The Stovebolt Site Forums The Shop Area Engine Shop Transmission options for 1963 GMC 1/2 ton 4x4

Hey all,

I got the SM420 trans and a 3:54 ratio. Engine is a 327.

I have gone through the tranny and all looks good but this thing really whines and rattles! At 65-70 mph it is just absurd how loud everything is and how revved the engine seems to be. It is in 2hi (NOT 4wd) and at 70 mph I am running about 3000 RPM.

My Question- What is a better transmission to put in there with the 3:54 ratio so this ole girl will quiet down enough to hear some Hank Sr. while driving down the highway?

Thanks
Shane Hamilton
Warrensburg, MO

how about an NV4500? I believe you will be looking for the dodge version, but the conversion may be rather expensive. Nobody makes a sbc bellhousing for the nv4500 that mounts to the frame like the original bellhousing, so an adapter plate will need to be used in front of the dodge NV4500. The pilot shaft on the dodge NV4500 is 1 inch longer than the pilot shaft on the gm NV4500 making this swap possible. This transmission should knock your rpms down to about 2300 at 70 mph. This tranny also has a granny low, but it isn't as low as the sm420, so there is a little bit of a loss in first gear, but a tremendous gain with 5th gear. You may find a transmission and a transfer case from a dodge and use them as a package, or find a 2wd transmission and move your transfer cas back to accomodate.

are you sure that it is a 3;54 ratio. what size tires are you running and what type of tires?
ron

Thanks for the responses!

Yes it is definitely a 3:54 gear ratio and I am running 31X16x10.5 tires. I looked at the NV4500 tranny and that looks like it will fit with the adapters. Actuall the bell housing that it needs looks like the bell housing that I have on mine already. The bell housing on my truck has four holes to bolt into for the tranny.

Why do you say a dodge NV4500 instead of getting a GM NV4500?

Also, my transfer case is divorced and it just hooks up to a u joint coming out of the back of the SM420 that has a seal and splines that go into the SM420. Is this how it would hook into the NV4500? Would I need an adapter for it too?

DO I need a 2 wheel drive nv4500 of the 4 wheel drive model?

Thanks so much guys!

Shane Hamilton

It just doesn't sound righ
ron

The NV4500 needs a speacial bellhousing, as it is a different pattern than a standard gm 4 or 5 speed transmission.

I have never put one in a 4wd, but I mention the dodge version because the imput shaft is 1 inch longer, allowing for a 1 inch adapter plate to be used between the bellhousing and transmission. The 2wd NV4500 is pretty long, so it might require pushing the transfer case back, which would make it necessary to make 2 or 3 new driveshafts for your truck. You would also have to modify the transfercase shift linkage.

By getting a dodge 4wd married trasnmission and transfercase, you will only have to have 2 driveshafts made. Also the shifters for the transmission and the transfercase should be pretty well lined up.

If you use the 2wd dodge transmission, you can use any year. If going for the 4wd transmission and transfercase, I believe you would need to use 94-97. The reason for this is for the mechanical speedometer. I am not familiar with the tranfercases that were used with the NV4500, so I really can't say how the speedo would hook up.

On my 1959 gmc 2wd, I used the 94-97 2wd NV4500 as that one is the easiest to hook the speedo up to. If you use a 2wd transmission, you would hook the speedo up to the transfercase.

Ok, enough blabbering... I really don't know the ins and outs of this swap... I'm sure these guys will be able to help. Good website too. http://www.high-impact.net/transmission_and_gear/nv4500.htm

After reading on the high impact site, maybe the 96 on gm 2wd transmission will work the best if you use the dodge imput shaft. I'm sure the tech support there can answer all your questions. Make sure you have your transfer case model number, and the distance from the back of the bellhousing to the front of the transfer case.