Help about this cylinder marks

[Eusebi]

I'm rebuilding a 2'7 engine and I've found this marks on the wall of several cyilinders. With the finger i can not notice crackes; its completely smooth.

Can someone help? It's a symptom of material failure?




Thanks!!!

[wdfifteen]

This should be interesting.
Is there any chance the engine sat at the surface experienced some corrosion, then the engine was run enough to polish it off? I've seen something kind of similar, though not as extensive, on cast iron cylinders.

[Walt Fricke]

Are these Nikasil cylinders (a magnet will be lightly attracted to the surface you show with those cracks) or Alusil (completely non-magnetic)?

Nikasil is a plating, so I could see perhaps it developing cracks, though I have not seen this. Alusil is part of the aluminum alloy itself, and I couldn't see it cracking, though that doesn't mean the cylinder surface couldn't crack.

Perhaps one of the engine builders here - they see lots more stuff than we do it yourselfers do - has seen something like this.

[SoyRacer]

Are the "shadow cracks" throughout the whole cylinder or piston height/lower position dependent per cylinder? I've seen this before.

Regards,

Rich

[Eusebi]

The cylinders are Nikasil (checked with magnet).
According to the bills that the previous owner has given to the present owner, theoretically these cylinders/pistons are brand new with 4000 miles of use!!
I have rebuild 8 engines before and never saw this.
These "shadow cracks" are visible in 4 of 6 cylinders and more visible in the highest position of the piston.

[mreid]

That looks like heat crazing to me. Did this engine overheat or was it run at high rpm for sustained periods?

[Eusebi]

I don't have information about a possible overheating. The present owner has recently bought the car and came to our workshop because the engine had oil leaks mainly at the base of the cylinders; when dismantling, the cylinderhead barrel nuts were very loose.

[RedCoupe]

Originally Posted by mreid

That looks like heat crazing to me. Did this engine overheat or was it run at high rpm for sustained periods?

+1 I've seen similar spider crazing on flywheels where there was serious clutch plate slipping going on. Wonder what the rings look like?

[Eusebi]

At first look the rings seem to be in good condition. But i'll check them exhaustively this week.

[mreid]

Originally Posted by Eusebi

I don't have information about a possible overheating. The present owner has recently bought the car and came to our workshop because the engine had oil leaks mainly at the base of the cylinders; when dismantling, the cylinderhead barrel nuts were very loose.

This could explain the heat crazing. Extra air is like a blow torch on the cylinders. An intake leak usually burns valves, pistons, and rings. A head leak usually burns cylinders and rings.

[JFairman]

Does look heat oriented.
I wonder if those cylinders were bored and re plated or is it original Mahle nikisil plating.

[tom1394racing]

Try cleaning them up with a Scotchbrite hone. See what they look afterwards.

[Eusebi]

I'll try what happens with Scotchbrite. Also to check if the cylinders are brand new as customer says, according to the invoices that the previous owner gave to him or if they have been replated.

[Eusebi]

After analyzing the cylinders with Porsche specialist and leader workshop of Barcelona, the conclusion ist that the aluminium under the Nikasil coat is beginnin to cracke. So, there are not reusable. Nobody can identify the origin of the problem...
Thaks for your opinions

[Eusebi]

Sorry, for a bad explanation. It seems that a reaction between the aluminium and the Nikasil cover has produced; and the NIKASIL not de ALUMINIUM is beginning to cracke.

[Eusebi]

Photos made with magnifiing glass

[tom1394racing]

I recently found Nikasil delamination a brand new, unused 90 mm Mahle Nikasil cylinder. A local experienced 911 engine builder told me that he has also seen this. It seems that their current plating process may have some issues. I had mine re-plated by Millennium.