924Board.org

[Beartooth]

Well, not too much major I've been doing the last couple weeks. I've been driving it regularly, and that's been good... other than almost putting it through a fence last Sunday. One of the gravel back roads near me has a ninety degree turn immediately at the end of a long, straight uphill grade - you can't see it until you crest the rise, around 100 feet before the turn. I want to say there used to be a sign warning of the turn, but there definitely isn't now. I was cruising along, just thinking "that turn's gotta be coming up" when I crested at around sixty MPH. I slammed on the brakes with no hope of making the turn and did my best to steer between the fence posts, bounced over a hump built up at the outside of the turn - all the time thinking I was going through the barb wire fence 20-30' beyond - and somehow (maybe it was the hand of God) stopped only a few feet from the fence. Anyway, I haven't had time to tackle anything more than a couple small fix-its. I'm too busy trying to get my shop sided, roofed, and closed in before winter really hits. I did replace the broken door stop bracket and put some grease on the door stop check strap (the finger that sticks out of the door to limit how far it opens). It still seems to take a bit much effort to pull closed from fully open; not sure if that's what broke it to begin with, but I might replace it anyway.

I also fixed my dying cat fan (the radiator fan - and now the heater fan is starting to do the same thing). I found a thread about using a motor and fan blade from a Toyota, and happened across one on closeout. It turns out there's a difference between one for a manual and for automatic transmission, and the dimensions for the one I got (auto) listed were wrong. I think it could work if I ground the opening in the shroud/mount, but it can't use it with the original fan blade, and given the trouble, I decided to tinker with the original motor. I'd originally tried taking the old motor apart, but the crimpings on the end caps didn't want to come apart without more effort or destruction than it was worth. Ultimately, I got some 3-in-1 oil that specified "for electric motor bearings" and lubricated the shafts. The front is no problem, although you have to remove the fan blade. For the rear, I drilled a hole, then oiled it and put a little rubber plug in the hole. I drilled it dead center, but that might not be the best because the shaft is right there. Anyway, the fan works fine now. We'll see how long it goes; I might go to the trouble to use the motor I bought if it needs oiling frequently. One other note: the fan is noticeably weaker running off the original wiring than hot-wired. For anyone dealing with marginal cooling, it'd be worth starting with some heavier wiring and a relay. You'd need it anyway if you went to a more powerful aftermarket fan. Oh, and while I'm on the topic, be aware that on a 931 the clearance between the fan shroud and air filter box is minimal. I had to re-think the mounting on the aftermarket fan I used for a while because it initially stuck about an inch above the radiator flange and interfered with the air filter.

Anyway, I've put well over a thousand miles on it now, and it's working pretty nicely. It does sometimes have a hard start after sitting a couple hours, but no other drama with starting, running, and driving. The other thing that's a little alarming is the oil consumption. I haven't kept perfect track, but I'd say I'm getting 200-250 miles per quart. My first car burned a quart every 200 miles, which I thought was pretty bad, but it's arguably a much lower rate with over twice the displacement (and associated ring/cylinder area) and twice as many valves. Now, this is running 5W-40 (Mobil 1 euro spec). I just started adding 15W-50 instead of lighter oil, and if that helps, I'll just stick to it. The oil pressure, per the gauge, is at least 4 bar (hot) at 2000 RPM, and usually closer to 5 bar, so I wouldn't say the motor is in dire straights. There's also surprisingly little smoke from the exhaust (given the burn rate). Really, the only time I can see anything significant is if I look in the mirror while I'm hard into it, and that looks like black smoke - could be more from how rich it runs under boost than oil burning. It's not the end of the world - I'm not embarking on a rebuild just yet. That said, if the parts were as easy to come by as typical American iron, I might be considering it more strongly. I do know these consume some oil right from the factory, but the rate I'm seeing suggests pretty significant wear. I guess I shouldn't be surprised, let alone disappointed: given what I started with, getting this far is hardly something I could have counted on.
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1980 931 diamond in the rough

[Beartooth]

I did a leakdown test today, and the results were... a bit depressing. 55% in cylinder 4, 30% in two others, but one (3, I think) was astoundingly good at 5% (that's 95 out of 100 PSI on the gauge, then 70 and 70 on the next two, then 45). I'd be ok with 5% even on a rebuild unless it was a tight, high-dollar, mega-build of some kind (I've read of 1-2%, even <1% with perfectly machined cylinders and a modern ring package, but I bet 5% would be a typical number on a factory 931). For reference, using the same gauge on my 560 with well over 200K miles, I got a max of 12% and most cylinders at 6-8%.

So at least I know what the best case is, that you could have pistons that seal like new if you're very lucky (I guess four times more lucky than me) or have a very well cared-for engine. In my case, this completely explains my oil consumption, and may be why it doesn't idle as smoothly as I'd like. That said, I'm a little amazed I could have three cylinders at 30% (or worse) leakdown and still have it run as well as it does; if it had a really bad idle and was rough in low-RPM, part-throttle situations, I'd be saying "a-ha!" I've found several references to S1 engines particularly tending to break the top ring, and I assume that's the case (and probably more) on #4, and likely on 1/2.

