replacing only one piston

[Boosted LS1]

I find these things interesting as well. Healthy debate can be a really useful tool. Some people refer to 'hidden det' but that's usually when they can't prove actual detonation existed. Often, the only evidence then will be pressure related damage which may/may not have been detonation related. I once had a turbo piston that had pretty interesting caterpillar marks in the carbon. Some very odd combustion going on in there and it was probably on the verge of detonation.

[Boosted LS1]

227bhp said:

So i'm afraid the obvious question is: What causes the piston skirt to flex so much it eventually breaks?

Worn or overly large bores, tall weak skirts, a slotted oil ring land which leads to a weak skirt, a non fully skirted piston, poor rod angle :-) Galactic mileages etc. Some things are wear related, some are poor design/materials and some things are driven by costs at the design stage.

[Pumaracing]

Boosted LS1 said:

Me, I think it's a combination of either eutectic or hypereutectic pistons being unable to deal with a high pressure event in the combustion chamber but I'm open to other possibilities, always.

The pics make the cause very clear. It's a very poor piston design with the ring lands completely unsupported from below along a considerable length in the gudgeon pin area. Even worse you can see that the land area is actually undercut so part of the bottom land is sort of hanging out there in fresh air with no connection to the pin bosses either below or behind it.

Even with normal combustion it's a marginal design and with prolonged high output use or trace det it's an accident waiting to happen.

[PeterBurgess]

You are quite right, a most pleasant thread.

I mentioned earlier we have seen the cracks in the skirt from the gudgeon pin boss when pistons are too loose in an engine(bore too big and or worn out pistons), coupled with thrash (your concept of over revving) and even poor ignition ( maybe too much advance but not detonating) pressure load rocks piston and flexes skirt till it starts a brittle failure as Dave said.

Peter

[Pumaracing]

227bhp said:

I would appreciate some confirmation from the OP as to whether those are cracks around the pin, the one on the right looks very much like one to me, but maybe it isn't.

The one on the right is a screwdriver gouge. Possibly from someone trying to lever the gudgeon pin circlip in. The one on the left is a little casting imperfection from a mark in the casting mould.

Before answering this on page 1 I downloaded the pic into Paint Shop Pro. Rotated it 90 degrees anticlockwise so I could see it in the correct orientation and enlarged it to examine all the damage carefully.

[paulisa]

Close up of grudge on pin area the crack just looks like a casting mark either way it's knackered

[Pumaracing]

paulisa said:

As you can see from this pic the 4th land (below the oil control ring) and 3rd land (above the oil control ring) are completely unsupported either from behind or below. The piston is hollow behind them. The 2nd land is part of the piston crown behind it but also unsupported below so it will be unduly flexible. This means that any abnormal combustion pressure on the top ring, or top land, has very little below it to resist breakage. Quite bizarre.

[Pumaracing]

227bhp said:

Thanks for taking time to post, now I understand PBs earlier 'inverted Vs' - I was taking that to mean the edges of the whole group of damaged 'lands being in a V formation, not just the angle on each individual one so now it makes sense.

On the piston in question here the lands become progressively less supported as we go down from the crown so the damage propagates out into wider and wider breakages and the edges of the breaks form a sort of continuous V. The lands break away back to their points of full support further round the skirt.

This will not necessarily be the case on other designs. Normally the deflection on the top land concentrates the force centrally onto the second land and so on. The second and subsequent lands may then break over a shorter length than the one above them depending on where they are properly supported but the key thing is each land will break with an edge angle that is wider at the bottom than top if the force is coming from above.

[stevieturbo]

Surprised no-one has mentioned that top ring.

Surface coating coming off....it's been detonating it's tits off.

[Pumaracing]

Boosted LS1 said:

Healthy debate can be a really useful tool.

Hmmm. What a curious concept. I'll have to give it more thought. I've always found it much simpler to rely on being infallible and then getting extremely tetchy with anyone who disagrees with me.

[Pumaracing]

stevieturbo said:

Surprised no-one has mentioned that top ring.

Surface coating coming off....it's been detonating it is tits off.

Those markings are far too regular in shape and also not extending to the top surface of the ring to be any evidence of detonation. They point to some defect in the ring manufacturing process.

[Boosted LS1]

Those rings have had a moly coating inlay or something similar on it. Thats why the black blocks look squared off. bits of debri may have worn off the inlay by scoring upwards/downwards in vertical sections.

Above the top ring, the piston looks fairly decent to me albeit covered in oil and carbon deposits. If there had been detonation on that ring I'm fairly certain the evidence would be visible in the crevice area of the piston.

Edited by Boosted LS1 on Thursday 16th July 17:18

[Pumaracing]

Boosted LS1 said:

Those rings have had a moly coating inlay or something similar on it. Thats why the black blocks look squared off. bits of debri may have worn off the inlay by scoring upwards/downwards in vertical sections.

They're moly filled grooves and the top and bottom of the grooves are clearly marked by the black bands. Some minor chatter in the grinding process has probably led to high and low spots in the grooves and we're seeing the low spots as black with some moly still left in them and the high spots as shiny where it's all worn away. Nothing to worry about really.

[Boosted LS1]

Pumaracing said:

They're moly filled grooves and the top and bottom of the grooves are clearly marked by the black bands. Some minor chatter in the grinding process has probably led to high and low spots in the grooves and we're seeing the low spots as black with some moly still left in them and the high spots as shiny where it's all worn away. Nothing to worry about really.

Which would indicate piss poor manufacturing or the top ring's not fully bedded in? I thought the moly was sprayed into the ring? Is there a grinding process after this? The shiny part's where the ring's fully bedded in and the dark areas have still to be bedded in. Then there will be a continuous shiney band around the ring circumference.

OP, what is the mileage of this engine and what are the 3 other top rings like? Thanks.

[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.

[Boosted LS1]

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

[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.

[Boosted LS1]

Thanks. That image on my pc isn't the best but they look nicely bedded in.

[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

[Pumaracing]

"Gratefully". Anyway.

Any engine builder who suggests that to get it running properly you need engine flush in a freshly rebuilt and supposedly professionally checked engine needs replacing before any of the engine components do.

That noise sounds fairly catastrophic. Microphones tend to distort what we hear directly so maybe it's harsher on the video than to the human ear but there's clearly either a non working lifter or some other valve train wear issue. Maybe a blockage is preventing oil reaching the lifters.