Truck transmission

[Ed Bashaw]

Help! I'm hoping there's someone out there that has spent somw time working on a 1962 Chevy truck transmission. this thing is jumping out of reverse. The rail seems to hold it pretty tight. This is a four speed. How do I identify it? Is this a common problem? In the process of backing up a ramp to dump a load of onions it popped out of gear and broke the 2 speed gear as well. This belongs to a large family of farmers and Dad just died with cancer and the boys that grew up with truck want to fix it. Thanks in advance foer any direction anyone can give. EGBinOR

 

[Anonymous-0]

Jumping out of gear comes from a couple sources. One is a failed detent ball spring on the rail. That's supposed to hold it in gear. I had that in a JD 112 transmission. New springs fixed it but I had to take it all apart. Some transmissions have those ball springs behind plugs on the side of the casting.

Another cause is a failed counter shaft bearing so the shaft can deflect under load and then that angles the gear to make it slide easier even with a good detent.

Another cause is the transmission built so the gear never fully engaged and so wore tapered.

All three can happen in 48 years of use.

Gerald J.

 

[old]

I'm guessing your talking about a 1.5 ton or maybe a 2 ton truck right?? Many things can cause that. Dent balls and or springs a bearing going bad and worn parts any and or all of them. What engine does it have?? The 307 or 309 V-6 can not remember which it is or maybe the 292 straight 6. I can almost bet it would be about the same one as in this 1965 GMC school bus I have

 

[Jeff B.]

The transmission you're working on is a muncie Sm420. 4 speed trans used in light trucks from 1948-1967. Don't know that they had particular problems with reverse, but my educated guess is either the fork in the tower is worn out enough that it's not getting all the way in, or the teeth on the gear are finally eaten through far enough that you don't get a good engagement. The reverse idler could be an issue, also. Since they aren't syncronized in 1-rev, after years of shifting the teeth tend to get chewed up. They last a long time, but at some point they're gonna give.

 

[87nassaublue]

If it's just a one ton, I'd try to just find a replacement. About twenty years ago, I and all my buddies were going from automatics to four speeds. I got a New Process 435 and Another friend got a borg warner t18. We were comparing them to the gm transmissions and they look like they will mate up with the gm bell housing. Back then we were paying about $250 for a good used one. That was way better than trying to repair or do a rebuild unless it's something simple.

 

[JDemaris]

Muncie SM420 with an 8 bolt top cover. Used for 30 years in civilian trucks and over 40 years by our military. Super low 7 to 1 ratio 1st gear. !st and Reverse are usually the first parts to fail on it.
Gearing: 7.02, 3.57, 1.70, and 1.00

GM started using the New Process 435 in its place around 1964-65. Also with the Muncie SM465 in 1968.

If it's 2WD, there are plenty of cheap SM465 transmissions around that would swap in pretty easy.

SM465 rations are: 6.55, 3.58, 1.57, and 1.00

<a href="http://s104.photobucket.com/albums/m162/jdemaris/?action=view&current=sm420.jpg" target="_blank">

</a>

 

[old]

Please correct me if I'm wrong but they pretty much used the same transmission in all trucks from the 1/2 ton up to the 2 tons when they where 4 speeds in the 60s and early 70s and even into the early 80s. I know the transmission I put in my 1969 Chev was a big heavy thing and was told it came out of a 2 ton Still have it and the 250-6 it was behind maybe one of these days I'll get my 3/4 ton 1969 chev built. Reason I say built I plan to put that 250 which has less then 3000 miles on a rebuild did it all my self and have that 4 speed and where the carrier bearing should be have a 4 speed aux tranny I have so I would have 16 forward gears and 4 back um ups What I want to do is make it look like a mini road tractor just wish I could do tandems on it

 

[JDemaris]

There are four different big heavy cast-iron four-speed truck transmissions with granny lows - that GM used from the 40s to the 80s. They look and act similar but are not the same.

Back in the early 60s, for 1/2 and 3/4 ton GM pickup trucks, there was only one. By the late 60s, there were two more being used in GMs.

