The Stovebolt Site Forums The Shop Area Engine Shop sm420 and 3:38 rear - play well?

Hi! My tan 62 is done!! I'm on the road and having fun. All is well, it drives like new (it had better!). During my ground up build I started down several paths: I installed an M4F cam w/ the intention of installing the dual intake and fentons later that I have on the shelf. It's running the stock 261 w/the correct big rochester for now. Behind it is the HD SM420 that came with it. I installed a 3:38 when I rebuilt the rear (from a 3:90), again w/the intention of running a saginaw 4 speed which I rebuilt but then sold. (I was going to do the Patrick's conversion.) The truck runs out fine, 1st is granny low, 2nd and 3rd go by pretty quick too (I can't imagine how quick they would peak w/ the 3:90) 4th gives me good hiway speed, I'm breaking it in so I haven't gone above 55-60, but it has more. I get this feeling though that this combo is mis-matched. Any comments? I still have my 3:90 gears or I could put a 3 speed in. Can a 3 speed be installed so that it could be shifted from the floor without alot of headaches. I kept the truck stock to avoid any cutting or mods that couldn't be put back to stock. I have access to a 60 truck and can pretty much do what ever I like. Thanks as always for your input.

I have 3:90 gears with the SM420 and have no problem cruising at 65. Its just a bit loud at that speed and 4th likes 55-60 best. I added a Gear Vendors O.D. which gives me the best of both worlds. I can go 80 -85 now in 4th over. 70 -75 is quiet and smooth.

"Can a 3 speed be installed so that it could be shifted from the floor without alot of headaches"

You have that already if you don't use 1st gear!

When I say mismatched, I mean that skipping first, making it a 3 speed & starting in second labors it a bit and wants to lurch if I don't finesse the clutch. An overdrive might be an idea if I go back with the 3:90. Does it require any driveshaft modification? Part of why I did not go with the Patrick's saginaw 4 spd was that with the lwb I have 2 driveshafts and the carrier bearing. I didn't want to hack up the shafts I had and didn't bother trying to locate a donor shaft. That and the fact that the 4 speed conversion was going to run over $1000. I backed out of that idea and sold the 4 spd I had rebuilt. You're right in saying that if it drives O.K. just leave it alone, I wanted to see what others thought about that combination. Thanks.

I had to have my driveshaft shortened. While I did it, I also had them(Drivelines NW) install a new bearing and new U-joints. They did it in 1 day. Great company! I typically start in 2nd gear. In fact, I can spin the tires effortlessly in 2nd, and again in 3rd just to show off. And that's with a mostly stock 235. Maybe you have clutch problems?

Sounds like a real good match to me. That 4 speed is rugged & has a good shifter.

3sp vs 4sp isnt an option.... the 4sp is the same almost as the three speed in 2nd 3rd and 4th.... think of the 4 speed as 3 speeds with granny low.... in other words second on the 4 speed is about the same as 1st on the three speed,,, same with second and third.... get it..???? so if ya put the three speed in it will still labor in 1st.... so if ya dont like the way it is changin the xmission wont do it.... its more the rear end and if you go with an overdrive then ya want the 3.90 probably....or it wont pull it in overdrive on a hill or headwind without labor the heck out of the engine....goin fast anyway....

I have an SM420 behind a GMC 270 running 3.08 gears. I find the combination very drivable just the way it is.

You mention a Saginaw conversion kit from Patrick's. I think you only need that if you are converting from a closed driveline, and your '62 should be an open driveline.

Hi. You are correct in saying the kit that patricks sells is for a closed driveline and mine is open, but the other kit he sells is for the stick/linkage conversion at about $280? That is what I was referring to, sorry. The ratios of 2 & 3 on the sm420 are noticeably different than the ratios of 1& 2 on the 3 speed. I posted to get opinions on the differences in driveability between the combinations. The guys in the know say what I've got now is O.K., that puts my wondering to rest. The only possibility I could have in my clutch action could be residual oil in the disc. Everything else in that area is clean & up to spec. Thanks!!

Unless you intend to pull a load with it I would leave it as is for a driver. Your right about the 3 speed v 4 speed ratios but if you do change it then yes, certainly you can purchase a quality speed floor shifter from your FLAPS. I doubt it would fit the standard 4 speed floor plate since 3 speeds shift on the side but Hurst/BM may make one that does.

All the aftermarket (and factory) floor shift kits I've seen for three-speed tranmissions mount at the tailshaft, placing the shift lever significantly farther back than the SM420's lever is located. Even in passenger cars, it's difficult to install this type of shifter with a bench seat, since the lever (in unmodified shape) will be too close to the front edge of the seat.

I don't see how it would be any different in a C10 without making major modifications to the shift lever or using bucket seats.

Finally, if 2nd gear seems too high for starting off without a lot of clutch slip, why not use it like a true 4-speed? I've considered using a 3.08 or 2.73 gear in my 64. If I were to do that, I'd definitely use the "granny" 1st as the starting gear. It wouldn't be a whole lot different than using a TH700R4 automatic with its extra deep 1st gear.

The spread between the granny first and second in an SM420 is big enough that I would not want to drive it that way all the time. You would be in first just a few feet and then shifting again, and then again to third real soon.

The spreads between second, third and fourth aren't great anyway (see Patrick's comparison to the Saginaw 4-speed) but the jump from first to second is much worse than any of the others.

Let me say again, that with a GMC 270 (sligthly larger than the Chevy sixes) my truck drives nicely with an SM420 and 3.08 gears.

I won't win any drag races, and the few times I've used it to pull a trailer it was very disappointing compared to my big block powered 1-ton, but running around town and light highway commuting it really is quite workable.

I'd forgotten about Patrick's shifter. It really is a good idea because it puts the shifter back near the stock location.

Ray: you are correct in that most shifters fit on the tailshaft but I have seen them made especially for pickups so that would have to make consideration for a bench seat. I cannot recall the brand name however as it didn't seem important at the time.

Happy 4th of July Weekend to everyone. See y'all next week.