putting in more detail. On checking plugs which were all good colour, I did compression test which showed excellent high readings on 1,2,4 but zero on 3 so the rest is history.
I believe the AE pistons come from UK so if anyone has a spare .040" o/s one I would be more than happy to freight it out to Australia. Ian

[richardcox_lotus]

I guess you can argue you were lucky the debris didn't take the valves / head chamber with it.

Hope you get it sorted soon
Richard

[rgh0]

Hi Ian

Before replacing just one piston you need to check the other 3 very carefully for wear, faults and cracks. You will also need the same type piston from the same manufacturer to match the weight probably. Unless you reuse the rings from the failed pistons assuming they are OK you will need to re- hone the bore to re-seat new rings and this may make the piston clearance to great so you may need a bigger piston and may thus need to replace all pistons to be able to match the weights, It all gets more complicated and the amount of work compounds.

Personally I would just replace all pistons and rings and re- hone the block. I gave Col the details of potential piston suppliers if you get a full new set

cheers
Rohan

[billwill]

Could it be that the valve to piston clearance is inadequate and when hot or high RPM the valve kept tapping on the piston until eventually it broke from fatique?

[oldchieft]

Burton do pistons for converting 711M blocks with the original rods and crank.

These have a lot of meat on the top.

if out and out light pistons for competition are not a requirement, then it might be worth looking at them.

Jon the Chief

[john.p.clegg]

Sorry Ian,all the +40 pistons are shallow valve cutout..

John

[denicholls2]

Could it be that the valve to piston clearance is inadequate and when hot or high RPM the valve kept tapping on the piston until eventually it broke from fatique?

I would make sure the valve springs are in-spec and the cam grind didn't leave this lobe high. The relationship of problem to valve is too curious to let pass without deeper investigation. And a scenario involving minor interference would suggest the other pistons may not be far behind and at least need close inspection.

Old springs get tired. Particularly if stored for long periods in the stressed position as is common to projects.

[denicholls2]

A ridge buildup at the top of the bore could also create stress here from top ring interference. You note a recent rebuild, I presume the bores were ridge-reamed and honed at the time?