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?

[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.

[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?

[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?

[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.

[Ebo100]

227bhp said:

I don't know why we have two offerings of pics of the same engine in this thread, is this the same person or two people?
Why was the cylinder head repaired? More to this story than initially meets the eye I think.

Edited by 227bhp on Wednesday 15th July 08:49

Let me try and fill in some of the gaps, for the purpose of this thread, consider me as the owner and Paul the engine builder (who has a better camera on his phone than me hence why I don't have any pics).

Original owner suffers misfire and coasts into a garage and asks them to check it over. The initial diagnosis for the misfire was put down to either a spark plug or valve. The head was removed for the previous owner (before my son). The bores and piston tops were examined and checked and thought to be good. This led to the thought that the head was at fault so the previous owner commissioned the head rebuild thinking the the loss of compression was a valve seat. Valves had new seats cut and lapped, head was rebuilt and tested. Previous owner then loses interest and sells the car to my son.
So we get the car thinking we had a simple job of fitting the head but check the bores with some white spirit and find piston / rings at fault leading to the strip down, subsequent findings and request for advice on here. Alot of the car is unknown and info is coming through third parties but from what has been said on this thread it looks like the boost could have been turned up and engine ragged.

[Ebo100]

PeterBurgess said:

I cannot see why over revving would do that to a piston? Overrevving usually seems to show up as shagged ring lands (worn out so too wide) and barrel pattern wear in the bores.

Nice pics of your broken lands, from your pics it still looks like an inverted V as the cracks do not appear 90 degree, but that may be the pic angle?. As I said earlier we were asked about the pics of the Mini piston and it cries out excess pressure from above. If the material is brittle it will fail very easily as shown, including the cracked gudgeon pin boss.

On a few of occasions we have seen wrecked ring lands/nipped rings when a rocker arm has broken on an ex valve and allowed excess pressure in the cylinder. On one occasion the customer had low compression on the cylinder after repairing the rocker assembly. He knew the top ring at least must have nipped, amazingly after 50 miles of use the ring must have made its own clearance again and compression restored...lucky bloke, if it was my engine it would have required a complete rebuild!

Peter

If we could only be that lucky. Does this excess pressure from the top suggest boost pressures then? If so then my plan to have it checked on the rolling road seems to be the way forward then after having the other pistons crack detected.

[Ebo100]

PeterBurgess said:

I cannot see why over revving would do that to a piston? Overrevving usually seems to show up as shagged ring lands (worn out so too wide) and barrel pattern wear in the bores.

Nice pics of your broken lands, from your pics it still looks like an inverted V as the cracks do not appear 90 degree, but that may be the pic angle?. As I said earlier we were asked about the pics of the Mini piston and it cries out excess pressure from above. If the material is brittle it will fail very easily as shown, including the cracked gudgeon pin boss.

On a few of occasions we have seen wrecked ring lands/nipped rings when a rocker arm has broken on an ex valve and allowed excess pressure in the cylinder. On one occasion the customer had low compression on the cylinder after repairing the rocker assembly. He knew the top ring at least must have nipped, amazingly after 50 miles of use the ring must have made its own clearance again and compression restored...lucky bloke, if it was my engine it would have required a complete rebuild!

Peter

If we could only be that lucky. Does this excess pressure from the top suggest boost pressures then? If so then my plan to have it checked on the rolling road seems to be the way forward then after having the other pistons crack detected.

[Ebo100]

227bhp said:

Does this engine have port or direct injection into the head?

Direct injection, it's the R56 turbo JCW

[Ebo100]

Explain what, excess pressure due to overfueling? Injector fault running lean?

[Ebo100]

GavinPearson said:

How modified from standard is the engine please?

No idea, it's JCW so running approx 210bhp but there is a forge dump valve but no other mods (standard air filter; exhaust etc)

[Ebo100]

@BoostedLS1 Sorry I don't have the piston here but I will see if I can get it back before the weekend. Total milage is 80,000

Good news tonight, I had the other pistons NDT examined at work and all are free from cracks so we are good to go. The machine shop were unable to hone the block today due to other (paying) commitments so fingers crossed it can be done tomorrow. Thanks for all the advice here nice to have an expert eye/eyes cast over it.

[Ebo100]

Boosted LS1 said:

Thanks, it'll be interesting to have a look at the other pistons, their top rings :-)

Sorry pistons back in the engine now but here is a couple of the top rings, hopefully enough detail for what you want to see.

[Ebo100]

Update time.

While I have been away this week my son and his friends have got the engine back together and back in the car, but not without issues.

The car was started up and has had a few short runs around town and then the oil has been dropped and a quality oil put back in. Thursday saw the car complete Mikes usual 70 mile round trip to work using only 2000-2500rpm. He says that on acceleration the car is pulling smoothly and feels ok, however on a constant throttle there is what sounds like a chain rattle that is rev dependant but disappears on acceleration and decceleration but returns on constant throttle. We didn't change the oil pump drive chain trying to save a bit of cash so this may have turned into a bit of false economy. Any thoughts? Looking at the haynes manual this chain can be changed with the engine in the car there is no tensioner on this chain so I don't think it is going to jump but at the next oil change we will look to change it if necessary.
The timing chain is all new with a second new tensioner so we don't think this is the problem.

Second issue is a top end knocking on start up that I think could be hydraulic tappets. The noise stays there as it warms up but does get better. I have taken a short video this morning starting the engine from cold does this diagnosis sound right or is there anything else I need to check?
https://www.youtube.com/watch?v=2P-0ClXdOho&fe...

I have found a hydraulic tappet cleaner in Halfords that I didn't even existed, has anyone used this, is it a good idea to use on a rebuilt engine? The valves were re-lapped before we got the car, would the tappets need to be removed for this operation? Will it be alright to use the car with a rumbling tappet?
The engine builder has suggested trying Wynns engine flush but reviews seem luke warm as whether this is a good product to use.

Any advice greatfully received

Edited by Ebo100 on Sunday 26th July 14:29

[Ebo100]

Pumaracing said:

Let's not forget that this car was bought as a non-runner with the engine already partially stripped. There may be a defunct lifter which the engine builder had no way of finding without replacing all of them.

Thanks for that.

Does the sound of the video point to a lifter then, or could this be something more serious, timing; bent con rod; bottom end?