jonvo

I bought a 968 motor with about 190k miles to put into my 944. Previous owner said the motor ran great, didn't burn oil, and had consistent compression in all 4 cylinders. I have all the paper work showing that the head was rebuilt at 160k. I want to put in new rod bearings/belts/rollers/water pump before doing the conversion, but my question is if I should re ring it? I have been reading a bunch of mixed opinions on re-honing alusil bores. Some say you can do it, some say you can't, some say it's a gamble, and some say if the bores look ok, you can re ring it without a problem.

The options I'm deciding on are to:
1. leave the rings and just do the rod bearings/belts/rollers/water pump
2. re ring without honing if the bores are in spec and visually in good shape
3. freshen up the bores with scotchbrite pads and the Sunnen silicuim mixture

What should I do?

V2Rocket

pull the head, if they look good and dont have any scratches deep enough to catch your nail on they are basically fine. alusil is actually a very hard, resilient surface, you have to try to ruin it. if they look good, just clean them up and use plenty of oil when installing the new rings and youll be good

Darwantae951

I'd have to disagree with alusil being very resilient. It may be resilient to wear, but not to foreign contaminants. Do as Spencer said and pull the head to check the bores. If they look fine, re-ring the rods and check tolerances on everything else while you're in there.

DarylJ

Woah woah....why are we pulling heads for visual inspections of rings/bores before doing a leak down test? And if that proves good we can progress past 1985 diagnostics and use one of them fancy new cameras to inspect the bores.

But hey....what do I know? If you guys like ripping heads off that have recently been rebuilt and having to skim them again (and hope the last shop didn't take too much off) just to take a look-see inside go for it.

And, yes, even if you don't have a camera it would be cheaper to buy than a new head gasket and the machine work. Way cheaper to rent.

m73m95

Just because the head comes off, doesn't mean it needs machined. Get a straight edge, and make sure the head is flat. If it is, throw it back on with a new gasket. Maybe lap the valves while its off....

And the rectal scopes aren't cheap....Not everybody owns one.

The Silicon in the aluminum is much MUCH harder than the steel rings, so if the bores are clean, new rings work great. The Aluminum however is soft, so don't knock any carbon chunks loose while you've got the head off, or it will scratch the cylinder wall. Make sure you clean everything before you put it back together.

DarylJ

Of course they don't always need to be decked, but it's a good chance that they will. So there's $100 at the machine shop. A head gasket kit is $200. And you need new head nuts. There's another $100 or more if you save and sand the washers. And cam gaskets. All to see if the bore and rings are OK. And you still don't know how good the rings really are on a visual inspection unless they are REALLY bad if you haven't done a proper leak down test (which you now can't do because the head is off).

Optical bore scopes are readily available for $120 and sometimes even less. You can even pick up decent ones with an LCD under $300, which what I said (cheaper than a head gasket and machine work). You don't need a $1400 bore scope to see what's going on in a cylinder.

The longer I do this kind of work, the smarter I've learned to do it. While there's nothing necessarily wrong with just ripping the head off (after a leak down test), it's clearly unnecessary if all you are trying to do see if you need to re-ring. If the OP is dead set on re-ringing, then whatever. But there was no evidence posted that is was necessary.

whalebird

Ideally a good compression test will determine if you need a leakdown. Sure do a leakdown. Often the cylinders may be scored in the lower halves of the bore which will lead to piston skirt wear. The pistons will deposite their iron coating into the cylinder walls and there is only one way to tell which is to pull the head. If you push the pistons out, you can mic them for wear and also the bores for taper. if everything checks out you're still screwed because the pistons and rings will never go back into the bores in the exact same position. Old rings/new rings are both going to have to reseat.
What to do???
The head will come of and go back on very easily and I would recommend a new head gasket if it's on the bench anyway. You should never have to "skim" a head if nothings been overheated and even then it should be discarded. Take it off and put it back on with new seals.
Make sure that the crank-thrust is in spec before you go to far in any one direction.

jamesjedi

If his compression/leakdown indicates rings could be replaced, should he do it, and if he does, do they have to be OEM?

whalebird

the general consensus with builders is yes, factory rings(not just OEM) are much prefered. There are others who make OEM style rings, but FACTORY rings are know to have the proper composition of carbon/chrome or whatever it is that's critical to alusil. Don't skimp on rings at this point.

m73m95

Whalebird has it.

At 190k, its probably time to go through the engine. These engines seem to have fantastic wear resistance, and I don't think 190k is too much to ask on a well maintained engine. However, if you're in there doing the bearings, and pulling the head, throw on some new rings.

Other than the aluminum getting scratched up, these engines are tough! I know a couple 951 guys that run 25lbs of boost (or more), on old blocks with new rings.

jonvo

From what I've been reading, the alusil cylinders are the wear item and not the chrome plated rings. Apparently, the rings are harder and the cylinders "break into the rings"

BTW, are Goetze rings appropriate rings to use?

whalebird

I think Goetze are OEM. However, the ones in the Porsche box are quality controlled to a higher spec dictated by Porsche. In fact, It is a legal violation of an OEM manufacturer to sell Porsche spec parts under their own branding. Factory is always better on critical components.

DarylJ

I'm not sure what that means. I find compression tests to be about as useful as idiot lights on a dash board. It tells you vaguely if things are gonna maybe work or not. A leakdown test, done properly (moving each cylinder through its entire stroke), will tell you a lot about a motor's condition. It will not, however tell you:

But now we're just guessing at what this guy really intends to do. It's one thing to consider tossing some rings in if it needs them. It's something else to decide to dig into it to see what else MIGHT be wrong.

I tend to rebuild for real or just let it go, and not something in between when I work on a motor that's already out. I decide after scoping and leak down testing as well as other typical testing without opening it up. If the thing is gonna run fine, I run it. If its not, it gets rebuilt properly. No sense in going cheap or halfway on a rebuild and skipping things just to end up having something you skipped go bad later and have to pull the motor again.

You/others may or may not agree with that, but its worked for me.

whalebird

You need to learn how to do a leakdown and compression test daryl, and how to interpret the results.
Tossing rings in it is exactly what we are talking about...and it is easier said than done. 190K miles is a good time to look at everything. A borescope and somekind of leakdown your describing will not remove the risk...just letting it go because you THINK it is OK will not work on any motor. Disassembly and properly measuring the components is the only way to know what a motor needs.
Anybody can put a junk yard motor in a car and expect it to run.

DarylJ

You're making some serious assumptions here. Starting with thinking that I tried to post exhaustive instructions on how to do a leak down test. I mentioned only one thing, something that I see so many people not do or not even know about.

Not every motor needs to be disassembled and miced, not even those with 190k on the clock. Plenty of these motors have gone a lot further than that without touching the bottom end.

If you retired dudes have that kind of time on your hands, have at it. Not all of us feel the need to go completely overboard.

And while you're at it, make a fresh pot of coffee. I think someone pissed in yours.