Scratch in cylinder
 

Hi
I'm rebuilding a 3.6 964 engine that is new to me. I tore down the engine based on oil and carbon buildup in the exhaust port and sure enough the valve guides was pretty bad...

But I also found this:
http://i1037.photobucket.com/albums/...psaa3610b5.jpg

I cant feel the scratch with my fingertips but it catch my nail, more so lower down than closer to the top. To me that says that it has been there for a while. I can't see any damage all to piston or rings.

I think it's OK to reuse but it would be nice to get opinions.

If you can feel it with your finger nail, then you need to remove that high spot, at the very least. How does the cylinder check out densionally, ie bore & taper, surface finish? What's your goal with this engine? For you or to sell? Weekend driver or track weapon?

For repair of the cylinder, some people have had success aith US Chrome. If you repair that one, they will probably recommend that the other 5 be checked and honed to get the surface finish for all 6 cylinders consistant.

I haven't measured them yet, still in the process of cleaning parts. Surface finish looks good otherwise.

How would I go about removing highspots? Should I hone the nicasil???

The engine is for my track day car, with just an RS carpet glued to the metal and solid engine/trans mounts I don't think I will drive it much on the streets. :)

Magnus,

If you hone the Nikasil bores (which requires diamond stones), you will increase piston-to-cylinder wall clearances and that will result in leakdown issues, ring sealing problems, and smoking. None of these things are desirable.

The proper way to fix this is by having the cylinder re-plated and finish-honed to fit your piston, if you can feel that scratch.

Thank you Steve!
I found a thread where Henry describes how he deglace the cylinders (which is how I have done it already) and why you should not hone them.

Replating might be the proper way, but that's not an option for me. Might be able to find another used cylinder.

As I said, the scratch is a lot less pronounced at the top, so I guess it's not a new damage.
How will a scratch like this affect the engine?

I'll try to measure the cylinders and ring end gap tonight.

I replated a single cylinder with US Chrome last year. They had no problem doing only one and didn't think it'd be a problem to only do one out of the 6.

That said, I did realize ahead of time i'd have a cylinder with fresher hatching that the others. I didn't think it'd be a huge issue. Can't say how well it's peformed over time since my engine suffered a bearing failure after only a couple hours of usage.

The only way you'll know if its a problem or not is by performing a leakdown test to see if you get any leakage past the rings.

In my experience, scratches that you can feel result in more blowby than is ideal, however it rarely affects power.

The cylinders measure ok as do the pistons and even the rings, so I think I'll gamble with the scratch. I will change the rings...

A different cylinder issue, all 6 cylinders has this kind of pitting on the sealing surface.

http://i1037.photobucket.com/albums/...ps705e8f97.jpg

The heads will be resurfaced but should I that done to the cylinders to?

That looks like fretting as a result of insufficient clamping force by the head studs.

Yes, I guess so. Unfortunately head studs is not in the budget... To be honest the engine wasn't really in the budget at all. :)

But should I do something about the fretting damage or doesn't it matter much?

The fretting is not very widespread so probably not a huge problem. If you were to surface the cyl tops, you would want to do a triple cut on the raised sealing surface, o-ring/gasket surface and upper mating surface to the head, all to maintain that existing gap. The cutting will effectively shorten your cylinder height. So you would expect to re-establish that cylinder height by using a thicker base gasket under the cylinder upon reassembly.

What kind of head studs are they? I assume all dilavar since it's a 964 engine? You could replace with all steel 993 studs and probably be better off going forward. Just my opinion based on what the 993 eventually ended up with. The debate of Porsches dilavar vs. OEM steel vs. aftermarket high end metallurgy always seems to come up. For a stock, street engine, I don't think you can go wrong with the factory steel studs.

It's all steel head studs. The Porsche 993TT is probably good but way to expensive, I have Supertec studs in my 3.2 engine a cheaper and at least equally good.
I was thinking about what happens when you cut the heads or cylinders. A 3.6 doesn't have any base gasket as the older engines??

Edit: I'm pretty sure they are plain steel on this 964 engine, haven't actually checked with a magnet, but they look like steel.

Well it does have a cylinder head gasket up top. I mixed up the o-ring location, top vs. bottom. I suppose the head gasket on top can vary in thickness? Though I don't see an alternate part number for the typical gasket. Seems odd there wouldn't be a thicker gasket to use, similar to how on the older engines you can use 0.25mm or 0.50mm base gaskets to set up your deck height.

If turning the cylinder tops a thousandth of an inch would clean off the fretting, I'd suppose that would not really require any compensation elsewhere. The CE ring ought to compress that extra thousandth? Then you'd be faced with the question of how much is needed to clean up the corresponding part of the heads. Another thousandth? Still going to have geometry and clearances adequate? Tiny increase in compression?

I just think of the fact that these are sealing surfaces, and to make them work better Porsche had to add the CE rings. When they dropped those rings for the 3.2s, they eventually decided that wasn't working as well as hoped, and went back to some kind of extra seal up top?

But maybe I am overthinking this, and if the motor wasn't leaking (evidenced by carbon deposits) combustion pressure here, it will seal up fine as it is?

But I'd want more than my guesswork to decide what to do.

The CE ring should compress the extra 0.1 mm I think.
I downloaded the 964 workshop manual and there it says that you can cut the head 0.1 mm two times, no mention about compensating for this.

Maybe I'm over thinking this, do the sealing surface need to be perfect?

Well, cutting the head won't affect CE ring crush.

If you cut one head, you need to cut them all. You can't compensate with a 0.1mm thicker base shim, because these don't come in 0.1mm increments unless you spend way more than having all heads trued would cost.

I don't know how good that surface really needs to be either, just that a pressure sealing surface is a pretty important one. But it was the cylinder top which looked fretted, not the head wasn't it? Or does the head have matching fretting?