The Stovebolt Site Forums The Shop Area Engine Shop 1941 GMC 1ton transmission

I picked up a 1941 1 ton GMC milk truck the other day and plan on restoring it. The engine looks to be stock and should be a 228 according to the serial number. The transmission is "toast". It appears that they had trouble with it and someone removed the top and left it open. Everytning is rusted together now. It has a covered drive shaft for about 18 inches. At that point it stops at a carrier bearing and is an exposed shaft from there on back. I don't have enough knowledge to know if my transmission is a 3 or 4 speed by looking at it. I would like to use a later transmission with syncros if it will bolt up.Can anyone tell me what years of transmissions would be compatable. If you want to see photos, go to http://www.geocities.com/milkman581 Thanks Roy

Late 54 or 55 GMC wwill fit and give you an open driveline
Is there a little trigger on the shift lever? and it is REALLY big then it's a four spd

Cool looking truck. Your tranny appears to be the old 4 speed "crashbox" type. Gears are non-synchro. You can update to a synchro (sm420) but you'll have to change the bellhousing because the bolt pattern of the 4 tranny mounting bolts is different. You may or may not be able to keep the partially enclosed driveshaft setup, not sure about that. 'course you could find another crashbox to put in there, it's a good unit if you don't mind double-clutching.

MM, yep! ugly truck. Yep! rusty trans! That looks like early 4spd to me with straight cut gears will be non-synchro. I was thinking that a later (48 or newer) SM420 was about same and can be had with enclosed driveshaft if you look in 1/2 tons with 4spds till guy above says have diff bolt pattern on b/house. Looks about same, must not be much diff. One thing strange, it has a taller shift tower maybe for shorter throw as longer lever and guys stood many times driving these delivery trucks. I think the B 27 9 mark is a date code. B=February 27=day 9=39? And the other # 590414 or 474 is the trans part #. Now to find someone with old parts book! It would be worth it to change bellhouse and newer trans to get synchros. Open drive would need whole rear end changed. Have to remove clutch and flywheel to get housing off, bolts are behind the flywheel, great huh? Good Luck!

Thanks for the reply...do you guys sleep? I see that some of your posts are from 1-3 in the morning!

They probably named these transmissions a "crash box" after the one I have. Most of the parts inside have been brazed or welded. Because the tranny and clutch are frozen, I have yet been able to determine if the engine is in the same shape. If it is toast, I will probably go with a later 235. If not, I may stay with the 228. If I need a later bell housing for a SM420, will it bolt right up to the stock block? Will I have to replace the clutch and flywheel with a later model also? The rear end on this truck does not use a closed drive shaft. Going to a later model tranny that does not have a closed shaft housing should not be a problem other than some drive line work at the half shaft carrier bearing. In looking through the junk yards (I call them gold mines) around here, how will I know if I have spotted the elusive SM420 transmission? Any hints on that would be helpfull. Thanks again, Roy

The transmissions that are syncro do not have a lockout lever on the shifter. This started in 1948. No one has mentioned yet that a 41 bellhousing has provisions for a clutch/clutch& brake mount (depending on size of truck) that the 48-53 truck bellhousing you will need does not have. There are several ways around this problem though.

Most any 4spd in Chevy truck up to 67 is a 420. Shouldn't be hard to find one. If it's open drive, go for that. One's on 2-tons have brake on back, can take off. thing you will still need 2pc shaft to get slip joint. OR, like my COE short wheelbase, it has slip joint bolted to back of trans and long part has the spline shaft on it to match. Harder to find, I'll tell you! Keep the GMC six, better engine. They are longer too, may have to modify mounts for Chevy to fit, but the rad clearance won't be problem if you use later six that way. Bellhouse between Chev and GMC, don't know? Didn't know about pedal linkage mounts either. Good auto and 250 six? :rolleyes:

Rust never sleeps I get a little!!