915 transmission newbie

[banjomike]

So my 1st gear is starting to get balky when cold, and it popped out of gear the other day. My mechanic says he thinks a rebuild is in my future. I'm trying to learn what I can about 915s so I can get the best out of this situation. I've only learned a few things.

There were 915s setup for mechanical speedos and electrical speedos. I've been told mine is electrical. I don't know if this is a huge barrier. If I found a good deal on a 915 for mechanical speedos could the tranny be modified? or my speedo? I also know that there were magnesium cases and aluminum cases. Mine looks magnesium to me. Does it matter much? I also know about the two ring and pinion ratios. I think I have the lower ratio (7:31?), if I don't 1st gear with the lower r/p would be good for pulling stumps. Anyway if someone could just point me to a place where I can read about these issues I'd be a happy camper.

In a perfect world if I do rebuild it instead of finding one to buy, I might want to make it fit me just a bit better. 1st gear has always seemed very tractor like. It has been useful in very slow traffic, but a bit higher ratio might be nice. On the other hand a taller 1st gear might tempt me to wring it out in 1st more often leading me back to rebuild terrirory quickly. Also a taller 5th gear would be nice for the commute and my gas milage. Finally on those few days a year when I get to autocross it I often wish I had just a bit more in 2nd. Maybe I just want an SC tranny to match my SC engine.

[IROC]

Lots of good questions. One thing that I will throw in is that the 915 in your car should be mag-cased with an 8:31 R&P. 7:31s only came in the '73 and '74 915s. It also *should* be one for an electric speedo, but not necessarily. My '76 has a mechanical speedo.

IMHO, if you are facing the dilemma of rebuild or buy a different 915 to swap in - I would strongly consider the rebuild route. It's not that expensive and you'd end up with a known quantity. Changing gear ratios can get expensive, too and you might find that what works great in some cases sucks in others...

One thing I often pondered (when my car still had it's 2.7) is a 7:31 and a taller 5th gear. That would have been really nice...

Good luck,

Mike

[3.2 CAB]

If you do decide to swap it out, try to find a good 915/67 trans. Has a little different ratios, plus an oil cooler.

[ChrisBennet]

I agree with Mike on the get it rebuilt recommendation. For someone of my means (or lack thereof) a rebuild is expensive but it's still a better value than buying another used transmission IMO. Think of it this way, you already have a used transmission, why would you want another one?
As for 1st gear, you can't change that without changing the mainshaft. (It's part of the mainshaft.) I really like that super low 1st for going slow in the paddock or in traffic without using the brakes.
-Chris

[addictionMS]

>If I found a good deal on a 915 for mechanical speedos could the tranny be modified? or my speedo?

It is difficult to modify the trani, there is a machinse spot on a electrical speedo trani where a sensor sits, it is a magnetic pick up, so there also needs to be the magnetic ring on the diff. To convert your speedo you will also need to "rewire" for the other solution.

>I also know that there were magnesium cases and aluminum cases. Mine looks magnesium to me. Does it matter much?

Mag is lighter, AL is stronger. unless you are racing it will not matter much. If you are racing, you will need to decide if weight or strenght is more important

>I also know about the two ring and pinion ratios. I think I have the lower ratio (7:31?), if I don't 1st gear with the lower r/p would be good for pulling stumps. Anyway if someone could just point me to a place where I can read about these issues I'd be a happy camper.

Pretty sure 1st is the same between 7:31 and 8:31, so 7:31 would be a little quicker off the line, but will require shifting sooner try reading
this there are stock gear ratios listed in the thread so you can see the differences between 7:31 and 8:31 first on both is 11:35

cheers

Jim