S3 2.25 petrol BANG! Help/opinions/yada please?

steve2286w · Aug 1, 2023

Still WOW!

, Did you find the section of missing top ring. Sump must have had a few bits in. It might be chicken and egg what went first , when I stripped mine the top ring fell out in two halves. The weakest part of the piston skirt is in the oil control groove with those oil holes every half inch or so, and it’s cracked along the centre line of a few of them I’m thinking a crack propagates from one hole to the next until forces too big and it all parts in the Big Bang.
I would carry on and dismantle and clean and measure as you go feel if any wear ridge at top , has it been rebored before or standard pistons

Tall Ratbag · Aug 2, 2023

The crown detaching from the top of series petrol engine pistons is not unknown - I've seen one before where the crown just stuck in its TDC position while the lower part of the piston went up and down as per normal as the engine cranked over - that cylinder just gave absolutely no compression pressure.

I think that it might be a fatigue failure, with a crack initiating at the corner of the ring groove, and growing until the crown goes its own separate. Do aftermarket pistons have a very sharp corner here, with no radius?

Castor · Aug 2, 2023

marjon said:
So how confident the others are not the same?

Full set for me TBH.

Your in it now far enough to make it easy.

J

I shall do that I think.
I'm assuming the big-end bolts are reusable..... ?

Castor · Aug 2, 2023

steve2286w said:
Still WOW! , Did you find the section of missing top ring. Sump must have had a few bits in. It might be chicken and egg what went first , when I stripped mine the top ring fell out in two halves. The weakest part of the piston skirt is in the oil control groove with those oil holes every half inch or so, and it’s craven along the centre line of a few of them I’m thinking a crack propagates from one hole to the next until forces too big and it all parts in the Big Bang.
I would carry on and dismantle and clean and measure as you go feel if any wear ridge at top , has it been rebored before or standard pistons

It's +40 on first quick measurement. No real wear, can't catch anything with a fingernail....
Its an MoD engine, so there won't have been any cost cutting with the bits. It's obvious that one quadrant had failed long before the rest though.....

Castor · Aug 2, 2023

Tall Ratbag said:
The crown detaching from the top of series petrol engine pistons is not unknown - I've seen one before where the crown just stuck in its TDC position while the lower part of the piston went up and down as per normal as the engine cranked over - that cylinder just gave absolutely no compression pressure.

I think that it might be a fatigue failure, with a crack initiating at the corner of the ring groove, and growing until the crown goes its own separate. Do aftermarket pistons have a very sharp corner here, with no radius?

This looks just like that. It's an MoD recon, so I guess they used good parts? But yes, good question, was there a failure from the ring groove root?

Castor · Aug 2, 2023

kermit_rr said:
Blimey. Has it overheated or had an oil starvation problem?
Hypothesis.. running too rich for too long.. piston ring seizes from deposits.. ring expands, can't move in the way it should and shatters the piston.

??

Nope. None of those, so far as I can see. Weird!

steve2286w · Aug 2, 2023

Maybe get away with just one new piston, and new big end nuts but check what the bolts look like
Turner engineering good for parts and also have some info on the petrol rebuild amongst others
really useful

News

Turner Engineering - Specialises in Land Rover Engine Parts & ✔ Shop for Genuine Quality Parts online & Enjoy Fast, Worldwide Delivery!

www.turnerengineering.co.uk

Castor · Aug 2, 2023

steve2286w said:
Maybe get away with just one new piston, and new big end nuts but check what the bolts look like
Turner engineering good for parts and also have some info on the petrol rebuild amongst others
really useful

News

Turner Engineering - Specialises in Land Rover Engine Parts & ✔ Shop for Genuine Quality Parts online & Enjoy Fast, Worldwide Delivery!

www.turnerengineering.co.uk

Thanks for that. I'm debating whether to replace all four...... dunno yet.

kermit_rr · Aug 2, 2023

Castor said:
Thanks for that. I'm debating whether to replace all four...... dunno yet.

I suppose the dilemma is.. how tired are the originals.. how good are the new ones? Often new ones can be a lesser quality, but that can be offset when originals are very old

steve2286w · Aug 2, 2023

Turners do a set for £120 and that’s good value as used in their rebuilds but bores need to be good
best strip check and record all measurements and condition of all the moving bits then decide
if bores u/s cant Recall if they do a +60

edit: +60:available too

Castor · Aug 3, 2023

steve2286w said:
Turners do a set for £120 and that’s good value as used in their rebuilds but bores need to be good
best strip check and record all measurements and condition of all the moving bits then decide
if bores u/s cant Recall if they do a +60

edit: +60:available too

All bores +040. All look good, but subject to another inspection with better light. I can't feel anything.....
I'm still dithering!

Castor · Aug 3, 2023

kermit_rr said:
I suppose the dilemma is.. how tired are the originals.. how good are the new ones? Often new ones can be a lesser quality, but that can be offset when originals are very old

Indeed. Old ones look perfect. This engine hasn't done many miles.

kermit_rr · Aug 3, 2023

Don't forget if you put new pistons and/or rings in then you'll need to re-hone the bores

Castor · Aug 21, 2023

Castor said:
Findings: No 3 cylinder. 2 very bent pushrods. Head looks superficially OK but I haven't dismantled yet.
And this. WTAF could cause *this*??

UPDATE:
After a short hiatus, I got the remainder of the dead piston out..... Looks like the initial failure has been there some time and is between the two oil holes at the bottom of the oil control ring groove. I speculate that there may have been a "cold shut" in the casting, or a sharp corner in the groove as a stress raiser. The bore seems OK, but at the top of the bore is +8 thou over what I'd expect for the +040 rebore it clearly had not too many miles ago.

Can anyone tell me if the gudgeon pins needs heat for installation? Some do, some don't.

steve2286w · Aug 21, 2023

The pins should just press in with firm thumb pressure lightly oiled, I just had mine rebored I could feel a ridge at top with fingernail and it was only about .005” from memory so went for a +20, as got pistons cheap off eBay , but +60 available if you go that route

Castor · Aug 21, 2023

steve2286w said:
The pins should just press in with firm thumb pressure lightly oiled, I just had mine rebored I could feel a ridge at top with fingernail and it was only about .005” from memory so went for a +20, as got pistons cheap off eBay , but +60 available if you go that route

Thanks. I have 4 new piston +40 assemblies from Paddock at £15 each, which is hard to argue with!

steve2286w · Aug 21, 2023

Yes that’s a good price but at .008” wear youre at the limit and one thou more the manual recommends a rebore , but it also says if at max rebore 40 thou liners should be fitted, but +60 available so how does that work

Castor · Aug 21, 2023

It's a very low miles per year utility vehicle that we use for beekeeping - if it was being used for anything more onerous, I'd go for sleeving it back to standard.

S3 2.25 petrol BANG! Help/opinions/yada please?

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