If it were American iron, the decision to pull it and rebuild it would be automatic. To be honest, I'm still leaning that way, but the lack of good options for pistons (given my budget) is a problem unless I can re-use mine. If they were all bad (or I need to overbore), I'm not sure if I could proceed, given what people are asking for USED pistons. I might do better finding a replacement motor at some point, but I probably have one of two 931s in the state of MT; maybe there's one next door in WY... Speaking of pistons though, if I could find S2 pistons, I'd actually be more interested. I'm not looking to push a ton of boost, plus I'm hoping, down the road at least, to convert to a microsquirt setup with EDIS and a knock sensor. Being at some elevation and tending to use the car as a runabout (when the weather is good), the gas mileage and torque advantage are worth something to me.

Anyway, for you guys who've turn down a motor like this, what did you do, and how did it turn out? Assuming the pistons aren't trashed, could it be that I could give the cylinders a light hone, re-ring it, and have a sound motor? I do see some possibilities for refurbishing the pistons: one can have ring grooves that aren't square machined and a spacer installed (opening some piston ring options), and the skirts can be coated to take up piston-cylinder clearance (to some degree). Tangentially, if I had the skirts coated, I'd likely ceramic-coat the crowns (Swain Tech offers both, and is well regarded). Unfortunately, if I had to go deep down that rabbit hole, I might again be pricing myself out of the market...

Well, what do you think? Is it as bad as I think, and am I dreaming to have some hope of doing a rebuild on the cheap? There is the temptation to just keep running it until it becomes unbearable, but at some point (maybe even if I baby it) it's going to shell something out, and might leave me searching for a lot of hard-to-find parts instead of a few...
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1980 931 diamond in the rough

[safe]

Where did it leak?
Piston rings, intake valves, exhaust valves? (listen to where it leaks)

I would expect a good NA engine to be less than 5%. I did a leakdown on my 911 -84 engine and it had <2 % on all cylinders with original piston rings and ~150.000 km, heads have been refreshed 80.000 km ago.
A friend have a 951 engine built for power, he had quite a lot more, 15% I think, but that had a much larger ring gap to handle the extra heat.

I've taken apart 5 NA 924 engines and only 2 of them have had pistons with usable ring lands.
If the ring lands are ok and the cylinders are still round within tolerance you could do a quick hone and some new rings. That's what I did with my turbo conversion.

The ring ridge (is that the correct English term for the lip/ridge left in the cylinders by the rings?) in the cylinders should be minimal to so it can be removed by the hone, otherwise the new rings can brake.

I would investigate more on there you have your leaks. Squirt a little oil down the bores to see if it improves (ring seal).
I would guess that your valves are a problem too.
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe

[Beartooth]

Thanks for the tips. I removed the oil cap and could hear the air leaking inside the crankcase. It was noticeable to begin with, and it got louder when I took off the oil cap. I'm pretty sure I could even feel the airflow out of the cap, especially on 4. Each time I checked a cylinder, I opened the throttle body (I couldn't hear anything from the intake on any) and listening at the tailpipe (again, didn't hear anything; maybe too far thought). I believe you've got it right, ridge is the right term for the lip that gets worn in the cylinder where the top ring travel stops. The tool for removing it is called a ridge reamer.

I've found a lot of things about what you should see for leakdown. Most sources say less than 5% is good to excellent. What's "bad" varies even more: anything from more than 5% to some suggesting 10-15% can be ok. I think I forgot to mention the engine was warm; probably not at full operating temperature, but I'd driven it this morning, pulled it in, and went to checking the leakdown shortly after.
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1980 931 diamond in the rough

[Mclaren924]

[Beartooth]

That's good news Mclaren, I'll keep it in mind. At this point, I'm thinking I'll tear into it this winter when the weather means I won't want to drive it anyway. I'd be very surprised if I've got four reusable pistons, but if at least half were sound, then picking up a couple more would make sense (even, maybe especially if one or two were oversize: then I could overbore the worst cylinder or two). I could probably scrape together enough to do a rebuild with custom pistons, but that might not leave me with much for other things. I might also consider the VW flat-top piston option: the loss of quench is definitely not ideal, but judging by the results others on here have achieved with them, it's at least a decent stop-gap solution until I could build the kind of motor I'd really like to. For that, I'll probably dig up a core to start building off of, which would allow me to build as time and money allow. It's too bad there's so few options, but hey, if it were as cheap and easy as a Chevy, everyone would be driving one!
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1980 931 diamond in the rough

[Cedric]

There's been a fair few s1s with cracked rings, including my original engine, alll top rings were in pieces. I dont know if i ever seen it on an s2, its possible they changed the material or supplier.

