Matt Sheppard

My '86 "parts-car" engine has 260K+ miles and I want to do the rod bearings, re-ring and take my time building my 2.7 - correctly this time. I already have the head redone and want to be as complete as possible without going overboard on $$ commitment - having to buy oversize pistons, bore block, hone, yada, yada.

I have used the telescoping gauge and micrometer method outlined in the Haynes manual with mixed results/measurements and without buying a bore gauge, I have determined that getting a relaible measurement this way is not feasible. The cyl walls look clean with no "finger-nail" scoring. Since I know that the cyl wall clearances are "bigger" than when new I want to at least close up the ring gaps that are prob. real well over-spec by now. I would like anybody's informed opinion on what rings to use.

Porsche sells factory rings in 100.0 mm and 100.5 mm. I would like to gap the rings myself so my main question is should the 100.5 mm rings work on a stock 100.0 mm piston but allow more material to do custom ring gaps on the worn engine? My logic leads me to believe that it wold get me where I want with a bit more tension against the cyl wall - which is not what seals the ring anyway, but rather the compression. I would think if I plugged in std. rings that my gaps would be significantly larger than necessary.

What is a good break-in procedure for re-rung engines? Oil for first few thousand miles?

Tia
Matt

dand86951

Matt, I would first check your existing rings for the end gap. See how that compares to the factory spec which is very tight. That will tell you how much cylinder wear you possibly have. Keep in mind that the circumference difference between the two is .0618 inches so you could have a bunch to file off. If your block is worn four or five thousandths of an inch on the diameter you could also have your pistons coated with a teflon moly skirt coating to gain back four to five on the diameter(two to three thick per side).

The best bet is to find some way to get a dial bore gauge into your block to know for sure. If you don't deglaze the cylinders some way you are going to have a more difficult time getting the new rings to seal. If you do go that way, make sure you break it in with non detergent no additive(slipery stuff) oil to help force the ring sealing.

aka 951

With 270k it probably needs new pistons also. The grooves in the pistons which hold the rings wear like the keeper groves on the valves. Wiesco and a few others are supposedly now making pistons for the nikasil liners in standard and overbore sizes.

dand86951

Just check to make sure that the pistons work in the Alusil bores the 951 uses. The 911 series car I believe use the nikasil process and they are slightly different. I would agree that probably your ring grooves may be worn but you can check that to Porsche specs with feeler gauges.

Matt Sheppard

OK, I'll measure ring land clearances on the top ring first, then figure out where to go. I beleive I can remove the top rings w/out breaking them, but with the mileage and age, I'm sure to destroy the oil rings if I try to take them out. I'm not opposed to just cleaning the piston tops and putting them rings and all back in the holes - this is not a total rebuild I'm doing here. The engine was strong still before it came out.

Has anyone ever rented a bore gauge before? If so , where and would they ship it? Big Bear aint exactly a metropolis.

On measuring the ring gaps - OK, got it. Will do. Still if I do rering, how can I get appropriate rings that I can gap? I can take 50 thou. off every ring with a dremmel wheel fairly easily and accurately and do the last 5 -9 thou. w/ a file. A lot easier than measuring bore dimensions with a tele-gauge). I suppose I can just buy STD rings, they will obviously be closer to spec than the old rings, but I dont want to take a step back in sealing if neither will do as good a job as the ones in there already.

What happens when one of these engines breaks a ring, what do people do? Hone and treat the block, get oversize pistons, rings and voila - $3K in parts and machining - come on. . .

What is appropriate for deglazing cyl. walls w/out the "special process". I've heard of scotchbrite pads from this board as a solution when reringing.

Thanks for the replies, really, but I'd like any real world data or feedback - perhaps compression readings from those who have just done a rering. I can not be the first here.

H2Opumpr

Just went through a similar process as you are doing. Started with a leaky pan gasket and progressed into something more...rod bearings, rings, rebuilt head
My motor has 170K and was running very strong. After looking the block over after I pulled the pistons, I had slight scuffing but nothing that would grab a fingernail, however I did have a slight ring ridge which concerned me. The bores were definately worn. Pistons were in great shape and the ring lands had no wear. When all was said and done I went with 100 mm rings and had the pistons coated by Swaintech to take up some of the clearance in the bore.
Compression after was 145,145,144,140.

I broke the motor in on a detergent based oil because I did not do a full rebuild. I went with 5W20.
Biggest difference in the feel of the motor is more response on the bottom end.(probably due to the head rebuild) I feel for the money I spent.... it was the best bang for the buck!
Good luck!!

