VCCA Home Forums TECHNICAL - GENERAL Trucks and Vans 51 3to 4 speed swap

my daily driver is my51 1/2 ton with a 3 on the tree.My tranny crapped out to day for good and recent searches at salvage yards for an original replacment have been unsucsesfull.Iwas under the asumtion that 4 speeds on the floor were only put in 3/4 tons.A friend told me they were options in 1/2 tons also.Is this true?Will a 4 speed be a direct swap with out modifying or changeing the original driveshaft?Are the 4 speeds syncronised?Any help woold be greatly apreceated as i hate walking in the winter.Thanks Ron.

The 4 speed was optional in 1/2 and 3/4 tons-standard in the 1 ton.To convert you will need the transmission and U joint.Also the rear extension may be different depending on what the 4 speed was in.Also the year as the mount could be different.

I would not consider putting a 4-speed in the 1/2 ton. The 4-speed has two gears you don't need. It is probably a bolt in, however it will require you to cut a hole in the floorboard.

Unless your 3-speed is total trash you would be better off rebuilding it. I would make a bet that everything you need to rebuild is available with a bit of searching.

Slo - I have several AD vintage trucks with both versions of trannys. They are interchangeable as far as bolting in, but as Ray mentioned, you will need a new floor pan to use the SM420 4 spd. The floor pans are plentiful in wrecking yards while the metal 3 spd pans are not. Don't cut a hole in your pan - rather save it for someone who doesn't want to use a repopped fiberglass one.

As far as driveability goes, the 4spd basically can be driven 2nd gear for starting and then 3rd and 4th for road driving. I prefer the 4 spd as you then have a granny gear for heavy loads especially when tooling around on the back 40. 2nd and 3rd gear are similar to the 3 spds 1st and 2nd gear as far as driving goes.

First and reverse are non-synchro, the other gears are synchronized. I believe that any SM420 from 47 to 54 from a 1/2ton - 1ton Chevrolet or GMC truck should fit fine. Just make sure it has the bell shaped tail shaft to accomodate the torque tube and you will be in good shape. The 3/4 and 1 ton trucks had a short enclosed driveline and the 1/2 ton GMC longbeds had a short extension shaft, but the trannys should all be the same.

Rebuild kits can be had from a number of sources and the 4wd crowd have put up quite a few websites with information on rebuilding these transmissions.

Have fun with that swap!

Thanks for the information gentelman,thats just what i needed to know.Im going to pull the 3 speed and crack it open on friday and see where to go from there.At first it would ocasionaly jump out of third.On the way to work yesterday it kept poping out so i put my hand on it and it stayed in gear.Onthe way home it wouldnt stay there even holding the shifter down. Ron

Before you get your hands greasy, check the linkage which may be so worn that it is the problem. If you don't find anything then you can get the dirty hands.

I got a pile of 4 speeds avaliable.

Email me for more info.

Back in HS I had a 57 BelAir with the 3 spd that would not stay in 3rd gear at times then got worse where you had to pull back on the shifter to get it to stay in. Check the shift forks inside the side cover for excessive wear. If I remember correctly the smaller fork was for 2nd and 3rd gears.

Back in HS I had a 57 BelAir with the 3 spd that would not stay in 3rd gear at times then got worse where you had to pull back on the shifter to get it to stay in. Check the shift forks inside the side cover for excessive wear. If I remember correctly the smaller fork was for 2nd and 3rd gears.

I checked the lincage and had just previously replaced the upper bushings.I pulled the side cover and one side of the 2nd and 3rd fork was almost completly gone.I put on a different side cover and it still jumped out of third.Ionly got a block and a half at 430 am on my country road and got pulled over for no tail lights.Imust have kicked a wire loose crawling in and out from under the truck.Ive sinse located a rebuilt 3 speed in south carolina and just need to get it hauled to wisconsin.Hey M.T.Thanks for the offer and good to hear from you.