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 70/70, 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 lands 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