300TDi knock noise

**Old Farang** · 26th March 2020, 02:55 PM

Originally Posted by Ronski

I'm going to be blunt here, but I worked in recond engine shop for years.

When honing and maybe putting in new rings/pistons, did you in fact remove the "ridge' at top of cylinder. There are tools for doing this.
This is clearly shown in your pictures, it was not done, DANGER, failing to remove this ridge, caused by wear of rings and bore, or more correctly, limit of wear of rings.
This leaves a ridge at top of bore, ie. where no wear occurs. BEFORE honing, this ridge 'MUST' be removed. Fitting new rings or pistons with very slightly
different thickness, 'aftermarket' rings/pistons' can hit this ridge. Some ring sets can be thicker than standard, And yes, knock noise, causing new rings hitting ridge, followed by deformation of the new rings, usually around gaps, then scoring of the bores. same issue at bottom of bore, and a reason why complete new rebore is recommended in cases where wear exceeds a certain amount, over simply honing and installing DIFFERENT pistons and or rings. Diesels have very high compression, not so noticeable on petrol engines.
Piston slap is obvious, just measure bore and piston, is it within specs?

Sound familiar,

Yes, a much better explanation than calling it "piston slap". Too much water under the bridge since I worked on small high speed diesel engines, but just how can you hear "piston slap" in such an engine? Those score marks are not the result of any "piston slap", but either a broken compression ring, or a incorrectly fitted gudgeon pin. (no idea if these engines have fully floating pins?) It is also unlikely to be heat related. As posted, remove the offending piston and all will be revealed.

**Finnius** · 28th March 2020, 11:24 AM

Originally Posted by Old Farang

Yes, a much better explanation than calling it "piston slap". Too much water under the bridge since I worked on small high speed diesel engines, but just how can you hear "piston slap" in such an engine? Those score marks are not the result of any "piston slap", but either a broken compression ring, or a incorrectly fitted gudgeon pin. (no idea if these engines have fully floating pins?) It is also unlikely to be heat related. As posted, remove the offending piston and all will be revealed.

Not much to talk about in terms of piston wear. This pic is of the part of the piston near the mark in the cylinder. There is only some thin scratch lines at this spot - no major wear patch at all like i was expecting.

The ring gap on the second compression ring lines up with the mark in the cylinder... Gudgeon pin is still in. Circlips are still in. The pin can float back and forth a couple of mm - i presume this is normal.

**Finnius** · 28th March 2020, 12:15 PM

I can move one or two of the remaining pistons side to side in the cylinder by hand

I read that the clearance shoud be 0.075mm - presumably measured on the skirt...

So maybe it's normal to be able to rock the top of the piston back and forth in the bore by hand?

**BathurstTom** · 28th March 2020, 12:39 PM

Originally Posted by Finnius

I can move one or two of the remaining pistons side to side in the cylinder by hand

I read that the clearance shoud be 0.075mm - presumably measured on the skirt...

So maybe it's normal to be able to rock the top of the piston back and forth in the bore by hand?

You should be only just able to get movement. Talking perhaps 1/3 to 1/2 mm or so. Any more and I would be worried. Dave (blcknight) would be able to more confidently comment.

Tom.

**Finnius** · 28th March 2020, 06:21 PM

Good news! It looks like i found the source of the knock. Piston slap on cylinder 2. The bore had only the faintest marks on it - but i could rock it sideways in the bore and produce a faint knock sound, so i pulled it. The piston has a fair bit of wear and even some pitting/melting looking spots.

'Old Farang' was right that the other marks on cylinder 1 must be from another cause - maybe some carbon buildup on the rings etc.
Bad news is that i have no idea why the piston slap has occurred in the first place. I checked the bottom oil jet and it was not blocked...

Perhaps it's a poor quality el'cheapo aftermarket piston that the previous owner put in??? These pistons are silver coloured all over whilst i have seen the other ones/Kolbenschmidt ones that are black i think?

**Old Farang** · 28th March 2020, 07:01 PM

Hmm, you have mentioned a couple of times about an "oil jet". Again, I am not familiar with these engines, but a quick check on the internet shows 2 different part numbers for these jets. They are piston cooling jets: ETC6531 for pistons #2 and #4: ETC6532 for #1 and #3. If this engine has been worked on previously there is a good chance the jets may be in the wrong position. edit: some confusion about which is which!

Other than that it has been fitted with either crap pistons, or the wrong size. My previous comment regarding heat damage was in regard to jacket cooling, not piston cooling.

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