In the us you have so many piston suppliers, making a custom set isnt unreasonable money wise. Or team up with others that needs them and make several sets.
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1980 924 Turbo
www.instagram.com/garagecedric/

[Beartooth]

From what I'm gathering, the S1s rings are pretty fragile, and I know my car lived a rough life. Maybe they would have been tough enough on a NA motor, while the turbo is too much for them, but it seems Porsche realized something was wrong and corrected it if S2s don't break rings nearly as much. Maybe it was DITC being able to protect the motor better, but I'd think you'd see more S1 motors with completely blown pistons if that were the case. It's hard to imagine an engine living on the edge of detonation enough to break rings while not causing significant damage to the crown. Anyway, I'll have to check with a couple places and just see what custom pistons would cost. I'm assuming somewhere in the range of $1500, but that's just a rough guess. It's a pretty unusual piston - you just don't find that big a dish many places - thus the lack of an off-the-shelf substitute despite many falling in the overbore range, and I'm sure the unusual dish would drive up the cost of a custom piston.
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1980 931 diamond in the rough

[Cedric]

[Fifty50Plus]

Diamond made some pistons for the 931 engines and some higher compression pistons for the NA engines. I believe Vaughan had some made as did I. Dan sold some of the turbo version Diamond pistons.
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car

[Mclaren924]

Has anyone punched the block out to .30 over and ran forged pistons? Tossing around ideas myself of forged pistons in my car and wondering if its worth my time going to max oversize for that bit of extra displacement? Either way block would be getting machined and honed so it wouldn't cost much machine shop wise, more worried about potential issues I could run into. I could be interested in splitting a set of 8 with you if we can get a piston that suits both our needs as well Bear.
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1980 931 "Salt" Bucket wannabe racer (not started)
1979 924 Sebring "Pepper" -Sold
1980 924 "Donnie" (Parts car)-DEAD
1977 924 Slicktop "Pennie" Bucket turned Silver Spoon

[Beartooth]

I've been digging, and found a few details online about 931 pistons. Let me know if I've got anything wrong here, but the ring package is 1.5 (top), 1.75 (2nd), and 3.0MM (oil ring), vs. 2.0/2.5/5.0 on the NA. The 931 ring package, as you noted Cedric, was pretty thin for its day. 1.5MM is pretty common now, but maybe it was a big step for Porsche back then. Additionally, the pin is offset 1.5MM, for unknown reasons. I found that, and a few more details here: https://www.porscherepair.us/porsche-924-turbo-service-2/pistons.html.

Ideola's pistons were made by Diamond, so that would be one place to look into. JE pistons seem to be the first go-to, so I'd check with them too. It does seem you either have to give them all the specs (and get them right, obviously) to have a set made, or it a couple places list them for sale, but are just a middle-man with the specs on hand to order a set for you. One manufacturer I'll reach out to if I get serious is a newer company, Gibtec Pistons. I'd think they're exactly what you'd want for a one-off set or small order; it seems they specialize in custom pistons, rather than being a manufacturer who offers custom pistons on the side. https://gibtecpistons.com/?page_id=77

I'd definitely be in on a group buy if we could get enough together to make a difference. I'm not sure we'd be able to get enough quantity to really open doors and/or drop prices, but it's worth a try. The trouble is, I'd think 50, or ideally 100 pistons would be the kind of numbers you'd need to get the most significant price breaks and favorable treatment. Bankrolling that many pistons makes sense once you get enough demand, but for the 931 it must be too sporadic, or somebody like ideola would still be doing it. That said, you'd think one of the manufacturers who's done it would put the pattern in their catalog so you could just order a set without having to supply the specs. I can't see much cost in keeping the specs on hand, and being able to just hit "buy" would get you some sales.

On the topic of the bore, what I found in other threads is people run into other issues before running out of "meat" in the block. It seems 88mm is the standard diameter for head gaskets, and given production error margins and such, you'd probably want to stay short of that, or have a custom gasket made. There doesn't seem to be anything to lose by going to the largest factory overbore size (87.5mm) though. That's about a .040 overbore. There was on particular post from someone who'd had a block sonic checked for wall thickness, and found enough material to go even bigger without concerns.

Kind of immaterial, but this discussion reminds me of something I heard about this summer: 3D printed pistons. They already exist, and are strong enough for most things (it sounds like they're not malleable like 2618 - probably more similar to 4032 forgings). The trouble is, while the technology exists, it's almost completely one-off, and maybe a decade away, at best, from sufficient production capacity to reach the likes of us. But it's very promising: you could have a piston that exceeds the needs of almost any street vehicle, in almost any size, for any application, for the price of a mass-produced forged piston. https://newsroom.mahle.com/press/en/press-releases/mahle-produces-high-performance-aluminum-pistons-using-3d-printing-for-the-first-time-76416
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1980 931 diamond in the rough

[safe]

Usually rings break from detonation or if they get unseated, when cylinder pressures gets past the top ring and lift the ring.

The pin offset is very common, its there to get the forces more inline with the rods and crank to lessen the wear between the piston and bore.

I have a block bored to 87.5 mm with new forged pistons. Its not put together yet, I'm still lacking new rods (and the stock engine is still holding together despite 29 psi...).

I think Gegge measured a block, didn't he? 88 mm would be ok, maybe 89-90, can't remember. Depends on what you want to do with it...
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe

[Mclaren924]

[daniel]

I use these in my race engine:

https://pauter.com/shop/rods/porsche/porsche-924-22mm-pin-rods/

Outstanding product and equally good people.
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Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet