What is the Best Way to Change SC trans to 7:31?

[pbs911]

I want to go with 7:31 gears found in the early cars for my SC, but I don't want to lose the tall gears of 4th and 5th. Here are the options i have found so far:

1. Just change the R&P in my existing trans. But then I have the lower 4th and 5th gears (4500 rpms at freeway cruzing, but I keep the electrical speedo).

2. JW recommended a 76 912 trans because it has the 7:31 R&P and electric speedo, but other internal gears and a 901 main shaft (?) have to be changed. Might as well rebuilt my existing trans and change the R&P, right?

3. Find a good used trans, numbers 915/08 and lower. It already has the 7:31 R&P and taller 4th and 5th that provide the same freeway crusing RPMS as my current SC trans. But it has a mechanical speedo and would have to changed to electronic. Can I just get a mechanical speedo from an early car and slip it into my SC?

4. Rebuild my existing trans with new all new gears and keep the stronger 8:35 (?) R&P. $$$$

I would like to do a buy and swap. And I don't want to spend any more than necessary. The most cost effective swap seems to be No. 3 if I can use an early mechanical speedo. Is this feasible or are other options better?

The trans will be bolted to my 3.0 with an estimated whopping 210 hp so the limited strength of the 7:31 R&P is not a major concern. Thanks.

[marcesq]

Paul:

I'm right behind you. I looked into this and it appears that adding a taller 5th gear and maybe a taller 4th would be the way I would go since my 915 is getting to the rebuild stage anyway.

No question that the 7/31 will add a little whoosh out of the box.

Hope some can shead some light on this.

Regards,

[1980SC]

From what I've read, the earlier 7:31 gearbox doesn't just bolt into an SC because the axles don't match. The earlier output shafts have 4 bolt CV flanges, while the SC's rear trailing arm takes the later style 6 bolt CV flange. I believe the axles are different diameter, so you can't just swap CV joints. I assume this is resolved by machining some sort of adapter plate. I'd really like to hear how it is done.

Our own "JanusCole" has swapped a 7:31 R&P into his aluminum SC gearbox even though he was told it wasn't a good idea to do it himself without the special tools. He also swapped in the doner transmission's 5th gear so it would be tall enough for highway use.

I think gearsets run around $500/set, so changing 2nd,3rd,+4th to whatever you want could be done for $1500 yourself.

Rob
1980SC

[Chuckar]

Any luck with this project / research. The 915 is out of my 80SC so I'm in the midst of this same decision?

What are the negatives of just puting in the 7:31, versus a regear of 1st - 4rd?

hmm


JB

[pbs911]

After a little more investigation, the most reasoanble way to approach this is replacing the gears and not going with the 7/31 R&P.

The 7/31 is weeker and 5th gear will cause the RPMS to run at about 4.5K while crusing on the freeway @ about 75-80 mph. Too high for me.

After reading a couple manuals regarding the gear replacement it does not seem that difficult. My plans are to pick up a spare SC 915 and do a rebuild and gear swap to get as close as possible to the 7/31 ratio's as possible on a limited budget.

[marcesq]

Ok Paul . . . now all we need is a tech article once you are done.

Good luck.

[TRE Cup]

You are correct Paul, the 7:31 with early mainshaft is a weaker combo. Prone to breakage or early wear when big horsepower /torque is applied. Jack Olsen's first gearbox had a lot of spalling on the final gears that essentially rendered it useless.
Make sure to inspect all the parts closely before reassembly and get them mag inspected for hidden stress risers (lil cracks).