70 Longhorn trans change

[bozracer]

I want to change my 70 longhorn from a 3 speed to a granny 4 speed.What would be involved as far as drive shaft length,bellhousing,mounts etc? Are there different granny 4speeds?The one I'm looking at came out of a '64 1 ton truck.

[stickshift_or_walk]

Bozracer,

Welcome to the board!

Your '70 should have a cast iron bellhousing, with a supporting crossmember that attaches to some ears on the side of the bellhousing. The 3spd hangs from the four bolts, and has no support under the transmission tailhousing. Factory 4-speeds came with the same configuration, just hanging from the bellhousing with no support. There will be a few issues to deal with.

First, your driveshaft will need to be lengthened, since the 4spd is shorter. Your 3spd has a slip yoke at the rear, and the 4spd likely has a solid (no-slip) u-joint trunion. If you're changing from a rear slip joint on the tranny to a non-slip mount, you'll need a slip joint at the center support, like this which is common in 3/4 & 1 ton trucks with 4spds:



Then you can find a longer front shaft that will mate to the u-joint on the rear of the 4spd, and swap the front half of the slip joint (the splined shaft, shown above) from your old front shaft to the replacement shaft. A competent machine or driveshaft shop can do this change for you. Balancing may or may not be needed, in my experience it usually isn't. Just remember, you need to have some slip in the driveshaft to handle the compression from suspension travel.

Second, the rear your bellhousing has two threaded holes at the top, and two open holes at the bottom. This is because the lower ears on the 3spd are threaded. The top bolts go in from the rear, and the lowers go in through the bottom front of the bellhousing. If your 4spd does not have threaded lower ears, you will need longer bolts and nuts for the inside of the bellhousing. Watch the length - things are pretty tight in there and you don't want anything hitting the flywheel or ring gear.

If you're looking at a GM donor, the '64 1-ton probably has a SM420 transmission, which has threaded lower ears. Another suitable transmission is the SM465, which came in later models. I used the SM465 in my '69.

One other thing to watch for is the size of the locating hole in the bellhousing vs. the front bearing retainer on the tranny. These sizes are listed on the above links, and easy to compare to your bellhousing.

Here is a good picture of various manual transmissions for comparison purposes.

Hope this helps. I've probably left something out, but someone will chime in and correct me or add on.

SSOW

[Longhorn Man]

A small detail on the longhorns...every one I have ever looked at has BOTh a slip yolk, AND the slip fitting in the center like this one.


I can't rmemeber off the top of my head if any of them were 4 speeds....but a trivial detail that may or may not matter.

[bozracer]

I've got to replace the clutch in this truck so now's the time to do the change over. I can't turn down this 4speed I found ($30 bucks w/shifter).The truck has 3 on the tree and it's begging for a granny4.The biggest thing I was worried about was the length difference so thanks for all the good info.

[JIMs70GMC]

Also need to find a high hump w/ shifter hole or cut the one you have. My 72 has a SM465 and is as described above. (133" wheel base like a longhorn)