Manual transmition

[me3]

I've got a stock 3 spd in the truck right know. Got an old 4 spd granny low its got a pto shfeel and is like 7 inches shorter. what do I do about a drive shaft. Going to the local salvage tomarrow a whole two miles away I love that place. gunna look for a drive shaft maybe anought trans. do I need some king of a slip in the drive shaft. the truck is a long box so can I find a drive shaft that long? peace Hart

[784X4GUY]

Why don't you just have a new one made for your specific length. They have places out here that will make you a driveshaft for round 300 bucks. Id go that route if I was you...Later Mike

[me3]

um 300 bucks lets see. I bought the truck for that that means the drive shaft would be worth more that the truck. nah

[784X4GUY]

Well I guess your gonna have to go hunting then. Good luck...Later Mike

[stickshift_or_walk]

Hart,

You didn't mention your model, year, or engine (V8, L6, in which mounting position), which we'll need to be more accurate.

Your truck probably has 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. You can replace that bellhousing crossmember and go with one under the tail of the 4spd like later trucks, but it's a hassle and really not necessary for your situation.

There will be a few other issues to deal with.

As you've noted, your front driveshaft will need to be longer, since the 4spd is shorter.

You definitely need to have some slip in the driveshaft to handle the compression from suspension travel. Otherwise, your u-joints won't survive. 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 no-slip mount, you'll need a slip joint at the center support, like this which is used in 3/4 & 1 ton trucks:



If the yard you use has lots of GM trucks, you might get lucky and find the right setup. You have a couple of inches to play with due to the slip joint. I do recommend you go with the center carrier bearing as shown above. It's a NAPA item. The part that gave us the most trouble was the bracket that mounts it to the frame - it's specific to 67-72, and different from the 1/2 ton. It's hard to tell from the pic, but the bracket just bolts on and is easy to swap out.

We couldn't locate a complete donor, so we pieced together what we needed. We found a longer front shaft that will mate to the u-joint on the rear of the 4spd, and swapped the front half of the slip joint (the splined shaft, shown above) from another salvaged 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.

You also will need to match your driveshaft yokes on the new setup to the 4spd yoke and your rearend yoke, or buy the conversion u-joints. We got the conversion joints from NAPA.

Next issue: 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.

It sounds like you could have one of the following trannies. The SM420 transmission has threaded lower ears. Another likely transmission is the SM465, which came in later models. It does not have threaded lower ears, and will need the longer bolts w/nuts as mentioned above. We used the SM465 in our '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.

You'll also need to find a high-hump tranny cover, and cut out the existing low hump in the cab. Those can be tricky to find as well, but you can usually find one on the parts board if not locally.

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

SSOW

[tphsmith]

umm, lived in Nebraska for 40 years. never seen a sign to hell Nebraska but always new I was there LOL