pistons strange fouled

[Eminent]

Finally, today i took the head from the engine.
As you can see at the picture's the surface from the pistons are partly fouled.
Most pistons are free from fouling at the side of the manifolds.
What is the reason for this? Does it say something about the big-end bearings or the smaller bearings in the piston itself?
Or is this is a totally normally for a car from '68?
The surfaces from the cilinders look good in my opinion, no signs of wear at all.
But thats is only with the naked eye ofcourse

[Benz Dr.]

The cylinders may not apperar to be worn but they also don't seem to have any cross hatch marks left.
The clean areas on the outside of the piston crowns indicates oil or blowby coming past the rings. This condition tends to wash a piston crown clean anywhere you have this leakage until it's just clean aluminium.

[Eminent]

Thanks as always Dr. Benz.
So in other words, the rings are worn and should be replaced and cylinders should be honed again.
Another thing is that i have an knocking sound....well the engine have. The bearings form the crank are renewed about a year ago, thinking that was the problem.
Can worn rings also be the cause of this sound?

[ja17]

Hello,

How was the oil consumption on this engine?  Is there a ridge at the top?,  Honing worn cylinders, even with new rings will not help.

As far as the noise, how was the oil pressure?  Remove the oil filter and tear it open to check to see if copper engine bearing material is collecting in the folds of the oil filter.

Precise measurements are most often needed to determine the extent of engine wear.

[Eminent]

Thanks Joe. In fact it doesn't matter how to solve this problem. I bought an overhauled short block from Cees last summer (a block you also have seen, he told me).
It was just curiosity what could be wrong with this engine and what should be done to solve this problem.

To give an answer at your questions. There is no ridge at the top of the cylinders. The oil consumption was not excessive, i never measure it. I don't drive very much with this car, so now and then when it need some oil, i filled it up.
I think it was normal for a engine of this age. Otherwise, i got the idea that the last time the consumption was more than 3 years ago when i bought this car.

The oil pressure was always good. Always on top except at idle. The oil filter is renewed not so long ago, don't think i will find particles in there.
As i told the bearings from the crank are renewed, the old one's showed slightly some scratches, defintly no copper shining trough.
Thanks.

[Benz Dr.]

New piston rings will only work if the piston is not worn out or the cylinder is still in good shape. It's not as good as a new piston in a freshly bored cylinder but it will work if the wear is minimal. The idea is to provide a new cross hatch pattern to the cylinder while removing as little as possible in the process.

The only way to determine what your next step should be is to pull the block and take it apart. Any knocking sound is probably coming from the bottom end of your engine. Tapping sounds are more likely to be in the valve train.
Cyinders should be checked for wear and size. Pistons need to be measured and inspected for ring groove and land wear. Connecting rods should be checked for out of round at the big ends. Also check for piston pin bushing wear. If the crank gear wasn't replaced ( I'd be surprised by that ) I would replace that.
You should be able to reuse the main bearings if they show no unusual wear - sort that out with your machinist. If you have to re-size the conecting rods, I would use new bearings so they conform to a perfect circle.   

[Shvegel]

I too agree that the clean area of the piston tops would indicate blowby due to leaking piston ring sealing. When the cylinders wear they tend to wear into an oval shape as the piston "rocks" in the bore when the spark plug fires.  That coupled with your description of a "Knock" in the engine would lead me to believe you are headed for a rebore. I would be willing to wager that the knock gets quieter as the engine warms up.

[Benz Dr.]

I too agree that the clean area of the piston tops would indicate blowby due to leaking piston ring sealing. When the cylinders wear they tend to wear into an oval shape as the piston "rocks" in the bore when the spark plug fires.  That coupled with your description of a "Knock" in the engine would lead me to believe you are headed for a rebore. I would be willing to wager that the knock gets quieter as the engine warms up.

And if it gets louder as the engine warms up, you have rod bearing problems.
Pull the spark plug wires off one at a time and the bad cylinder will usually go quiet. Of course, the  head is already off this engine but it's an easy test to sort out noises.

[Eminent]

Thanks, i will keep it in mind should it happen again.
But i think (hope) that the overhauled short block won't make that noise.
BTW, at idle the knocking noise was barely perceptible.
It was more under load when i heard it.

[Benz Dr.]

Sounds a bit like piston slap or a worn pin bushing.

[GGR]

It could be a piston pin. I had that on a citroen DS and it wouldn't knock at idle, would knock like mad at intermediary rpm especially at full load and then disappear at cruising speed.

