Bearing/Ring Re-Use Ever Possible?

[OK-944]

I'm not trying to sound cheap, but my current budget is somewhat limited. All of the engine rebuild info I can find basically states that new bearings (rods and mains) plus new rings - should always be installed after removing existing ones.

My question: is it possible to evaluate the condition of bearings and rings...and if all checks out - to simply re-install them?

Some history: I purchased this 85.5 three years ago, and now its been sitting for two years. The PO had replaced the clutch and belts, but not the waterpump. No other history available. ODO showed 70K (certified by the PO's dealership) but within a week the ODO started cutting out intermittently...so who knows? Car looks well kept otherwise.

My gut tells me that I should do a complete reseal (top and bottom)...and that removing the engine will make this easier...plus give me more room to attend to other stuff in the engine bay and give all a good cleaning.

While I'm in there, I'd like to check the condition of the pistons, rings, cylinders, and bearings. I'm also thinking that before committing myself to new rod and main bearings and new rings - I should be able to clean them up and compare them to specifications (plus check and measure the cylinder walls), and that if all is within spec I should not need to replace them.

I suppose I could make life easier by first doing the top end work, then check compression and maybe do a leakdown test...but seeing as how the cam cover sprang a leak shortly after I changed the oil (and by the sludge coating the bottom of the oil pan) - I'm guessing that no matter what the true mileage, and no matter what a leakdown might indicate...that this engine has never had a reseal and its just a matter of time.

A related question: would it be possible to detect an amount of bearing free play which would indicate a rebuild...without removing the bearings?

Again, I'm not trying to be cheap...I just want to know the extent to which I can evaluate the true condition of my engine - without committing myself to parts which may actually not need replacing for another 50K or so miles.

Thanks!

[krystar]

u need to plastigauge it to see how much wear there is left. the con rod bearing clearance is 0.34mm-0.92mm or 0.013"-0.036".....i guess u could feeler gauge it..if u could get it in there.

[944 Ecology]

Don't be so cheap.

Fix it right the first time or burn it for the insurance.

[HondaDustR]

Do not remove the pistons unless you are planning to replace the rings and at least re-lap the bores to expose new silicon. These are Alusil blocks and do not do well with new rings in used bores, much less used rings reinstalled. I'm pretty sure rings break in to the exact shape of the cylinder down to the microscopic level especially with these sleeveless aluminum bores and are virtually impossible to reinstall them exactly as they were. Read this thread, as it's got some good info on that issue.
http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/523851-enigine-rebuild-questions.html

As far as bearings go, some say they're a one time install and some don't. I would change the rod bearings at least, since they are known to wear out after 100K miles or so and they're relatively cheap ~$70. While you're in there, you might as well pull the main bearing girdle and inspect the mains. They're not really prone to massive failure like the rods can, esp #2, but they might be worn to the point of replacement. Plastigage them and see if the oil clearance still measures up (0.020-0.098mm). There shouldn't be any discoloration visible indicating it wore through to the copper layer or steel backing.

[Rasta Monsta]

I like your idea of pulling the motor, popping the head off, inspecting the bores for obvious damage, resealing, replacing rod bearings, and popping it back in (especially if that 70k is accurate. . .compression should be fine).

[924RACR]

I agree, plan on rod bearings, but if leakdown/compression is OK, or at least doesn't show ring wear, leave 'em and the mains. You might also evaluate valve sealing, possibly re-lap the valves while the head's off. It's just time, costs virtually nothing to DIY.

[oliver88]

Please clarify why we can't put new rings in without doing a re-lap on the bores...I've never heard that before. We've put plenty of rings on and never even honed the bores.

