Is this what pistons hitting the head looks like?

tetlow · Post by **tetlow** » Tue Jul 10, 2007 8:13 pm

I have just stripped my motor and found this.

http://s138.photobucket.com/albums/q279 ... ine012.jpg

There are no marks anywhere on the bores, chambers and top of piston except the flat top piston edges and here on the heads.
Each cylinder is roughly the same. Some of the cuts are deep, .5mm or more. The top outer of each piston and bottom area around the outside of the chambers are pitted to hell.

I have 9.75 pistons and had the heads skimmed before the last rebuild

Do you knowledgeable gents agree with my diagnosis?
Will just fitting a composite gasket solve the problem?
Regards
Dave

seight · Post by **seight** » Tue Jul 10, 2007 8:43 pm

I don't think the pistons stick out of the bores at tdc do they? So there's no way they can hit the heads unless you have had the block rather than the heads skimmed - I think.
Perhaps you have something in the inlet that has come apart, hence the pattern on all the pistons/cylinders?
Mike

Post by **Coops** » Tue Jul 10, 2007 8:47 pm

that is due to foriegn bodies in the cylinders,
i had something similar on my old heads which was due to a piston ring breaking up,.
was the marks there prior to skimming?

tetlow · Post by **tetlow** » Tue Jul 10, 2007 8:49 pm

The odd thing is there are no marks anywhere else in the chambers or on the top of the pistons.
If something was loose in the cylinders then I would assume the pistons would be marked all over their tops.
Some of thes cuts are 5 mm long and nearly 1mm deep!!!

tetlow · Post by **tetlow** » Tue Jul 10, 2007 8:52 pm

The heads were like new when fitted.

tetlow · Post by **tetlow** » Tue Jul 10, 2007 8:55 pm

One odd thing though, the rear 4 pots had both inlet and exhaust valve bottoms black, the front 4 had the exhaust valves pink.
I assumed this was because I have not head the Webber set up yet.

bill shurvinton · Post by **bill shurvinton** » Tue Jul 10, 2007 9:23 pm

There has been some form of contact. Either something is very wrong with the engine Geometry so the piston is hitting the head or something has gotten in there that shouldn't. As it is all 8 pots then I would first investigate geometry. Crank engine to TDC and check that

1. the pistons are not proud of the deck (rod length)
2. There is minimal change of piston slap (pistons too small).

If neither of those then we are looking possibly at detonation damage. Now I am not an expert in things that cause detonation, but I know that squish can be critical and this is around the squish region. Clutching at straws but unless pistons are hitting the head then I cannot think of anything else.

tetlow · Post by **tetlow** » Tue Jul 10, 2007 9:37 pm

The engine is mechanically Ok I think.
Originally I had leaking head gaskets so removed both heads and had them skimmed, no marking then.
It seems odd that the only marking is the top outer of the pistons and corresponding part on the heads, of course Bill's idea of detonation would be in this highest pressure region I assume.

The marks look like a nut has bounced around!! I assume the valves would be harder metal than the head but I would have thought that the chambers would be marked all over and the top of the pistons would be pitted all over too.

Could all this be due to having too much skimmed off.

The Original Tom · Post by **The Original Tom** » Tue Jul 10, 2007 10:13 pm

Booger me!

If the pistons were hitting the heads I'd expect to see a sort of shape to the denting, not just random scratches! Definitely looks like foreign-body damage, I've never seen anything like that...
BUT
The combustion chambers in the heads look fine, which would seem to indicate pistons touching.
I know you say the engine is sound mechanically, but here's a list of thing's I'd do.
1) get a straight edge (steel rule?) and hold it over the piston holes. crank the engine by hand to see if any hit.
2) if they do hit - worry. The block has either been skimmed too much or you have the wrong piston/rod combo.
3) if they don't hit, remove te pistons and rods and check for slap. Either from the main bearings being shagged or the small-ends being oval. If the gudgeon pin holes have elongated in the head then inertia could cause the pistons to move more than they should whilst flying up and down. This won't be noticeable when turning by hand because you need the speed to make them come proud of the block mating surface.
4) Check behind the valves. If some of the valve seats have broken up and dropped inside this could happen???
Can't really think of much else...

My exhaust valves were all sandy coloured and my inlets were black when I did mine, dunno why, they just were...

Post by **katanaman** » Tue Jul 10, 2007 10:36 pm

Thats had something in there possibly just large enough to only hit on the edge of the piston. Once it moved further in it wasn't large enough to make contact. You would have to skim the block 40 thou before the pistons would hit even on tin gaskets, skimming heads doesn't bring them closer. Take the valves out and see if there are any witness marks on the valve seats. Only other thing is detonation which has been mentioned but on a rover something still has to be pretty wrong somewhere to get that damage.

stevieturbo · Post by **stevieturbo** » Tue Jul 10, 2007 10:46 pm

Looks like detonation to me.

If the pistons were hitting the heads, aside from the noise, where there is contact, the head/piston would be clean, as carbon can never build up.

Foreign bodies are another matter, but the damage wouldnt usually be limited to those surfaces alone.

Detonation is a really weird thing, and how it marks things inside the engine ( assuming it doesnt actually destroy them )

HairbearTE · Post by **HairbearTE** » Tue Jul 10, 2007 11:47 pm

Very interesting picture. I would say Marki is spot on with his assumption that the damage has been caused by an object just big enough to be trapped in the quench area of the chamber. It definatly looks like physical contact as opposed to detonation. If a ring has broken then there may still be particles of it embedded in the chamber and/or the piston. Was the plug in that cylinder in good condition? A spark plug electrode might be just the kind of size we are looking for if indeed the damage has been caused by an object. Can you post a hi-res close up shot of the damage?

stevieturbo · Post by **stevieturbo** » Wed Jul 11, 2007 2:10 am

HairbearTE wrote:It definatly looks like physical contact as opposed to detonation.

I dont have any pictures handy, but detonation marks would really surprise you !!

Eliot · Post by **Eliot** » Wed Jul 11, 2007 6:18 am

Spark plug electrodes was my thought too.

tetlow · Post by **tetlow** » Wed Jul 11, 2007 6:59 am

The following gives shows a bit more. Just to show one side. Both sides the same. I will try a high res later.

Tha engine has always run noisily, quiet on tick over but I have put it down to the 4 at least exhaust gaskets blowing as the cause. I am trying to get the headders skimmed, no luck yet!

The cuts look to me as if a ring had bounced around but in all 8 cylinders the same and no marks in the piston dish!!

I wll check the piston travel but I am sure it is normal. The block as far as I know has never been skimmed.

I think Marki's theory of somthing just big enough to get trapped between the piston edge and head sounds good, but to get this in all 8 pots would mean I hand full of Sh*& thrown into the carb. I have never run without the filter.
Dave
http://i138.photobucket.com/albums/q279 ... ine013.jpg