Piston/Ring Failure
Turbo Mini Forums

I am curious as to how this may have happened :( so any suggestoins from people who know about this sort of thing will be appreaciated.

Right, the cracked ring (1st ring) was on piston number 4... pistons 3 and 2 were fine, no bits missing from the pistons and no cracks in the rings. however piston 1 also had a cracked 1st ring and a rather large chunk of material dropped off when the piston came out! (see pics) Also on the one with the bit of the piston that fell off, the con rod was stiff and wouldnt swing more than 40 degrees and would seize etc? (too hot?)

When the pistons were in the engine they did seem a very lose fit in the bores which made me think that the bores are too big for the pistons? but ive just measured it and theyre 73mm (1360 evolution pistons from minispares). i havent measured the pistons yet (no vernier or micrometer, theyre at work)...

The bores of the engine themselves are fine, no dodgy markings or scratches!

Anyways im just curious as to how this may have happened so that i can try and make sure it cant happen on my new engine with more expensive/better parts!

P.S. the only reason i took the car off the road is cause it spat a load of oil out from somewhere whilst i was coming off the motorway and it all landed on the turbo manifold and vapourised instantly creating a massive cloud! lol then i drove it home and here it is! i think it may have come from the filler cap area...

what boost ignition timing, fuel an cr were you running?

first thing i'd say is its due to det, but the piston dosen't look eroded above the top ring.

piston clearance wise, you can end up with a lot of gap at the top of the piston, the clearance is measured down at the bottom of the piston under the gudgeon pin, you will find the croun will measure a lot less than 73mm.

I think its worth pulling off the intact rings putting them in the bore and measuring the end gap, if its small (sub 12thou) it suggests that the rings may have broken due to expansion.

Pistons 1 and 4 run hotter, especially 4.

So, as Joe above, I would suggest measuring the ring gaps from what you have left.

But was there any other signs (ie, audible noise) of detonation ???:

Schrödinger's cat - so which one am I ???

When mine blew up due to det.there was no audible warning and no obvious top land erosion.Just big cracks in the second land.

'Where does the engine go?'

As others said. Detonation is one suspect, but more likely ring end gaps far too tight.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Ive not had any det problems with this engine build, i was running 6psi of boost for 4,000miles then 9psi after that ( was tuned in between)...

Cr was about 8.8:1 then went upto 9.2:1 when i got a new cylinderhead after i blew a hole in the first one...

the new cylinderhead has been on for about 3 months...

the timing was spot on (tuner said) even under boost it was producing the correct amount of advance. and i couldnt hear any det sounds..

AND the gap in the rings is about 2mm not 12thou.... :(

PLUS the rings were slightly bent and they moulded them selves into the piston and with piston 1 it sort of pushed the lip at the edge where the ring sits up/out and created a burr??? lol hows it done that?

the engine (in total) has done 6,000miles from ALL new parts...

Possibly bore wash, removing oil from the ring land and the ring then wore it down, eventually cracking, have seen a similarly shattered top ring/sloppy groove in one of my 1275's .. but it had done over 100,000 miles and i know for a fact it was too rich as it was one of the first cars i "tuned" haha.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

yeah i think that might be the more likely culprit? hmm ive had nothing but disaster with this engine, the first one had a cintaminant in the oil which wore out everything... (all new parts) so had to rebuy $$$... now this! only 6k miles and over 4k spent bah!

2mm ring gap? How on earth did you manage that?

25 thou ! That's far to excessive.

WWW.TURBO-MINI.COM

73mm piston in a 73.5mm bore?

it seems to be a 73mm bore, i havent properly measured it yet, ive only used a steel ruler i will get my vernier on monday and check it again. and at the time i diddnt know what the ring gap should have been but that mistake wont be made this time :)

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

not sure what it actually was, i remember him saying it was what it should be, which all i can think of is what the standard metro turbo was set at

Th problem with high load high speed det is that you cannot hear it without some sort of audio device.

yeah, i do have straight cut drop gears so i probably wouldnt be able to hear it :( although the pistons are in very good condition! theres no pitting? maybe i should send you one so you can have a look at them?

Not sure why straight cut gears have anything to do with it? and no thanks.

http://www.mlcmotorfactors.co.uk/Trouble%2...rs/PistonTT.pdf

drop gears are loud? so will make it harder to hear det?

it would seem that page 9 is what problem i have, high CR and possibly over advanced ignition, not sure about det but that seems to be the cause i would suspect, thanks for the link :)

We've already stated the cause. It is either ring gaps or detonation. There were no if's or buts.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will