3 speed to 4 speed swap

[gregg72340]

hi Guys

Interested in swapping my 68 GMC with a 292 from a 3 speed to a 4 speed. As far as the driveshaft goes is there any differences, or can I reuse what I have? Also want to know what 4 speed should I use and what years I might find them in. Is there an overdrive version?

Thanks for the help

[oldblue1968chevy]

4spd like that originally came in these trucks or a car 4 spd?

[FL71C10]

I asked a similar question a few weeks ago and got some excellent answers. Here's a link to the thread.

http://67-72chevytrucks.com/vboard/s...d.php?t=407083

[capev86]

i don't see any value in a 4 speed over a 3 speed unless it has a granny gear and you have a super soft (highway) gear in the back. a 5 or 6 speed is much better. i have the 3 on the tree in my 72 burb and the only thing it needs is an overdrive (will be fixing that evehtually w/ a vintage borg warner unit). if it had been a 4 on the floor....it would have been gone months ago in favor of a nice modern stick with an overdrive. i also like having a synchro first gear.

[gregg72340]

Ok I have a follow up question to my initial question here guys, I picked up a 4 speed complete for $30 from a guy cleaning out his barn this week. Will a 4 speed work with a low hump 2 wd truck or do I need to swap in a high hump?

Thanks
Gregg

[rsavage]

If it is a truck 4 speed you will need a high hump. Car 4 speed, like an M20 or M21, will go in with a low hump.

[oldblue1968chevy]

so is it a car one for 30 bucks!!?!?!?!?!?!??

[gregg72340]

Its a truck one.....oh well guess Im puttin a hump in

[gregg72340]

Ok I swear this is my final follow up question......can I use my 3 speed beel housing or is the 4 speed one different?

Thanks again guys

[raycow]

Originally Posted by gregg72340

can I use my 3 speed beel housing or is the 4 speed one different?

If you have a stock 68 bellhousing now, any 68 or later truck 4 speed (SM465) will bolt up to it without modification. If you want to use a 67 or earlier truck 4 speed (SM420) or any year car 4 speed, you will need a spacer ring like this.
http://dennymac.com/mod.php?mod=user...103&page_id=54

Ray

[gregg72340]

One more question can I reuse my 3 speed driveshaft or is the 4 spd a differnt length

[rsavage]

The truck 4 speed sm465 has a yoke bolted to the tailshaft much like the yoke on the differential. A two piece drive shaft is used because the sliding portion is on the rear section after the carrier bearing. I think the 4 speed truck transmission is shorter than the 3 speed. I have one of each out of vehicles but didn't measure so I am not absolutely positive. In any event, the driveshaft, if a two piece will have to have a u joint that adapts to the bigger transmission yoke and your driveshaft front yoke and it may need the front tube lengthened. If you are going to use the truck 4 speed, you will need the 2 piece shaft. A car 4 speed, like an m20 or m21 is pretty close in length to the 3 speed, but will need a fine spline slip yoke for the transmission (27 I think) as opposed to the 11 spline yoke on your 3 speed. The drive shaft, if I remember correctly, had to be shortened slightly on my 66 which was a 3 speed with a one piece shaft.

[raycow]

Originally Posted by gregg72340

One more question can I reuse my 3 speed driveshaft or is the 4 spd a differnt length

Short answer: You will need a 4 speed driveshaft, or the front section of one if you have a 2 piece shaft now.

Long answer: The 4 speed shaft is longer than the 3 speed, but the main difference is in the way it is constructed. The 4 speed transmission has a yoke rigidly bolted to its output shaft. It looks something like the pinion yoke on the rear end. The front U-joint bolts to that yoke. To accommodate length changes, the 4 speed shaft has a slip joint behind the front U-joint. This slip joint serves the same purpose as the slip yoke on the 3 speed shaft, but is not interchangeable with it.

Ray