930 build, Mahle cylinder marks need some advice

[dobrjan]

Hi guys,

While in the process of checking valve to piston clearances, I noticed some marks on the cylinder walls. Pretty much all the cylinders had some marks. I decided to strip motor back to reveal the cylinders and pistons to get a better look. .

Background on build.
Full rebuild (spun big end bearing on 4

Heads were fully reconditioned by reputable shop here in Melbourne.
Case and crank were worked on by a local machine shop that works on air cooled Porsches.
New Mahle 3.3L pistons and cylinder sets are used.
For assembly all pistons and rings were removed as they come installed in their cylinders. These were cleaned with carb cleaner and alcohol and oiled before installation. Ring gaps were also set to face 12, 10 and 3 o'clock. Some marks are on the bottom which makes me think its not from a ring burr or edge. I did not grind the rings.

During the piston assembly process I did not notice any marks when proceeding from one piston to the next and going through TDC for each piston. Also did a few rotations of the crank to see that everything was ok. However this would have been only a few rotations.

It is when I performed the timing and valve to piston clearances when I noticed the marks that run the entire way of the piston travel. I have attached some photos. My machinist is having a look as well, he thinks it is a bit of ring rub but will confirm tomorrow. To note none of the marks catch my finger nail or have any edge. However the shiny appearance almost looks like they would particularly if you are looking through a tiny spark plug hole. All 6 cylinders have some form of this. Cylinders 1,2,4,5 have more bold looking lines while the others have slight lines - harder to see and photograph.

After some advice here, is this something normal, given that during timing and valve to piston clearance I would have turned the motor over at least 20 or so times. Or should these be perfect without any marks. I initially thought it was debris, but I have been extremely anal with cleaning things multiple times and using anti lint wipes. Motor gets covered with new plastic when not worked on.

Your thoughts and advice would be greatly appreciated. Thanks everyone.

[Dpmulvan]

That’s no good, Did you measure everything before install? Looks like ring gaps are scraping cylinder walls. Did you have wrist pin bushings replaced?

[dobrjan]

Thanks for your reply Dpmulvan.
Yes rods were checked and new bushings installed.

I checked the bores of the new cylinders, all were perfect at 97mm. Unfortunately I did not do a ring gap check as I read that Mahle assemblies are already gapped correctly. Still I probably should have done a check to confirm. The marks are around the entire circumference of cylinder wall. My ring gaps were set near the top of the motor, the marks pictured are more at the bottom, plus there are a lot more marks that are really faint - more marks then ring gaps. If the rings are indeed too large I wonder how they came assembled in their respective cylinders.

Wondering if anyone else has had this issue with these sets?

[Dpmulvan]

Originally Posted by dobrjan

Thanks for your reply Dpmulvan.
Yes rods were checked and new bushings installed.

I checked the bores of the new cylinders, all were perfect at 97mm. Unfortunately I did not do a ring gap check as I read that Mahle assemblies are already gapped correctly. Still I probably should have done a check to confirm. The marks are around the entire circumference of cylinder wall. My ring gaps were set near the top of the motor, the marks pictured are more at the bottom, plus there are a lot more marks that are really faint - more marks then ring gaps. If the rings are indeed too large I wonder how they came assembled in their respective cylinders.

Wondering if anyone else has had this issue with these sets?

Measure your wrist pin bushings make sure they’re not cockeyed.

[icarp]

First thought is the hone looks too rough, Mahle likes an RA of 7-10, you say they are new Mahle.
The walls should be well oiled when rotating the crank.
Never assume the rings have been gapped , Get the proper ratio for each ring and set the gap.
They look like witness marks more than a scratches, hence the look of dragging the ring on the hone
Just some thoughts
Ian

[Zuffenwerker]

Short answer: it’s normal