Advice sought on 915 rebuild scope creep...

[Quasimoto]

I've read quite a few threads about scope creep when dropping the engine on these cars, so thought I would seek some advice to help shape my plans.

Car is an 84 M491 with 57k on the clock. Engine runs very well, with no smoke, however, the 915 definitely needs a rebuild. The synchros are marginal at best and gears whine a bit more than other similar aged cars I've been in.

Is it reasonable to think that I could pull this thing out, fix the transmission and zip it back up, or is it a case of once I'm on the slippery slope that there's no where to quit short of a complete overhaul? The car could use a restoration at some point, but in the near term, I'd love to fix the trans and drive it for a season or two before committing to a larger cleanup.

I've attached a couple of pics that may complicate question a bit. There are two cylinders that weep a bit of oil at the base. Never enough to leave a drop on the floor, but I'd like some opinion over whether this is normal, or indicative of a stud problem.

Caveats:
My background is American musclecars, so I know I will have to use a mix of DIY and professional help to get a result.

Although I have some records from purchase, I'm not confident about the actual mileage of this car. On one hand the engine is sound, on the other hand, could the synchros be gone at this mileage?


Any thoughts or advice are appreciated...Thanks!

[offroader1006]

The synchros could easily be gone by 57k. It wouldn't take 2k, with a bad driver.

I feel like, if you're taking it out, go through it once, fully, and be done. Replace whatever it needs and drive with peace of mind. They are really simple gearboxes, just have a few quirks.

[Trackrash]

Aside from a complete nut and bolt restoration you can think in terms of these cars in their basic components. Assuming your car is basically rust free and you are not wanting a color change, I would just attend to the issue at hand.

There is no reason why you can't rebuild the trans and be done with it. However, with the motor out it may be time to take care of some maintenance. How old are the rubber fuel lines on the motor? Good time for a valve adjustment? Fix a couple of oil leaks? What is the clutch like?

To answer your question about the weeping cylinders, when you adjust your valves check for broken head studs. Now would be a good time to check. Otherwise that looks to be fairly normal seepage.

If the other systems are in good order, like brakes, suspension bushings, or shocks, leave them for another project. If it ain't broke, don't fix it.

[Bill Douglas]

I think the gearbox just needs new oil. I use swepco.

[evan9eleven]

Scope creep? More like scope avalanche... ask me how I know.

If you are going to do synchros, you really should replace the sliders too, as these and the synchros wear into each other over time. Change just one and the shifting may not be optimal. Once you get into the box you will also see what else is worn. Differential housing bearings may need changing, if the pinion shaft bearing race is loose the case will need machining. And so on. You really don't want to tear it apart again anytime soon, so if you can justify the cost it is definitely better to rebuild it all at once.

As another poster mentioned, there is lots more to consider while the motor is out. Fuel lines, motor mounts, CV joints, shift coupling, valve adjustment, cleaning and sealing up leaks, etc.

[175K911]

Originally Posted by Bill Douglas

I think the gearbox just needs new oil. I use swepco.

Excellent advice! Before going to the trouble and expense to pull everything apart, try a simple drain and fill of the transmission oil. Who knows what fluid is in there right now. It could be original and just plain worn out. Or someone could have changed it using the wrong gear oil like Mobil 1 or many others. Order up Swepco 201 from our host here, and give it a try. As someone who has tried other fluids in the past, I learned many many years ago that this is right fluid for a 915 gear box.
https://www.pelicanparts.com/More_Info/SWE-201-80W90&SuperCat=Y&SVSVSI=.htm

[Walt Fricke]

If my engines were as clean as that I'd think I'd died and gone to heaven.

Checking for broken head studs, for sure. If you find one or more, now you are in for more engine work. I was actually able to replace a broken stud with the engine in the car, but only because of which one it was - with some grinding on some surrounding aluminum I could get a vice grip on the stub, but it might have been the only one in the whole engine where that could be done after the air deflector was removed (a chore in itself on an assembled engine).

As for the 915, the three shift synchronizing parts are the slider, which mates with the "dog" teeth pressed into the actual gear. Both of those pretty much wear together, and if one is buggered, the other is also. As the synchro ring (the other main part here) wears it looses its ability to synchronize, which leads to wear on the teeth, so that is the third part.

But why not try draining and looking at how much fuzz there is on the magnetic plug, and seeing if new Swepco makes a difference. I'd not put a lot of money on that, but if it works it sure beats the alternative.

[rwest]

Scope creep doesn’t have to happen; the “while you’re in there” thought process is really not a big deal as you can remove the motors on these cars pretty easily/quickly, so just do what you need to, drive it and evaluate over time.

Now, if you’re paying a shop to remove the engine, then you might want to do a bunch of simple things since you paid to have the thing removed.

[Quasimoto]

Great feedback thus far...there needs to be a "like" button for posts...

Should have mentioned that I've already gone partially down the 915 rabbit hole...

1. Refurbished frozen clutch linkage, new bushings, Omega spring, etc.
2. New clutch cable and pedal bushings.
3. Wevo Shifter and PSJ
4. Swepco - a bit of fuzz on the magnet, but particle size was super small...no big stuff at all. Maybe had been done before I bought the car, dunno.

All of these items made nominal improvements in shifting and driveability, but for example, the 2-1 downshift is only possible at crawling speeds, and then only gingerly.

My next item was to be mounts, but I think I finally accepted that the box needs to come out and be rebuilt, so thought I would do that then.

[Bob Kontak]

Originally Posted by 175K911

Excellent advice! Before going to the trouble and expense to pull everything apart, try a simple drain and fill of the transmission oil. Who knows what fluid is in there right now.

