4 cyl Piston wash - anyone heard of it?

[AndrewP]

Evening everyone,

 

I've had the good fortune to come across another sprint engine for my Dolomite (as the current installed engine has spun a bearing )

I've begun a precautionary strip down to make sure its a healthy engine, and a friend, who is very knowledgeable with all thing diesel and marine, came round, took one look at the piston crowns and bore wall tops, and explained that the rings were probably seized (not rotating) and/or worn out and it was called 'piston wash' where the hot gases can escape past the rings in spots, and cause the carbon sealing ring to burn away and the crown of the piston in the area where the gas flows, to be stripped clean.

 

Below is a pic of what it looks like, but I thought Id see if the folks here concur and have heard of this before (piston wash)? and could perhaps explain how piston rings can cause this?

It seems (thanks for Google) to be more associated with over fuelling rather than compression ring issues, but not everything you read on the internet is true...

 

 

I`ll be putting a new set of rings in anyway (and honing the bores) but any thoughts?

 

Thanks

Andrew

 

 

[Peter Cobbold]

Andrew,

Not a term I've come across. Googling, it seems to apply to two-strokes. The wash occurs where the transfer ports are located, and is greatest when the mixture is lean because less fuel carbonises on the crown.

http://www.greenhulk.net/forums/showthread.php?t=10089

So it cant occur in a four-stroke.

Maybe what your are seeing is 'blow-by'. Worn rings allowing hot gas to pass and heat up the periphery of the piston ?

 

Peter

[john.r.davies]

Yes, excessive fuelling can dilute the oil that gets up onto the rings from below, leading to bore wear and poor sealing by the rings. Any petrol smell in the sump oil?

At the same time, rings normally rotate slowly, at about one thousandth of the rev rate, and in a neglected engine carbon deposits can inhibit that and seize the rings. Seems to me the two diagnoses are mutually incompatible, but either way, this is a tired, worn engine, but that is no surprise!

 

There is certainly evidence at the top of the bore for bore wear. If you can feel a ridge with fingertip or nail then just new rings may be inadequate, and a rebore and new pistons may be needed. But that cannot be a surprise in an old engine!

 

John

Edited August 14, 2016 by john.r.davies

[MadMarx]

I agree with piston wash but from water combustion.

There might been a slight water leak or crack that let intrude water to the combustion.

Water cleans the piston and make it look steam cleaned.

 

I've never heard of the other thing your friend said.

 

You see, this is a 5 year old piston with water intruding and detonation.

See the clean surface between the valves.

 

Edited August 14, 2016 by MadMarx

[stillp]

I've only heard the term in reference to excessive fuel washing the oil from the bores.

 

Pete

[Motorsport Mickey]

Yes as John says fuel wash is caused by excessive fuel in the chamber which can cause the cylinder wall oil to be washed off excessively, thereby causing the rings to "burnish" (polish) the walls and cause compression blowbye.

Excessive radial piston ring wear can also occur with this (check out the search function for this diagnostic report) and it often also causes a glazed bore.

To help you decide where you are, measure the piston diameters in 4 positions around each circumference, the piston measurement is the largest dia you can find (they are made oval), and also measure the cylinder bore diameters again in 4 positions. These should be round and the measurements will help you find if there is excessive wear between the piston and bore. Road going engines should be about 3 thou and competition engines can go up to about 5 thou (but with a reduced life for mileage). Check for a wear ridge in the cylinder bore (maybe up to about 4thou) if so it will need a "ridge dodger" top ring which has a 10thou step machined into it to prevent it hitting the cylinder ridge and breaking. If ok hone the cylinder walls and replace the piston rings.

 

Mick Richards

Edited August 14, 2016 by Motorsport Mickey

[Eddie Cairns]

Turbo diesels and both direct injection petrol and diesel engines tend to get higher fuel contamination in the sump oil.

 

If you look at oil specs most long drain service interval cars if not specing a specific oil will use an A3B4.

 

The A3 being a spec for petrol engines and the B4 needing being additives to try and cope with piston blowby on these engines. Older A3B3 does not have that extra additive.

[stillp]

Andrew, are all the cylinders the same?

 

Pete

[Nick Jones]

If you are talking about the clean half moon at the top edge of the piston (and possibly another at the bottom) I think what you are actually seeing is the "squish shadow".

 

If you look at the head you see that there are two matching half moon shapes in the combustion chamber that are level with the head surface. With the piston at TDC the clearance between them and the piston is only the thickness of the gasket plus a few thou. Their purpose is is to "squish" the mixture into the centre of the chamber at high speed in the last stages of compression before the plug fires and the high velocities that occur tend to keep the pistons cleaner at that point. From what I can see in the pic the pistons and bores look good.

 

Nick

[AndrewP]

Gents,

 

Thank you all very much for the great replies. Im very thankful there are so many knowledgeable people around to diagnose what would otherwise be a nondescript clean spot on the piston crown. Even more impressive given the distance of 14,470 kilometers away!

 

So, to answer a few points raised.

 

There is a barely detectable lip on the top of the bore that I'm pretty sure will lessen more when honed. Nothing that you can catch a thumb nail on sort of lip so Im confident that I`ll be OK with standard rings.

I`ll take the time to clean out the ring grooves correctly and gap the rings before re-inserting them equally spaced to maximise their chances of bedding in well.

 

The sprint engine is notorious for blown head gaskets due to the rubbish design that the angle the upper head studs pull the head onto the block. The picture that MadMarx has posted looks like it has produced a very similar effect, so I`ll look closer when I get the piston out. Its very possible that the old head gasket could have been letting in a small bit of water but as the engine is an 'unknown' I`ll have to wait and see. The head is nice and flat however and the old gasket didnt show any obvious signs of failing.

 

The engine is in great condition overall. Even the original vandervell bearings have plenty of life left in them.

Hopefully a hone, new front and rear seals and a check of the oil pump tolerances and it should be good to go.

 

Some more pics:-

 

All 4 pistons...think Pete asked if all the pistons had the same markings. Id say yes, within a degree or so.

 

 

The head design is as Nick has described, which has these two 'flats' covering the edges of the piston.

 

 

Interesting (to some degree) that one side is affected more than the other, but I assume this is due to the side showing the most symptoms is also the inlet side (so no hot gases etc).

 

I don't know if its sheer dumb luck, but with all my cars I have never had a major failure of any block component, so apart from the odd head rebuild and a Vauxhall 1.3 engine rebuild a long time ago, I haven't had much of a chance to get into a rebuild.

It certainly beats the months of welding/metalwork and now the scraping off of underseal off my TR6 shell...for the time being at least

 

Thanks all very much once again.

 

Cheers

 

Andrew

Edited August 15, 2016 by AndrewP

[MadMarx]

If I look on the bores I see that the head gasket contact surface is not shiny at all places.

There are darker parts of the surface.

cyl. 1 - at 6 o'clock

cyl. 2 - at 2 o'clock

cyl. 3 - at 6 o'clock

cyl. 4 - at 6 o'clock

 

This could be a sign of water seeping through.

Edited August 15, 2016 by MadMarx

[stillp]

I was wondering if water had been sitting on top of the pistons while the engine was standing.

 

Pete

[saggy]

Not heard of piston wash. Bore wash though if it has been very rich and if timing off then possibly even detonation - all very bad - you would smell petrol in the sump oil and you would get oil passing the rings and see a lot of deposits in the head and plugs - which you haven't got so I would rest easy