Replace piston rings or not
 

I am pretty sure that I need to replace piston rings, but posting this just to make sure that I am not missing anything.

Background: engine is 1978 911SC 3.0, 180k miles, totally stock, it looks like I am the first one to open up the engine - under each washer the metal isn't just clean, its shiny. It was running really well, but I have the cylinders off to replace broken cylinder head studs. It did not smoke on start-up unless it sat for more than a week. Plugs were generally clean, but the car is new to me so they might not have many miles on them. Insides of intake runners were really oily though. I had not done a compression test, but the leak-down test generally good, mostly at 4% but one cylinder at 7% and another at 11%. I will keep the car generally stock, and it will be entirely street driven (except that I might do some PCA driving events).

The cylinders look great inside, crosshatching throughout. The gap between the piston and cylinder is too small for me to measure with a feeler gauge. I don't have the bore gauge to directly measure the cylinder, but measuring it the wrong way (calipers!) showed like-new measurements top/bottom.

Piston ring end gap on the top two rings is very large though - over 1.25 mm. Based on wear limits, seems like the only choice is to replace them, and clean/"deglaze" cylinders. Is that a correct assumption, or is there any reason that I would want to re-install the old rings.

The reasons I ask is because of reading discussions on new rings having gaps larger than specs (to reduce chance of too-tight rings from damaging cylinders), and getting new rings to settle in with the old pistons without honing (just "deglazing"). Still, 1.25+ mm seems excessive and at this point I am planning on new Goetze rings.

Pardon the long post, and thanks in advance for your opinions!

It sounds like you are doing the work yourself. Street car? Are you trying to minimize costs? Are you entertaining ideas of increasing performance "while you're in there"?

Compression ring end gaps of 1.25mm are way beyond spec. The wear limit is 0.80mm.

If you want to minimize down time (get it driving this spring) and cost, then new rings and deglazing should be as far as you go. Do not split the case. Since you have the cylinders and pistons removed, take them to a Porsche shop and at least have them measured to be sure they are within wear and ovality specs. Or buy a bore gauge off Amazon for $60 and check the ovality yourself. The cheap bore gauges are not extremely accurate in absolute measurement, but they are good enough to compare the top-to-bottom and side-to-side dimensions for ovality.

Of course, get the heads inspected too, and count on replacing the valve guides and seals, and getting valves and seats refaced.

Now for the "slippery slope" part: If you have to replace the pistons or cylinders, you can keep costs down by finding used but within spec replacements. You don't even have to replace all of them, just the ones that are out of spec. BUT, for about $5K you can get all new Mahle Sport p/c's and increase the compression from the anemic 8.5 or so that the early SC engines used, and increase the displacement to 3.2 liters with the larger diameter versions. Add an M1 or similar cam ($1000), and you can bump up the power by 15-20%. You should also do some cheap tweaks to the CIS and distributor to realize those gains. Next step up would involve higher compression pistons and a more aggressive cam. That would also require a new intake system, either carbs or an EFI. Those will add many thousands of dollars more.

Others, particularly the pro builders, should weigh in here too.

Thanks, PeteKz!
Yes, doing the work myself, except that the heads are off at the shop getting rebuilt now. Not trying to pinch pennies, but new P&C is something I would rather avoid. And if I did get new ones I would probably stick with stock or close to it to avoid the slippery slope - at least with this car, a Targa that will see most of its miles cruising with the wife. Someday I will get a coupe, and that will be more performance-oriented, so I will hang onto your suggestions to apply to that car in the future.
I think that I will go ahead and get the dial bore gauge so that I will 'know' exactly what I am dealing with.

To determine if your cylinders are Alucil (you shouldn't re-ring or Nikasil OK to re-ring), count the number of fins on your cylinder. If the number is 10, so sad, they are Alucil. If 11 fins, so glad, as they are Nikacil and can be carefully deglazed. But not with conventional stones !

(in Nigel Tufnell voice) "This one goes to eleven!"
Thanks for the tip, brighton911! Many people describe deglazing with Scotch Brite and hot soapy water, so was not planning on the Stones (conventional or otherwise).

So wrong In regard to re-ringing...

With the proper Sunnen hone process Alusil can be re ringed. We have built plenty of motors with Alusil with 100% success....

Think about how many other motors used Alusil that have been rebuilt over the years...

Cheers

The other thing to check is the piston land clearance. If you re ring and deglaze etc there will be more friction/grip on the cylinder and piston rings. If the land clearance is out of spec you can get yourself in trouble going that route.
Alan

Thordyke, with the successes Jeff has had with re-ringing Alucil, my advice is not accurate.

Good to know about Alusil, I don't have to deal with it in this case but will look into it more just out of curiousity (and the possiblity of running into it in the future).
@Alan L, is "land clearance" the same as "side clearance"? If so, I think that I measured it as OK with the existing rings, but will check it again and for sure will check it with new rings. Good point, and thanks for that!

1.25mm ring gap can’t be right, I bet your actual ring gap is closer to .017 or .018.

Show us how your measuring ring gap.

100% correct, many BMW blocks are Ausil.
http://forums.pelicanparts.com/uploa...1709830839.jpg

@Dpmulvan, just to make sure we are talking about the same thing with the gap, I am talking about the end gap (where the piston ring ceases to be a circle). Measured the gap by putting the ring into the cylinder up to about even with where the block would be level with the cylinder, and putting two of the thickest feeler gauges (0.60mm and 0.63mm) together into the gap and still having extra space. You can tell visually that the gap is greater than 1.0 mm.

Feeler gauge between the ring and the top, or bottom of the land (groove) it is sitting in.
Alan

Checked several pistons last night, some that have already been cleaned and some that are as-removed, and using my thinnest feeler gauge (0.04mm) I can barely get the gauge into the top (compression) ring, and not at all into the lower two rings. Looking at the chart, seems tighter than they should be when new, right? They do move relatively freely in the lands though, I can rotate them by hand.

That sounds like good news.
Alan

Curious that your end gaps are that wide, and the other parts measure tight. I would get the cylinders measured for ovality. Otherwise, it looks like you got fortunate.

Yes with the end gap being so worn, you would have expected something more worn than new spec on the lands.
As long as they have been cleaned and measured right, he should be good to go in that regard.
Alan

Thanks guys! I appreciate you looking at this with me!