+1

In the wayback, 97-ish before Pelican, I read of SWEPCO.

I bought some. Yes, I did. SWEPCO 210. Not 201.

When I sent the trans to Mayo in '08, Tom P told me that 210 was great for gears but not what you want for the 915 synchros. (Perhaps it was Matt Monson in a later post after the dust settled?)

Anyhoo, even with the best intentions, less than optimal fluid can be utilized.

All of that said, Mayo performed the labor for $750 (2008 money). I pulled the engine and UPS'd the trans to them. $2,700 all in including 201 and shipping. A couple of $200 parts on top of the synchros and a new TOB fork.

I am saying this because the cost of the grunt work for a skilled technician is not that bad. Meaning, don't bother with doing it yourself. Just find the right one.

[Jack Stands]

I’d also recommend shipping it to a shop that does these all the time. My ‘84 Targa needed a first gear synchro replaced, but we also did the clutch, etc. as the car had 134,000 miles on it. I would do the fuel lines on the engine, valve adjustment, etc. as it will be much easier as long as the engine is removed.

[Walt Fricke]

If the 2 to 1 downshift was the only real issue, I'd not bother much. You don't do that too often, and not in a hurry. The 2 to 3 upshift is where you are apt to rush things and damage stuff. Incrementally, to be sure, but it adds up. Porsche redid the teeth on the 1st gear side of the 1/2 slider to try to make this downshift work better, but I think it can still be a problem due to other, more obscure, wear.

But if you get a groink noise on just about every 2-3 upshift, or something like that on some downshifts, the wear is adding up. If you train yourself to double clutch (not just heel and toe), you can get along for quite a while with a non-functional synchro system. Eventually, wear from those teeth rubbing on each other before sliding into position will cause it to pop out of gear on acceleration, though.

[Quasimoto]

Both 1-2 & 2-3 have to be done fairly gingerly. Double clutching, or even just a pause at the gate seems to help, but cumulatively these things kind of take some of the pleasure away. I've driven another 915 that wasn't nearly this cranky to get along with.

(Not minimizing anyone's input on this at all, I just think I've reached the event horizon where I think the box needs work)

[MichaelSJackson]

915 synchros don't like slippery trans oil, so Mobil is like the worst. Make sure you're using the correct Swepco. You probably don't know what the PO put in there?

Everybody complains about the shifts. Even at its best, the 915 is balky compared to almost everything. The 2-1 shift can't be done without rev matching. Just always stop in 2nd; then shift to 1st.

Always double clutch upshifts. Or try to learn to wait for a count of one while passing thru neutral. Naw, double clutch.

But maybe your synchros do need to be fixed. Since you're there, be tempted to replace/upgrade the clutch and pressure plate. But definitely spend on a new throw-out bearing. You'll kick yourself if it goes and you have to drop the engine again.

Scope Creep (while tranny is open): Wevo gate shift, LSD, different ratios

minor creep without dropping the engine or tranny: new short shifter, new shift bushings under the shifter, new coupler at the back of the tunnel, new stiffer engine mounts

While engine is out: adjust the valves

[Matt Monson]

Originally Posted by Quasimoto

Both 1-2 & 2-3 have to be done fairly gingerly. Double clutching, or even just a pause at the gate seems to help, but cumulatively these things kind of take some of the pleasure away. I've driven another 915 that wasn't nearly this cranky to get along with.

(Not minimizing anyone's input on this at all, I just think I've reached the event horizon where I think the box needs work)

What rpm do you make your upshift at?

[Bill Douglas]

I still think the gearbox has too few miles on it to need a tear down. Why not put some swepco or other correct grade mineral oil in it, drive it a while then drain that and put some more in. In other words flush out any additive that may be making it slippery that is not letting the syncros work. It's a cheap first try at fixing the problem.

[Coloradocurt]

I note in your thread that you mention having a muscle car background, so that could be a source of the problem (either in your situation, or more generally with many 911/915 drivers). Detroit muscle car floor-mounted stick shifts are intended for the driver to just grab the shift knob and "yank" it into the next gear (such as in drag racing). Over multiple decades of driving 915-equipped 911's I've never needed to overhaul the tranny, and I attribute that to a shifting style taught to me. Rather then maintaining a constant amount of force on the shift knob through the entire motion, I just reduce that pressure slightly as I pass through neutral (and to this day I use that motion even on my slick-shifting Honda Civic Si and S2000).
I also agree about trying the Swepco 201.

[Mike Andrew]

Scope creep seems to be the norm for most of us. I think that it just happens as we find little niggling things and think, "why not"? Our cars are old and all have little issues.

My synchros were bad at 38K, especially 1 - 2. Pulled everything and sent to John Walker year three of ownership.Certainly Matt could go thru it as well. I just did not want to do it myself and screw something up resulting in another drop and sending it out to a pro. Seems like you and I both know our limits with a wrench and when to use a pro.

I would try new lube and see if this helps as it is clearly the least expensive option and may well solve or mitigate the problem.

Also, even after rebuild, I almost never do a 2 -1 downshift unless creeping and with a good blip.

[Quasimoto]

Some more background in post #9

Thanks all!

[175K911]

Originally Posted by Quasimoto

Some more background in post #9

Thanks all!

I read that, wasn’t sure if that meant you’ve already done a drain and fill with fresh Swepco 201?
And I’ve never owned a 915 that was easy to downshift 2-1 without being at a crawl or dead stop. And I’ve had 3 911’s I bought brand new- ‘77 911S, ‘83SC, ‘86 Carrera. None of them could be downshifted into 1st at any other speed that absolute crawl.

Would be interesting for someone who’s put lots of miles on a 915 car to try yours just for another point of view.