 

[87nassaublue]

Jdemaris, you nailed it, gear ratios and all. The main way you could tell the difference if you're in the truck is where reverse is. If you know what your looking at you can recognize them looking at them from under the truck or if it's out on the ground. Seems like the transmission lengths might have been the same. I know there were some variation on ujoint sizes. Since we were doing change overs, we had to have drive shafts made. Napa took care of our special ujoint needs.

 

[old]

So then do you understand what I want to do with this 69 3/4 ton and doing the 16 forward speeds etc. Sure wish they made a tandem set up for the smaller trucks like they do/did with the bigger ones

 

[87nassaublue]

Old LOL, that is one heavy transmission you're talking about. I look at one once thinking I'd maybe try to stuff it in my truck. I just shook my head and walked away. I can't imagine wrassling that thing up into the bell housing LOL.

 

[old]

Well the normal 4 speed sits on the bell housing then you have the aux tranny where the carrier bear should be. I have this 1969 Chev 3/4 ton truck and it has no bed on it right now. I also have this 250-6 cylinder engine that is bored 40 over head milled 10 or 20 do not remember which and it has less then 300 mile on it. I would love to if I ever get the time and $$ to do so built a mini semi sort of truck with the parts I have. The double 4 speeds would give me 16 froward and the 4th gear in the aux is an over drive so I might just get fair mileage. But the old truck does have it problems like A arm bushings and bad and it sort don't have a title but I have a title for another 69 so not a big problem really. Also would put dullies on it just for looks and a 5th wheel plate and then of course I would have to rebuild a trailer I have so it would be a semi type for it

 

[JDemaris]

The Ranger aux. transmission is made just for that purpose. But, it's pricey unless you can find one used. I got mine for $200. It installs between your four-speed trans and ballhousing. Since it installs before the main trans. it works great for 2WD or 4WD.

"The Ranger Torque Splitter is an auxiliary gear box that bolts in front of the standard Chevy, Ford and Land Cruiser 4 speed transmissions. This fully synchronized unit allows you to split your gears at any time. Just push in the clutch, grab the Ranger's shift lever, (You will have two shifters: one for your main trans and one for the Ranger) and shift. At any speed. The unit is available in either a 27% overdrive or a special order 17% underdrive.
(Please call for availability on the 17% underdrive models.) Through the years it has gone through several design changes. Today's unit is cast aluminum, measuring 7-1/2" long. The gears are helical cut and the shifting mechanism is a short throw manual shift. The unit is built to handle 420 ft./lbs. of torque and has a G.V.W. rating of 25,000 lbs. This unit is ideal for towing applications or improving gas mileage by lowering the RPMs."
http://www.high-impact.net/transmission_and_gear/rangeroverdrive.htm

 

[Allan in NE]

Replace those big iron mainshaft gear bushings.

Allan

 

[Allan in NE]

Replace those big iron mainshaft gear bushings.

Allan

 

[old]

I think I'll stick with my 4 speed aux that I have. Shoot I have 4 of them all in big gas burner rigs but one is pretty small and will not be all that hard to set in place of the carrier bearing on this 69 Chev 3/4 ton. And I know they will hold up with not problem since the one I plan to use has a Ford 534 V-8 for a power plant

 

[Ed Bashaw]

Thanks JD and others.. I tore into it today. I hadn't noticed earlier but the bushing on the reverse slider was loose and floating around. Looks like I'm going to need the reverse idler because someone cobbled it up last time and now the gear teeth are worn at a slant and starting to roll over a little bit. Hope I can find parts!! Thanks again to all of you. EdBinOREGON

 

[Ed Bashaw]

Thanks again guys. Problem was the reverse slider bushing wore half out and was sliding around and sometimesm woludn't let the reverse slider completly mate up, thus it walked out of gear. All I need is the reverse slider & gaskets. Does anyone know where to purchase the parts? All I seem to br finding is complete rebuilds.. This truck probably goes about 300 miles a year. Thanks for any and all help! EdBinOregon