The Stovebolt Site Forums The Shop Area Engine Shop SM420 rebuild - is this trans now junk?

My truck has an SM420 4-speed. It was loud (yes, even louder than a normal 4-speed) in gears 1-3, and it was not synchronizing properly shifting into 2nd. So I pulled it apart today.

It looks like at some point in the past, the 2nd gear synchro retainer ring broke, and the pieces got lunched in the gears. Most of the pieces were in the bottom of the trans, buried in gunk, so I think this happened some time ago. The 2nd gear synchro was kind of flopping around in there when I looked inside. The retaining ring also ended up taking a pretty big chip out of one of the gears on the countershaft:

[img]http://rhoville.com/mattsnaps/sm420-chip.jpg[/img]

I figure if I rebuild this transmission and put a new snap ring in, that will probably solve the synchronization problems I've been having. But thanks to the really big chip in that gear, I also figure it is going to be loud forever. It doesn't pop out of gear or anything.

So here's my question for the group: If you had a similar transmission, would you put a rebuild kit in it, or would you take this as a sign from above that it's time for a 5-speed or automatic trans swap?

Follow-up question: Anyone got a countershaft assembly for sale?

P.S. - happy Thanksgiving to all Stovebolters and their families!

Those 4 speeds are a dime a dozen. I wouldn't spend any money on that one unless you're going to replace that gear set too. Just my opinion though,
Scott

Those transmissions are popular enough with the jeep crowd that there are places selling parts. It should not be real difficult to find a replacement gear.

But if it were me I'd look to find a replacement transmission. With a little looking I'd expect to find one for less than $100 that you can just bolt up and go.

I've seen those transmissions given away by guys who took them out.

Another thing you might want to check, especially since you mentioned that the tranny is noisy in all gears except for 4th, would be the roller bearing on the inside of the input gear. This bearing allows the top shaft to run at a different speed than the input shaft. Sadly, the input gear is the outer race for the bearing and if this bearing surface is worn it may mean replacement of the input shaft.

I tend to disagree here. The SM420 is an old transmission, and just bolting up another one might come with its own host of problems. Mine too likes to grind if I shift too fast. The synchros are obviously worn, but otherwise no other problems that I know of. I'm sure most of the old trannies out there have similar problems.

The only way to guarantee "new" operation is to rebuild either your original tranny, or go get a used one and go through it. Unless grinding gears do not bother you...they bother me though.

Hi, thanks for all the help so far. People do sell lots of parts for these transmissions; I'm currently exchanging email with Novak (novak-adapt.com), as they seem to do a lot of business in SM420 parts for Jeep rock crawl people.

That's a great tip about the input shaft vs. output shaft bearing; it hadn't occurred to me that of course those two shafts spin at different speeds exactly in gears 1-3. However, those bearing surfaces look great, no clogs in the oil supply holes, etc. So, here's hoping it is just some other bearing. It occurrs to me that, since the chip is on the 2nd gear driving gear on the countershaft, that's not likely the source of the noise anyway.

I should have mentioned up front that I want to get this guy back on the road soon - my parents are visiting for Christmas, and I really want to drive them around in my truck. So, what the heck, I will do the rebuild on it, put in a good 2nd gear if I can, otherwise just see what happens.

I'll let you folks know how it turns out!

You might want to check out Kajunjon.com. He's out of Texas. I bought a rebuilt 3 speed from him for $250.00 works well. Super knowledgable and will put it all together for you, all you have to do is install and add oil. He also has slip yokes too. Might be worth checking into as he advertises the four speed on his main web page. Hope it works out.

they hav not made those trannys for quit some time and sources for good parts are slim. the jeepers are hording all they can scrounge up.
good luck

Well, I finished putting the transmission back together tonight. It looks awesome all cleaned and painted. I checked operation on the bench in all four forward gears, plus reverse and neutral. At first I thought shifting in 3/4 was broken somehow, but it's just a bit tighter than it was before - I guess that's what the long shifter arm is for. Heck, the speedo gear even works!

Now I just have to finish up the clutch replacement, get everything installed, and I can find out whether I was a fool to rebuild this transmission in the first place!

The rebuild was not really rocket science. I admit there was some fantasizing about a Goodwrench 350 / auto trans combination while cleaning the case, but the Simple Green fumes snapped me back to my senses.

Watch this space for further news. I'll post before/after pictures soon too.

Hoarding SM420s? I can't get rid of any extras I have...just collecting rust.

Go hook up with some rock-crawlers - it is a desirable transmission for them. They like the stout construction, short front-to-back dimension, and low 1st gear ratio.

Lots of Jeeps out there with crate 350s and SM420s behind them.

OK, here are the before/after pictures. Enjoy!

before picture
after picture

WOW-that looks awesome! Hope it works as well as it looks! Wonder if it might be worth filling it with synthetic 90 wt.?
Speed

Novak suggests using GL-4 gear oil, because the lower sulfur content is easier on the synchros. They know more about these than I do, so I'll trust them on this one. I figure it will be fine with whatever GL-4 oil my FLAPS has on the shelf. But, if I do run across some Red Line MTL (which is also GL-4 spec) I won't turn it down, as that stuff has been very good to the stock transmission in my 220hp turbo Miata.