I've got a 79 911SC, 80K miles with the fabled 915 transmission. 1st and 2nd are so notchy & baulky (sometimes) it's almost like I've never shifted a "standard" transmission car. I've gone to Swepco and haven't seen much improvement. Yesterday, I was downshifting into first, which is always a puzzle for me and the tranny, and I had about a 750 millisecond grind. A nice, smooth, even grind, with no real peak.

Question: This is the first "sustained" grind since I went to swepco about 3K miles ago. Do I need to change the fluid due to all the new little pieces I've created in the box?

 

yes you do...after you rebuild your 915

 

Swepco really is the best gear oil for the 915 transmission. It sounds like yours may be due for a rebuild, and if you're not already, you should be double clutching on downshifts (not really neccessary on upshifts to higher gears, ie, 3rd to 4th). Although, once you start double clutching, it's easiest just to do it all the time, and it certainly doesn't hurt you on upshifts.

 

I've been fearing the 915 rebuild topic....Does anyone have a ball park figure on what I should pay for a rebuild?

 

Mr. Gregory,
Why does double clutching work? Is it just the time delay that helps?

GeorgeMa
83 911SC (also difficult to shift)

 

George,

Double clutching does two things. First, as you mention, the act of clutch in, shift to neutral, clutch out, clutch in, shift to new gear, clutch out, in itself tends to slow things down just a hare (sounds like alot, but with practice doesn't take much to do), which the 915 transmission seems to respond to. A double clutch on a downshift is more important, however, than on an upshift. This is because, executed correctly, when the shifter is in neutral and the clutch is out, you want to blip the throttle to get the revs up. Just a blip. This helps to match the speed of the transmission in the new gear and the engine, and makes for a smoother power transition. Of course gear syncro's and the clutch take up much "shock" during down shifts. Double clutching with a blip just smooths things out for the transmission.

 

George,

Bill is right on (as usual), my 915 crunches every time I downshift to 2nd, unless I double clutch...and match the revs before releasing the clutch. On the track I typically downshift from 4th to second skipping 3rd and double clutching to avoid the crunch. I too need to rebuild my trans and replace the synchro's but right now there 's just no time, until then I'll be doing the double !

 

Expect to pay around 2K for a 915 rebuild. Less or more as it is opened and examined. If the hubs are notched, you can file them for a fix. Good time for a new clutch BTW. One thing not mentioned is the shift fork: this item is behind the plate help on by four nuts on the underside. Drain the trans oil, undo the nuts, and pull this plate (car in neutral). On the plate you will find (is it three?) nuts holding a fork: these bolts go loose and the shift fork wobbles internally, causing crappy shifting. Disassemble the fork from the plate, complete clean and dry, Loctite bolts to studs (DO NOT OVERTIGHTEN) and replace plate with gasket (new if the old one tore).

Now if you plan on rebuilding your 915 some time in the future, you should first ensure that your shifter ball socket bush is good, the shift coupler as well, and that the shift linkage has been correctly adjusted. Next, check that clutch pedal/cable/horseshoe spring is in good nick. These contribute greatly to smooth shifts.

But the grinding, that may be an internal job. My 915 was a bear into 2nd and 1st, and I replaced/performed everything listed above before I finally gave in and had the trans rebuilt by a professional. It shifts like new now, with some notchines that is the nature of the beast.

Jw

 

Thanks for all the great input. I'll be saving my pennies (dollars, actually) for a rebuild soon.