The Stovebolt Site Forums The Shop Area Driveline 1942 3/4 Ton With 4 Speed Questions

Hi,
Just pulled the engine from my '42 and was looking close at the transmission. I never really got far along in the rebuild of this truck to pay much attention to the transmission and just discoverd I have the 4 speed and it is original to the truck. Right now it is pretty caked in grease so I will have to clean her up to find some casting numbers. What model of 4 speed trannys were used on 42 trucks? What are the pros and cons and anyone with experience driving one of these want to share some personal opinions. Thanks

It's known as a 4 speed crash box because there are no syncros. Shifting involves double clutching to bring the main shaft to the speed of the flywheel. It takes a certain feel to get it right.

If you have a 3/4 ton, then you have an open rear end. You could swap that 4 speed out to virtually any other transmission with the right adapter plate. However you would have to figure something out for the parking brake.

The old sliding gear transmissions are noisy, and as the previous post states, you will have to learn to double clutch which isn't that difficult. If you want the feel of a 70 year old vehicle, keep what you have. If you want something more like a more like a newer model, you will want to make some changes. The transmissions were very tough and if it always had oil in in it, it will probably last as long as you want to drive it.

Thanks for the reply guys. Appreciate it. I am definately going to keep this truck original...even the old crashbox! That will be part of the charm of owning and operating this old truck the way it used to be. I was curious though was the transmission made by GM and what model number was it (i.e. sm420 etc.) Thanks again.
Chris

It is referred to in the old books as the GM corporate sliding gear or spur gear transmission. I think it was just recently nick named a "crash box" because people my age were expected to learn to drive them without crashing the gears.

ken are you sure your not talking about a two speed rear when shifted incorrectly ? they crash

as for the pros and cons from my perspective is the pros outweigh all the cons with an improvement or they would have made them longer. i operate both types regularly and have no issues with either one. the biggest issue i have is the noise the crashbox makes in all but fourth gear. realizing it has straight cut gears i understand the noise. the pro is the reverse lockout mechanism. ive embarrest myself too many times playing with the lever and not beeing able to have it in the proper gear when the light turns green. the lever also makes for a good conversation piece.

What kind of noise? Mine sounds like worn bearings in second and third. Is this the normal noise you speak of?

normal. ever see an old movie and you can hear the gears wind out in the tranny with a little grind in between. thats what you got.

The biggest disadvantage of the 4 speed non synchr trans is that if you are traveling at a hefty speed and need to brake quickly, you are messing around with getting the engine speed correct, to the detriment of you actually having a foot left to stab on the brake pedal with. So either you rely on all brake, with the truck still in top gear, or you gear down properly and rear end the car in front of you. Mind you you, if you are a putterer, then this will never be a problem, but with the 3 on the floor, you can downshift AND put the binders on hard, and successfully haul that truck down in a hurry. I ran my 302 with OD in my '45 for many years with the 4 speed until I switched back to the 3 speed a couple of years ago. It is much easier to drive in everyday traffic, which i do. The one thing i miss about the 4 speed, is that nice low first gear for crawling around the back yard with.

What do you mean "messing around with getting the engine speed correct"?? Never having driven one of these myself yet, isnt it possible to just throw the tranny in neutral or just shove the clutch to the floor then use the brake to bring the truck to a stop??? I guess I am not understanding.

Thanks for all the responses thus far.

I'm not sure I understand that post either. I have never had the urge to find a lower gear during a panic stop. I just leave them in whatever gear I'm in and apply the brakes. A person can always shift her into neutral once your road speed is down to where the engine is at an idle. If you slam on the brakes hard enough to kill the engine, you have locked up the rear end and have found yourself in a skid.

Some guys like to downshift and use compression braking along with the wheel brakes. Unless the truck is loaded pretty heavy, and running at a low enough speed that 3rd. gear is actually an option, just stick it in neutral and climb onto the brake pedal. My sports car driving buddies used to get into the non-synchronized 1st. gear in their MG's and Austin-Healeys by locking up the rear brakes momentarily and jamming it into 1st. while the driveline was stopped, but they scattered a few very expensive gearboxes in the process. 6:1 compression on an antique 216 doesn't supply a lot of extra stopping power anyhow!
Jerry

Thanks jerry!