The Stovebolt Site Forums The Shop Area Driveline Tranny won't go into reverse (SM420?)

Hello. I'm new to the world of antique trucks. I've always wanted one - and I managed to purchase an early 1947 GMC EC152. She's beautiful. I got her running to discover I can't seem to get into all the gears - I'm pretty sure I can only get into 1st and 2nd. I was told it's a 4spd. The most concerning is I can't get her into reverse. She has an external/manual reverse lockout. When I lift the lever I can feel the shifter is both able to move further to the driver's side, and it appears to prevent engaging/entering forward gears.

According to the Novak website [https://www.novak-adapt.com/knowledge/transmissions/manual/sm420.html?srsltid=AfmBOopMeuxxgSA7UhXQ6AS99zZs4Tn8BcvHkhlbEPG8jxDI_AHx39GX] it appears to be an SM420 (side numbers are 590474 - top cover is 590511). Many websites indicate the SM420 only came with an external lockout (so maybe it's not a 420).

So my questions are...
1) How do I positively identify this tranny?
2) Does anyone know of a reason that I can't get into reverse (places to look or items known to fail)?

Warpig - 1st off, Welcome to Stovebolt! Your 4 speed is the earlier "Crashbox" unit, not an SM420. Non-synchronized, straight cut gears and best shifted using the "double-clutch" method. It sound like you may have some internal issues with the top plate shift mechanism and/or stuck gears. You're going to need to remove/disassemble to see whats going on. You could remove the shifter top plate for an intial inspection.

Thanks for the info. I've struggled to figure out what tranny this is. When I got her the clutch was stuck - but came free easily. I've rewired the truck (when I got her the guy removed all the wiring and butchered most things). Now she's running an alternator with 12v, new instruments, etc. She's got all new glass and so much more.

The attached pick is from before I got her.

I'll answered my own question...It's a GM transmission GKT-134 (not muncie, etc).

Warpig - You hit the nail on the head, but most of us still will always refer to them as "Crashbox's" - lol.

I can't find info on the "crashbox" but there are enough similarities to the SM420 to at least guide me a bit. I found some sweet manuals on the SM420:

https://kinzers.com/don/landcruiser/transmissions/

When I opened the tranny... moisture. It appears the trans may have been opened before - about 1/3 of the top-cover gasket was missing. The previous owner said the truck sat in the garage for 10+ years - even the clutch had frozen to the pressure plate by this time. So it's no surprise that the tranny is a little locked. Luckily it seems to be superficial at this point. The shifter itself seems to have gotten dry of gear oil and suffered the rust. I think I can get things freed up again (PB Blaster, wire brush, elbow grease).

With any luck... I'll have her moving around my driveway this weekend!!!

Here are some pics (in the event you see something)...

Here is a page from a 1942 shop manual that covers the rebuild of your transmission;
https://chevy.oldcarmanualproject.com/shop/1942/42csm0719.htm
At the bottom of each page click on "next page" to continue reading.

Kent

The best penetrant is a 50/50 mixture of acetone and automatic transmission fluid.

Fill it up and let it sit for a week.

These trannys are really simple. If you follow the directions in the shop manual it takes less than an hour to disassemble and the same to reassemble. The only critical assembly requires the use of a feeler gauge at one step (free play between a gear cluster and case). I am confident a skilled person could assemble one in less than 20 minutes.

I have this tranny in my '37 and '42. I have owned the '42 over 30 years and at first I looked for a 3 speed, which is synchronized in 2nd and 3rd. But soon I learned to double clutch, and more importantly, to match engine speed with gear speed in whatever new gear I was selecting. I like driving these trucks now as it is part of the charm and character.

Kent

Here's another item that may help you in digging into the GKT-134 transmission. Fellow Bolter Brad Allen has this original Chevy filmstrip linked that details the removal, disassembly, inspection, repairs and reassembly of the 1939 GKT-134 which is I think almost identical to your slightly later unit. Lots of details pics and instructions.

@Kent: Wow, that is useful. Thank you. I found that the forks were frozen to the shafts via rust. I cleaned the rust and applied a light cover of grease and tapped them back and forth a bit till they moved by hand. I dry-fit the cover assembly back on the tranny, and it feels like I'm going in all the gears now.

@Dan, Thanks - very cool pics. That'll be useful when I start examining those u-joints and trunnion.

Just in case you get an urge to replace the crash box with a SM420 you'll find it to be longer than the crash box, more than can be taken up with the drive shaft. I tried putting a crash box into a 48 half ton and couldn't do it. I wanted to replace a tired three speed and SM420s in those days were still in current production so I couldn't afford it.

@1Ton Tommy: I had that thought (replacing the crashbox with a tranny that has synchro). I imagined there was a difference. Good to know.

I'm happy so far with the crashbox. I haven't done much more than testing it in the driveway. I think I could stay with it... there is something about the sound and feeling of driving a vintage truck... nostalgia I suppose. But I can also imagine myself getting sick of double-clutching, grinding, etc. But who knows.

I have 3 trucks with "crash box" trannys and have driven quite a bit in traffic. Because you have to pause slightly when going through the gears to allow the engine rpm to come down to meshing speed you will find people running up your rear because of your slow acceleration. Sure, you can accelerate once in a gear, but that slight pause between gears seems to annoy people behind you. They will pass you but you catch up to them at the next stoplight. For me, with lots of traffic it is hard to hear the engine rpm and my shifting is not as crisp as I would like, slowing me even more.

Long commutes in traffic with a daily driver might get tiresome but that does not describe my situation.

Kent

@Lightholder's Dad - I look forward to it. People will be an a$$ even if you can accelerate quick - I try not to let other's influence my driving. The "crash box" will make me take my time I'm sure.

My slow takeoff surprises some people but most give me space and understand that the truck isn't brand new.

Allowing it to roll back a little on a hill before letting out the clutch keeps them off your tail.

@Otto - LOL. I recall learning to drive a standard and I can tell you those steep hills were stressful.

So if I gathered right, the "crash box" is unsynchronized. Mine gets a bit annoying because of grinding gears. From a complete stop in neutral then shifting to first, I get a "mild" grinding. 2nd and 3rd may or may not grind depending on engine revs. or truck speed. Is this normal? It is not the original transmission but looks and fits like the original to my '54 235.

And here I thought I was the only one who knew about the above pentron replacement. Good job Otto!!