replacing only one piston

[Ebo100]

My sons Mini Cooper S has dropped a piston, we are unsure of the cause but suspect is has been over revved (thrashed) by the previous owner. We have stripped it down to a bare block and we are satisfied that the bores are ok. The cost of a set of new pistons is nearly £700 which coupled with all the other components we need to renew takes the bill to over £1100 and will cripple my son.

I know this is not good practice but we are wondering if we could get a way with changing only one piston with a full set of piston rings for the other pots. I plan to take the other pistons to work and have them dye penetrant crack detected.

Without knowing the exact root cause does anyone have any thougths?

[Boosted LS1]

Why would you change the rings on the other pots? They're already bedded in.

As for replacing one piston, if it weighs the same as the others and is dimensionaly the same then there shouldn't be a problem. If the bore needs a hone you'll need a fresh ring pack.

[227bhp]

Wtf is 'dropped a piston'?

[stevieturbo]

Ebo100 said:

My sons Mini Cooper S has dropped a piston, we are unsure of the cause but suspect is has been over revved (thrashed) by the previous owner. We have stripped it down to a bare block and we are satisfied that the bores are ok. The cost of a set of new pistons is nearly £700 which coupled with all the other components we need to renew takes the bill to over £1100 and will cripple my son.

I know this is not good practice but we are wondering if we could get a way with changing only one piston with a full set of piston rings for the other pots. I plan to take the other pistons to work and have them dye penetrant crack detected.

Without knowing the exact root cause does anyone have any thougths?

Explain the damage better and history prior to the damage....ie how did he do it ?

Are we talking the hairdresser mini's, or a proper classic mini ?

And there is absolutely nothing wrong with changing a single piston if that's all the engine needs.

[Boosted LS1]

227bhp said:

Wtf is 'dropped a piston'?

I suspect the OP means an exhaust valve :-) could be an inlet though.

[Ebo100]

Boosted LS1 said:

I suspect the OP means an exhaust valve :-) could be an inlet though.

No I really mean a piston. Its a 2007 JCW cooper S that son bought with the head off so we don't know what has caused the damage. It was stripped down because of a misfire, the compression test showed lack of compression in No1 cylinder. We have stripped back to a bare block and found No1 piston has a piece missing around the gudgon pin and ring landings and the gauze in the oil pick up pipe holding the remanents of the rings and a piece of the timing chain guide. The head was sent off by the previous owner for a rebuild and has had new valve seats lapped in and was then pressure tested so I have no concerns about the head.

Thanks for the advice; my experience goes back to the A series and Ford X flow where a set of pistons was fairly cheap and so usually changed as a set. The cost of the Cooper pistons is just less than £700 so we don't want to change the lot if we don't need to as that is a months wage to the lad.

[Ebo100]

Boosted LS1 said:

Why would you change the rings on the other pots? They're already bedded in.

As for replacing one piston, if it weighs the same as the others and is dimensionaly the same then there shouldn't be a problem. If the bore needs a hone you'll need a fresh ring pack.

The plan is to hone the block, the bores look in good condition with no visible signs of scoring but while it is out and we have free access to a machine shop we thought it couldn't hurt to clean it up a bit and put a set of rings in for £90. How it has managed to lose part of the piston and half the rings without damaging the bore I don't know but it could have been nasty.

[stevieturbo]

Ebo100 said:

The plan is to hone the block, the bores look in good condition with no visible signs of scoring but while it is out and we have free access to a machine shop we thought it couldn't hurt to clean it up a bit and put a set of rings in for £90. How it has managed to lose part of the piston and half the rings without damaging the bore I don't know but it could have been nasty.

If the other bores are good, I'd sooner leave the factory hone and rings in there.

But you'd need to thoroughly inspect the other pistons, if one has suffered damage like that, good chance there others have damage too. Make sure they are the correct shape and not cracked anywhere.

When back together, it will need checked by a competent person to ensure it is running correctly so it doesnt blow up again.
Really...you could be into a lot more than £700 even after the mechanical parts are rebuilt

[Ebo100]

stevieturbo said:

If the other bores are good, I'd sooner leave the factory hone and rings in there.