[Eminent]

Today, when taking a closer look, i noticed that there are some marks from the inlet valve at top of the pistons.
Specially the number 1 piston has it, see the attachement.
Could this be a reason for a knocking sound also?
Thanks.

[Benz Dr.]

I noticed that too but thought you were already aware of it.  Check the thickness of the cylinder head which should be at leat 84.00 mm. Another thing to check is whether the cam timing is out one tooth. Minium valve to piston clearance is .9 mm at 5 degrees ATDC.

[Eminent]

I measured the head at several points, the average thickness is 84.6. So no problem.
The chain is replaced almost 2 years ago now. The old one was about 10mm longer.
When replacing the chain there was a moment i couldn't turn the engine any further. I solved this by putting the chain one tooth further.
The "0" on the damper and the mark on the camshaft was correct after this, assuming the old chain was off 1 tooth indeed.
Or something happend when replacing the chain.

[Benz Dr.]

I made your picture larger so I could see more detail and it doesn't look like it's actually hitting the piston and it's more likely hitting the deposits on top of the piston. If the valve was makig contact with the piston you would really hear that and there would be a clean area right down to the aluminium of the piston.

 You have plenty of thickness on the cylinder head so I don't think that should give you any problems. Was the engine smoking or using a lot of oil? There seems to be a very heavy concentration of carbon deposits on the piston crowns. It looks as though it became so think that the intake valve/s started to cut into the carbon. This would indicate that the valves may be very close to minimum specs re valve to piston clearance.

I believe the intake valves are supposed to be recessed no less than .5 mm below the parting surface of the clinder head. Use a depth gauge and check this. If the valve/s are not recessed deeply enough you will need to have that done. I would recommend having the seats cut and not ground so that the depth can be kept uniform.
Cutting the valve seats deeper changes the valve rocker geometry so you may have to sort that out as well.

[Shvegel]

Valve marks on number 1 might also be an indicator of the big end or wrist pin being loose.

[Eminent]

Making progress today. Everthing what is attached to the short block is removed, except for the starter.
So i'm almost ready to take it out and put the overhauled block in. The head is ready within a few days.
New valve guides, valve seats and exhaust valves wil be placed. The inlet valves were not that bad to renew them.
Ofcourse the stem seales are renewed. See what to do with the camshaft. It has small pitting on some lobes. I think i will bring it to the machinst. When the head is placed it's not possible to remove the camshaft anymore.
Also see if i need an offset woodruff key. The head was 84.6 en will be slightly decked.

I also cleaned the surface of piston 1. The piston is damaged by the inlet valve.
When measuring i came to a depth of 35/100mm (!), the difference between point A and B.

[Eminent]

Well...all pistons have some damage from the valves beneath the fouling. Not that much as the #1 piston, though.
Cause the valves didn't show marks form being hit by the pistons and that the head is decked before ánd that there is no blank metal on the pistons where the valves should be hit, makes me think that there happend something in the past.

When i push the pistons sideways to the cylinderwalls i noticed a lot of movement. So the decision to put an overhauled engine in is correct i think.

[ja17]

Hello Eminent,

If your chain was advanced one link off  before the tear down, this could cause the impressions on your pistons. Lateral movement of the pistons in the bores is not a valid test for cylinder wear. Measuring the bore or the ridge at the top of the cylinders is a sure indicator. Clean all carbon of the ridge first. Compare it to the allowable specs.

[Benz Dr.]

You should be able to use those pistons providing there are no fractures in the aluminium.

[Eminent]

Well, it's been weeks now that i swapped the engine and so far so good.
Tonight i looked back at the pictures on top of this topic.
Some of the members said that the clean area on top of the pistons is caused by worn piston rings.
I sure believe that. But as you can see the pistons are all blank at the same side.
Because of that my thoughts are now that it has to do with the injectors.
And that one piston is more fouled than the other could be a sympton that there are injectors who are not working well.
But hey, i'm not a pro, so what do you think of that?

[Benz Dr.]

The thrust side of the piston is on the manifold side of the block which also appears to be what I'm seeing on your piston crown. This would be considered typical.

[Eminent]

Ok, sounds plausible..and proofs that there are pro's here indeed

[ja17]

Turn the engine a bit so that the damaged piston is moving downward. Stop before it gets too far. Now push down on  top of the piston. If it moves down a bit, you have a bad bearing.