Matt Sheppard

Wow, just what I was looking for. Thanks H2Opumper! Did you scuff the bores w/ scotchbrite as well? 170K and good ring lands - man that's good news!

Any other experiences?

I did an archive search last night and noticed Keith in MO did his rod bearings but not the rings and ended up regretting it - so he says. I suppose for $70 to simply get new 100 mm std. rings is a worth it investment.

Swain-tech coating - for gold coat and PC-9 is like $250 w/ shipping. That one I'll have to think on.

H2Opumpr

Glad to help!

No, I didnt use scotchbrite on the cylinder walls. I thought it was risky, even though the Alusil bores sometimes have a hard time with new ring seating..... I took the chance by not scuffing the bores. All seems to be well.

Looking at the old rings they seemed to be in pretty good shape. My main concern was with metal fatigue breaking a ring .. potentially causing massive destruction in the cylinder. Replace them, they are cheap!!

Swaintech just coated the piston skirts with PC-9, only 85.00 shipped to Sacramento, I know that the iron coating on the factory pistons is old tech, and was wearing off.... it was a no-brainer for me. Didnt go with the ceramic coating on the crowns beacause I had heard it doesn't last that long and can flake off. I definately didnt want ceramic flakes floating through my turbo.

Can't think of any other pitfalls. Sure is nice to have a leak free motor
Cheers!

Perry 951

My "new" block had 40,000 on it and was in great shape. I went with standard rings and the pistons that came with the block. So far so good. I'll do a compression check next week. Before run in I had 115, 119, 118, 126. I did not touch the bores.

Most important, check every tolerance possible. If you cannot get a micrometer, take the block to a shop to have it measured. (also a good chance to check the deck for squareness) This will give you an idea on what rings you need, 100 or 100.5. If the ring lands are within spec, toss the rings on the pistons, put it together, and break it in with the cheapest non detergent oil possible.

Initial run in should be no load, 15 min of run time, keeping the motor between idle and 2500. Always keep the revs changing while doing this. Let it cool all the way and go for a 15 minute drive with light throttle. Return, change oil, and let cool.

Next 1000 miles.. try and modulate the RPM's as much as possible, running it to 5 grand and 3/4 throttle. For the next 500 miles, a few short full throttle blasts to redline are acceptable. After that, change oil and drive as normal

Matt Sheppard

Thanks Brian

Well, I checked the rings tonight. The ring lands are fine (could get a .003" feeler in and it's tight, can not get a .004" in). Ring gaps on the rings are surprisingly not that bad for such high miles - though out of spec. .026" was the worst compression ring, spec being .007" - .017" for those.

That's right, I could just coat the skirts and polish the piston tops - for $85, that sounds like the right play - thanks!

The big surprise, the engine had spun the #4 rod bearing. #2 bearing had some funky wear in it, but #4 was definately spun. THe tangs did a number on the rod big end, but the journal on the cranks was unscored and seemed OK. I miced all the rod journals and they look to be within spec or a few 10,000ths under based on my feel with the micrometer and its calibration. They were all within 5 10,000ths of each other, #4 being the smallest. No detectable taper. I will need to have the rods resized on the big ends and rebushed so I think I will shorten the rods by a little bit - .015" as the block has a noticable step in each bore where the rings stoped at TDC. It seems a little insurance to keep the new rings from comming into contact with that step at higher RPMs.

I'm just amazed at how well this thing handled almost 300K miles! It makes other engines I've rebuilt seem like lawnmowers.

Matt Sheppard

Perry 951

At which oil change should I switch to detergent oil, after that first 15 min drive?

aka 951

Matt, sounds like you're having some pretty good luck. I guess it makes sense that if a 951 lasted 250k it must have been well maintained. We'll have to meet up at some point. I'd like to check out your rides and show you the Turbo S I just picked up.

Perry, those numbers seem REALLY low. Were they before the rebuild? If they are after then you may want to do a leakdown if they don't come up. I'm assuming you have stock components in the block if I remember correctly. Anyway hope it works out.

Perry 951

Matt - switch after 3000 miles.

Erick - My compression numbers were taken after the motor was rebuilt, before it was run. That is about what I was expecting. 120 is the target on a new motor with new rings and such. Once the rings bed in, it should be back up to 145.

alengyel

Matt, total seal makes a kit for the 944 for $184. I have heard good things from others, meaning ford people, on increased power. The best thing is no gap to worry about. Anyone else heard anything????

Perry 951

I was told by a few tuners not to use them for a few reasons.

1- A little blowby is good
2- I don't think they are compatible with the Alusil block.