The Stovebolt Site Forums The Shop Area Driveline SM420 Question

Will a SM420 trans bolt up to a 1942 Chevy 6 cylinder engine bell housing that has a 4 speed in it????? Thanks!!!

Marv.

If it has the original bellhousing it won't, the bolt pattern on the SM420 is different.

You will need to locate a bell housing from something like a 48 truck. This bell housing, however, lacks a place to bolt up your brake, clutch pedal assembly (if you have 1/2 ton, 3/4 ton no problem). The next issue is that the SM420 is approximately 2" longer than the original 4 speed crash box requiring a shorter torque tube and propeller shaft. I am doing this on my 1939, and have gotten this far. There may be other issues that I have not yet addressed.

I guess I should have made my question clearer, the truck is a 1942 one and a half ton ( 1-1/2 TON ) with the rear motor mounts on the bell housing. What other bell housings will work with SM420 that has the motor mounts on the bell housings. The engine I am putting in is a 1962 235 6 cylinder. I have a open drive shaft no torque tube. Hope this clears things up a bit. Also what years, makes and models trucks did the SM420 come in. Thanks again!!!!!!

Marv.

Bellhousings that accept the 1942 mounts and SM420 transmission are 1948-53 truck.
SM420 started in 1948 and I believe stopped in 1972.

That really sounds interesting Barry. I've never heard of any
SM465's being installed in and AD before. Are you sure about
the only mod being to bore the pilot hole in the bell housing?
Have you installed one and if so I'd really like to see a
couple of pictures of it.
I just scrapped out an SM465 that I'd been dragging around for
10 years. Those are really a nice transmission and should be
much more readily available since they were made all the way
up till '91. Using it with the stock drive shaft could present
a problem so I would think in some cases you would need to have
a custom drive shaft made.
Novak has some interesting history and information on using this
transmission.
http://www.novak-adapt.com/knowledge/sm465.htm
DG

I am pretty sure the floor cover would need modifying,they have a tall cover.

I didn't notice but are the gear ratios the same as the SM420?
Of course I understand that 4th. is 1:1 just wondering about
the others. I've really found no problem with 2nd., 3rd., to
4th. in the SM420. Have only used 1st. sometimes like around
the grounds at a car show but even then, 2nd. is fine usually.

DG

SM465 Gear Ratios are:

First Gear - 6.55:1
Second Gear - 3.58:1
Third Gear - 1.70:1
Fourth Gear - 1.00:1
Reverse - 6.09:1

SM420
1st Gear Ratio .... 7.05
2nd Gear Ratio .... 3.57
3rd Gear Ratio .... 1.70
4th Gear Ratio .... 1.00
Reverse Gear Ratio .... 6.78

Don't gain anything there do ya Tim! tks

DG

Denny,

No gain/advantage: it looks like using 2nd gear in either transmission to "take off", unless you have a low-ratio rear end, or you want to burn rubber, or you need to pull a car up a ravine, or you are entered in a slowest vehicle race.

Just in case someone with a 1947 truck and an "oddball" 4-speed transmission reads this in the future: The SM-420 transmission might have been manufactured by GM from 1947-67 (as shown by Novak), but it was used in 1948-1967 model Chevrolet trucks (probably/maybe in late '47 production, after the 1947 4-speeds ran out of inventory).

My understanding is that the SM465 is not only longer but bigger than a SM420, I was hoping not to do any cutting of the floor pan. Is there any other transmissions that would replace the original 4 speed in my truck that have synchros in them and is not any larger in size? Also what years and makes of trucks would they come in. Thanks to all for your help!!!!!!!

Marv.

Don't know if this is relevant but I've seen threads discussing using early S-10 T-5 transmissions behind the 235 engine. I believe they are 5 speed fully synchronized with better ratios. A search on using T-5 should being them up.

Hi Marvin
I have made this same conversion many times. The key is to get a 1947-53 Chevy truck 6cyl bell housing BUT the 3 point mounting bracket for the clutch and brake pedals must be added. The bell housing to frame mounts are stock items.....there are two different mounts and I think you will need the ones for the 1942 crossmember.I do have one of these modified bellhousings in stock if needed...$240.00 plus shipping form California
Brian

repguy
Thanks for the offer to sell me a modified bell housing but that is out of my price range at this time. Has anyone seen or made a bracket that will mount the clutch and brake pedals once a 1948-53 bell housing is used???? Send me pictures or links. Thanks to all so far!!! 1-262-392-2989 or [email protected]

Marv.