The Stovebolt Site Forums The Shop Area Driveline SM420 question

Can someone tell me what is a SM420 transmission? I have a 4-speed manual. Is that a SM420?

If it came in your '55 2nd series and your truck is 2 ton or less it is an SM420.

OK - Should have been more specific. I have a 1946 1/2Ton with 4-speed. Just new and I've seen SM420 a few times and wondered what my transmission was.

Sorry John. I don't know why I thought you had a 55.2 truck.

GM started using the SM420 transmission in 1948, so if original your '46 would not have a SM420. I don't know what they called the earlier 4 speed.

The prior-to-1948 four speed transmissions did not have synchronizers on 2nd, 3rd and 4th gears like the SM420 did. You'll have to learn to double-clutch.

John, The earlier 4 speeds (before SM420) are known as "crash box" transmissions. None of the gears are synchronzed, with straight cut gears that you will need to learn how to "double-clutch" to shift smoothly. The pic below is a screen grab from a 1941 Chevy Truck dealer handout. Your 4 speed is shown on the right (1946 is basically same design as 1941). Lots of posts on this type of transmission as well as some helpful hints on "double-clutching". Here is a great earlier thread on "double-clutching" that may prove useful.

Is it just preference if I should keep my original "crash box" transmission or upgrade to a later transmission with synchros? If upgrading, are there any direct bolt in transmissions that work with my 1946 torque tube? Sorry for all the questions. My truck is currently dissembled and just trying to figure out options.

Just read the earlier thread that Dan provided above. Wow! What a challenge it would be for me to learn to operate a crash box. Think I'll stay with my SM420

I found I needed to double clutch the 4-speed in my '48. (Worn synchro gears?) My left knee started to suffer with the extra clutching. So, during a shift, I just pause in the neutral position for an instant before putting it in the next gear. That got rid of any grinding. I don't know why this works for my '48 but I'm happy.

John, one option is to find a 3 speed transmission from the same era. That is almost a direct bolt in requiring only the proper pedal bracket or one can add shims to the bracket (stack of washers) to account for the fact the 3 speed tranny is narrower. The 3 speed transmission will be synchronized from 2nd to 3rd. These transmissions were the standard for the 1/2 ton trucks of that era so there is a good chance you will find one.

Kent

Yes. What a challenge on these crash box transmissions. Now that's driving!

I drove a Model A Ford with a 3-speed straight cut non-synchronized transmission for years. Like Wally said, slow and easy on the shifts was the trick, letting the engine slow down before engaging the higher gear on upshifting, or revving the engine a touch while in neutral on downshifts made for a clash free shift. It's even possible to shift without the clutch with enough practice (called float shifting). Matching the gear speed with the engine speed is the trick there.

I drove a 1934 Brewster, which is essentially a Ford with luxury coachwork. It had a non-syncro transmission but it was one of the smoothest manual transmissions I've ever driven.

You could just east the shifter into position (no jamming it in or forcing it) with a little pressure and the gear would drop right in. I was amazed.

I attempted to put a four-speed crash box from a 46 or earlier truck into a 48 that came with a three speed. The crash box was too short and I'd have to have moved the rear axle forward. So I gave up. The junkie wouldn't take it back so I had and extra useless transmission for years.

I still have my stock '46 4-speed which I'm planning to use again. I also have a couple other transmissions in my shed, but not sure what they are. I believe one is a manual from an early '50s truck and the other looks to be an automatic. As my project gets further along I'll try to post some pictures to see if anyone can identify them.

If your using the wrong gear lube you will grind the gears...you need to be using Mineral Oil, it's not as slippery as the modern oils.

Mike B

Mineral Oil? What? Is that what the manual calls for? I was thinking 85-90w or I've heard some say the same as the differential?

Tractor Supply sells the correct oil for these antique transmissions under the name Traveller ISO 90 Ford Tractor Transmission Fluid Mineral Oil.

https://www.tractorsupply.com/tsc/product/traveller-ford-tractor-hydraulic-oil-2-gal-t806600

To answer your question, the 1948-1952 Chev. Truck Shop Manual states in General Lubrication, section 0-5 that;

Transmissions - S.A.E. 90 Straight Mineral Oil gear lubricant.
S.A.E. 90 "Multi-Purpose" gear lubricant.

Rear Axles - S.A.E. 90 "Multi-Purpose" gear lubricant.

Transmission lube does call for either Straight Mineral Oil or "Multi-Purpose" gear lubricant, but I don't believe "Multi-Purpose" lube contained sulfur compounds back in the 50's. It was probably rated API GL-4 which doesn't have sulfur compounds in it and is OK for transmissions with brass components. "Multi-Purpose" nowadays is generally API GL-5 which should NOT be used in our transmissions. It's also too slippery for the synchronizers to work correctly. API GL-5 was introduced in the 1960's. It's hard to find a M-P gear lube that's GL-4 or lower.

OK - I'm learning new stuff all the time! I had my 4-speed transmission serviced/rebuilt and it is dry right now. I'll make sure to fill it with the correct oil.