But you'd need to thoroughly inspect the other pistons, if one has suffered damage like that, good chance there others have damage too. Make sure they are the correct shape and not cracked anywhere.

When back together, it will need checked by a competent person to ensure it is running correctly so it doesnt blow up again.
Really...you could be into a lot more than £700 even after the mechanical parts are rebuilt

All good points; which is why I started the thread as I was not sure if it was false economy, the remaining pistons will be coming into work for crack detection for piece of mind and the engineer in the machine shop has checked everything with internal & external micrometers.

How could I check what boost pressure it is running without doing a full throttle rolling road session? The car is fitted with a forge dump valve which suggests that it has had the boost/wastegate played around with.
What is the likely cause of the damaged piston, excess boost, over revving or maybe lean mixture?

[DVandrews]

I would leave the other three bores and Pistons as they are if they don't exhibit any problems.

Glaze bust the suspect bore with a flex home or similar and fit with a new Piston and rings. If you are using a secondhand piston then ensure that the skirt is lightly conditioned on the thrust faces with P2000 or a fine scotchbrite and then thoroughly cleaned.

Dave

[Ebo100]

DVandrews said:

I would leave the other three bores and Pistons as they are if they don't exhibit any problems.

Glaze bust the suspect bore with a flex home or similar and fit with a new Piston and rings. If you are using a secondhand piston then ensure that the skirt is lightly conditioned on the thrust faces with P2000 or a fine scotchbrite and then thoroughly cleaned.

Dave

Thanks for that; but we have ordered a new piston from BMW/ Mini. I take it this will be ok to use straight off without conditioning?

[paulisa]

Ei .

This is the damaged piston as can be sean the top land is perfect and piston looked like all the rest from the top

[stevieturbo]

It just shouts detonation.

It could be many things, bad tuning, a fault, crap fuel, other problems

Hence you will need to get it to a competent person to inspect, diagnose and repair where necessary once it is running again so it doesnt happen again

[paulisa]

No marks or signs of detonation on any of the piston crowns i just spoke with someone who knows the last owner
He told me he used to rag it about and liked revs so it could just be over reved

[Pumaracing]

Finally, some hard core internal engine stuff that's in my wheelhouse.

Looking at the damage this appears to be a stress fracture propagating downwards from the top ring area and breaking the lands in their weakest position over the gudgeon pin end. The most likely cause is detonation. Another possibility is over heating causing the top ring gap to close up although the latter would normally also result in bore and piston scoring.

Google suggests that broken pistons on this engine are not unknown. The design appears to be rather weakly supported in the fracture area and could be unduly susceptible to detonation fracture which a stronger piston would shrug off.

I would examine the cylinder head and piston crowns very carefully for signs of detonation damage.

If everything else is ok then one new piston and a light bore honing should be sufficient. I'd put the boost pressure back to std though if it's been tinkered with.

[Pumaracing]

paulisa said:

No marks or signs of detonation on any of the piston crowns i just spoke with someone who knows the last owner
He told me he used to rag it about and liked revs so it could just be over reved

That's nonsense basically. If an engine can't withstand full throttle and maximum rpm without falling apart unduly rapidly it's the fault of the engine designer not the person using it. Boost modifications that exceed the component design parameters are another thing altogether of course.

[PeterBurgess]

The pic isn't brill but the fracture points of the ring lands look like text book inverted 'V' showing the force has been delivered from above the piston. Most likely causes are; too much ignition advance, detonation, pre ignition or too high a CR.

Peter

[Ebo100]

Thanks for the replies everyone. I've spoken with a rolling road place this morning and I think when Paul and I put the engine back together it can go and have a mild RR session and check for boost and ignition problems. Incidently they suggested having the injectors checked which could be giving a weak mixture which I hadn't thought about.

we'll follow the advice and go with the single piston replacement and see what the RR shows.

[Boosted LS1]

To me the piston smacks of excessive cylinder pressures. The cause could be any of the ones previously mentioned but it looks like a cast piston so it's strength may have been marginal in the first place. I wouldn't run a lot of boost on those, maybe 5 psi or so.

[PeterBurgess]

That's what it comes down to Mike, too much pressure/speed of pressure rise from above the piston from whatever reason.

Peter