The Stovebolt Site Forums The Shop Area Driveline SM420

After a recent outing (short 25 miles out and back) I noticed the tranny starting to get difficult to get into second gear, not impossible, but needing a hard pull. Second to third is fine , third to fourth is fine and vice versa is fine till third to second.
Not being real mechanically inclined was wondering if there’s any suggestions as to what is going on……
Up till this trip it was working reasonably well for as old as it is

Thanks

You're talking about shifting from "granny gear" to 2nd, right? When I try that, I coast to a stop before I can make the shift. So I've always started in 2nd gear. My 1st or granny gear is a parking brake for me.

I drive mine the same as Wally. But if you have a heavy load, start out in granny, then shift to second.
If you start out with a heavy load on a hill, stay in granny until it starts to level out some.
Also, try not to slip the clutch very much taking off in second.

Don

Sounds like problems shifting down from 3rd to 2nd. At least that's how I'm reading the post.

"Second to third is fine , third to fourth is fine and vice versa is fine till third to second."

What kind of oil is in it? Certain oils don't work as well as the proper mineral oil does.

Cmayna is correct it’s down shifting to second from third . And its generally just before a complete stop.
Maybe I’m just driving it differently lately. I very rarely use the granny first gear, usually when I pull into the garage to park.
I’m due for oil change so I’ll be sure they take a look at the level.
Like some of these old trucks, there are various oil leak issues. 🤷‍♂️
BTW, I tried to reply directly to the individuals, but appears that cant be done?

Yes it would be good to know what type of oil you are using. I use NAPA or Tractor Supply 90W mineral oil. I have no problems down shifting from 3rd to 2nd when coming to a stop.

Lumbersawyer has a good question. The SM420 synchros work best with mineral oil. Hypoid gear oil or synthetic oils are too slippery for the synchro to work well and any oil with sulphur may attack the bronze synchros. If you are not sure I would drain your oil and replace with 90 wt mineral oil. I did that with the SM420 in my COE and it had an immediate, albeit slight, positive effect. Your 2nd gear synchro may just be worn out which happens from too much down shifting into second.

Kent

I have the same issue with the SM420 in my '58 1.5 ton. I have changed the gear oil, the old stuff looked like mud. I used a mixture of Diesel Fuel and ATF to clean the internals. I placed the Truck on jack stands and allowed the Transmission to turn in gear at just an idle with no load. I didn't do this for very long, 5-10 minutes at the most. I left it open to drain overnight and refilled it with 90 wt. mineral oil from Tractor Supply. I did work a lot better, but I suspect, as Kent suggested, that the second gear synchro is really worn.

Related question - I don’t seem to have major synchro issues. I do have some gear noise in all gears but 4th.

My hunch is that an SM420 is never a silent unit, even in perfect condition, but that’s only a hunch

Anyone care to comment?

My biggest issue with my unit is the super-floppy shifter. Need to address that at some point.

My SM420 purrrrs like a kitten. Maybe I'm just used to it?

Mine isn’t terrible. Just a minor bit of whine.

I suspect that being a 1.5 ton farm Truck, "Thor" probably spent a lot of time in second gear..

Concur with LD on the 2nd gear synchro. If it shifts fine double-clutching, or at a walking speed roll that would confirm it.

Even after an overhaul, my SM420 always had a little bit of gear noise, but not objectionable.

It's had 90w hypoid gear oil in it since before I owned it (I changed it back in the '90s and drained old sulfur out) so shoot, I thought that was what they were supposed to have!

When I downshift from third to second, there's always a little whine as the synchro catches up, I just don't force it and then it falls right into gear. Just like how Dad drove his (same make model year and drivetrain) dump truck. That downshifting synchro sound meant we were turning up the street to our house and were almost home.

So picking up this thread. I’ll be going with 90 w mineral oil for the tranny.
Question, is the same oil fine to top up the rear end ? It’s a little leaky leaky ……

Thanks to all……..

Though I use 90 w mineral oil for my sm420, I use Valvoline high perf. 80w-90 gear oil...... for the rear end.

FWIW ... my SM420 (paired with a 261 and running 17 in wheels and a 4.11 rear) in my '49 1-ton is quiet, even cruising at 65 MPH. I only hear a little noise in 1st gear and reverse. My truck is a low-mileage (probably less than 75k miles) 1-ton. Its had nothing but mineral oil in it since it was built.

I would not mix mineral oil with hypoid gear oil. It may be compatible, but I certainly wouldn’t take the chance of reducing the gear oil’s compression rating.

Depends on the rating of the mineral oil. These trucks ran on rear axle gear oil from before GL-5 was developed. If it's straight Mineral oil without a rating like GL-4, then the service manual says
"CAUTION: Straight Mineral Oil gear lubricants must not be used in Hypoid rear axles or two-speed rear axles.

Save your mineral oil for the tranny. It's probably less expensive to buy the prevalent GL-5 gear oil to top off your rear axle.