[HondaDustR]

Well, it can be done and apparently has been done without anything bad resulting, but it's kind of a crapshoot, since with Alusil, the bores kind of polish smooth as they break in. New rings need a relatively rough surface to break in well or they might just glaze and never quite seal perfectly. Read the "re-ring Alusils?" thread linked in the engine rebuild questions link above for the whole dillema, as well as the other info in the thread. Some had success, but others in the 911 forums had 10K miles on their rebuilds and were still smoking. It probably depends a bit on how many miles the bores have, but it's not worth the risk of less that optimal compression and oil consumption. You can't hone them like Nikasil and cast iron bores. I went over mine with a 3 arm hone wrapped in scotchbrite pads lubed with mineral oil, followed by felt pads with Sunnen AN-30 paste and mineral oil to thin out as needed, and the motor never smoked and pulls very strong. I guess do what you know works and/or are willing to risk.

[Capt Squid]

You did not indicate why the car has been sitting for 2 years.

If you can get it started, do a hot leakdown and compression test. If it will not start, a cold leakdown, while not great, may still give you an indication of engine condition.

The head is the heart of these engines. Have a machine shop do a valve job and seals and check the head for trueness. If you plan on spinning the engine up to red line on a regular basis, then change the rod bearings and check the mains. Otherwise you will probably spin #2 rod bearing.

If the cylinder walls look good and you have good compression and leakdown, leave the rings alone.

If you plan on driving it like you stole it, then you need to go through the engine first.

[OK-944]

Thanks for all the great replies!

While I don't plan to redline this car on a regular basis - I lack an accurate history, so my inclination is to go ahead and replace the rod bearings. Besides this, I think I'll clean up, check, and reseal everything, remove and rehab the head, and check the condition of the cylinders, and if they look good, I'll leave the mains and pistons/rings alone for now.

While I'm guessing that I could do the above with the engine in the car - I'd much rather remove it, both for the easier access it will give me, plus allowing for easier/better access to and better evaluation of other components. Also much better in terms of cleaning everything up. Plus, as I've never removed an engine before, I look forward to the experience and knowledge that I will gain from this. (and who knows, I may just decide to go ahead with mains and rings after all). Thanks again!

[wild man]

Originally Posted by OK-944

new rings - should always be installed after removing existing ones.

At the risk of sounding cheap also, the word that everything hinges on is "should", not "must". They would not use that particular word if there wasn't some leeway available there.

I have already done a complete build of a motor using NO new parts - only the parts off of 3 different blocks that had miles on them. That was over a decade ago, and it still runs. But I would recommend replacement of the rod bearings and checking the mains with plastic gauges, at a minimum.

[HondaDustR]

Definitely do a compression test, or even better, a leakdown before tearing it apart. It's your only good chance to know just how good the rings still are. Less than 100K is still pretty young for rings and bores in these motors if they've been taken care of and not raced/abused/redlined to within an inch of their lives. Oil consumption is another big one. My original motor always had decent compression, but would devour oil at 250 mi/qt, and that was with 150K mi. It was kind of a declining thing, as it would go 1500 mi/qt easily when I first got it at 90K mi, but I rev it freely every opportunity I get. I thought it was valve stem seals, and the valve guides were kind of shot, but I wouldn't doubt the oil ring was worn and not working as well. The plugs never looked oil fouled and it didn't leak anywhere near enough to account for that much oil. Unfortunately, I killed it after replacing the valve stem seals due to a series of unfortunate events involving wads of paper towels left behind, so I'll never know exactly what it was.


I always wondered about doing a hot compression test. Since you're not supposed to remove spark plugs from a hot aluminum head (although I have before), how are you supposed to do a hot compression test? Loosen the plugs to finger tight, warm up, then remove for testing?

Originally Posted by wild man

At the risk of sounding cheap also, the word that everything hinges on is "should", not "must". They would not use that particular word if there wasn't some leeway available there.
.

In this case, "should" seems more like a consensus. Sure, everything's been tried, but what's the best compromise? If the rings still make good compression, leave them alone and do not remove the pistons. If they don't, or you just want to start fresh WYIT, remove and replace them. Simple as that. Deves rings are only $60 a set from Pelican and they are a high quality aftermarket product. Prepping the cylinder walls for new rings is whatever gamble you choose, as there are many ways around that that have worked or not to varying degrees, from nothing to re-sleaving with iron along with the right pistons.