The Stovebolt Site Forums The Shop Area Engine Shop SM420 trans

I have a 66 Chevy 3 on the tree. I want to put a 4 speed in it. I can get a SM420, do I need a bell housing and drive line and clutch linkage or will it bolt up to the bell housing that is there?

Thanks

I bet ya a dollar itll bolt right up but as for the length of the 2 trannys??? Theres enough parts out there that it should be pretty easy to swap. The only hard part would be the length of the trannys and whether or not youd need to shorten/lengthen your driveline.

Difference in clutch linkage? I doubt it, but there could be.

Jeff

What about the clutch its self will I need a different one. And auto parts might know.

Be sure to get the bellhousing for the 4 speed. Some of the 3 speed housings have a smaller bearing support opening than the 4 speeds and won't let the trans full seat against the housing. The clutch splines should be the same, diameter may be different. The length difference would depend on which three speed you have,
Scott

You are also going to have floor clearance problems. Factory 4-Speed trucks had a taller, remove-able floor hump. I've seen some in trucks with the original low floor hump, but it doesn't fit just right. Maybe some will jump in and have some suggestions other than cutting and welding in the correct one.

Bill

Just to make sure... Are you aware that you will only be gaining a granny 1st? The SM420 4th gear is 1:1, just like 3rd gear in your truck.

It that floor hump is removable then I should be able to get on and put it in my truck. Maybe some welding. Any suggestions with that?

I am aware that there is only a first that I won't even use but I like the idea of the truck tranny instead of the hurst shifter.

If you are off road, especially with a heavy load, you will find the benefit of the granny gear. Heck, I even use gear 1st on the road. I would put one in my 1/2 ton with a 3-Speed column shift, but "guess" what is holding me back?

...the floor hump.

I am open to suggestions tho as mentioned above. Maybe someone out there can give us a good idea on how to make it fit right without changing the hump.

Bill

The 4-Speed floor hump is remove-able. The 3-Speed hump is part of the floor.

Also it's not just the hump, the floor in front of it is also taller to accept it.

Bill

How about a BFHammer?

I went out to my truck this morning and it has a pretty big hump already I don't know why you would need a bigger one. I will go to the wrecking yard where there is a truck with a 4 speed and look at the hump before I buy it. Or if someone else knows the answer?

C90-Bill do you know about my floor having a hump, is the hump to small and will the right hump go in my truck or not.

I did that conversion on a 1972 pickup.
Got the transmission complete / clutch linkage / drive shaft / and the floor cover. Put the cover on my floor and marked it out. Then measured in to acount for bolts and cut it out. Weld nuts to flat washers then bolt down cover then weld the washers to the underside of floor, that was done. took that apart installed trans and stuff and drove away. Yes it was that easy.

Well now I am back to thinking do it. Thanks underdog I will keep looking into it more. Has anyone else change out the tanny in a 66 chevy 3 on the tree.

The small transmission tunnel cover is spot-welded in place and can be easily removed by drilling out the spot-welds with a spot-weld cutter or a 1/4" drill bit. The large transmission tunnel cover is secured with capture nuts welded to the underside of the floorboard and bolts from the topside. The hump is not a problem in doing this conversion, it just requires a little more work and some additional parts, if it is not already a large hump cab.

In any case, good luck with your project and I hope this information is helpful.

Tony Smith

Thanks 4-5-6 Chevy Trucks, I was wondering if it might be the right hump because it is pretty high up and I can't see it being any higher. But there is a wrecking yard here that has a 66 so I will be checking it out. Ether way it doesn't sound to hard to do.

I did the 3 to 4 speed swap in my 64. I scrapped a 78 for it's 350 and SM465. I used the removable hump from the 78 in mine. It almost fit, just some small mods with a hammer and a mig welder and I had a new floor,
Scott

I went out and looked at my 66 and there were not any spot welds it is all one solid peace. Scott, did you have to cut the hump out of your 64? If so did you use a cutting torch or a saber saw?

Mine was removable as is my 63,
Scott

If your 63 is removable I will have to go look at mine again because there shouldn't be a difference. It looked like one peace

Climberman,

I just was over at Jolly's GMC site and saw just what we are looking for. He has a step by step procedure converting a SM 318 3-Speed to a SM 420 4-Speed, with great pictures to boot.

Check it out at: http://www.6066gmcguy.org/3-to-4-speed.htm

Bill

Thanks Bill, this answered all of my question and it is certainly doable. My problem now is that I don't have a donor truck. I don't think that someone will want to ruin their cab to sell me the front part of the hump. I will call some wrecking yards

Thanks man

I have a front hump cover, if you need one. I will send you a PM.

Tony Smith