The Stovebolt Site Forums The Shop Area Engine Shop Downshifting the SM420 ('51)

Seems I can't downshift into 2nd gear, it just grinds. Haven't tried double-clutching with a rev in neutral yet, but I doubt that it would be necessary with a properly working box.

Am I correct in my assumption that the trans needs fixing?

Any thoughts on this?

I was havin the same issue, could never get it to drop down into 2nd or 1st when slowing down.
In my gearbox only 3rd & 4th have synchronizers, so you have to get the truck slowed down enough to get it to drop into 2nd & 1st, double clutching helps alot too (even MORE fun when you add shifting the 2-speed rear end!). The shifting instructions say to double clutch it as well. I've had to practice, but have gotten it down pretty well.

So a syncro can be made to only work on an upshift, but not on a downshift?

When I downshift with mine, I give a little shot on the gas between gears. There is enough drag with the clutch in to make a difference. My synchros will work on their own but I like to take some of the load away from them to help them last longer.

Woody my Bro had a International ....late 40's ..I tried as long as he had it to downshift to the low gears ..he could do it "ALL IN THE TECHNIQUE" ..I COULD NOT !...If I blew a shift it was come to a stop an start over ...lol
Soo I FEEL YOUR PAIN ...lol

Downshifting by the Double-Clutch method just takes practice. My suggestion is to just start off by double-clutching your upshifts. In every manual that you drive, just keep hesitating in neutral and depress and release the clutch. Once that pattern is down, now just apply the same method in reverse, only this time while hesitating in neutral with the clutch out, just give it some gas. The hard part is, just how much gas to give it. When you are rolling slow and need to downshift, a little tap on the gas is all that's necessary, but the more speed your rolling, the more gas is needed. There is a kind of a narrow window as not enough gas and too much gas will both make it grind. Also some transmissions are harder to downshift shift than others. The greatest importance of downshifting is to put no strain or "lugging" on your engine. The only alternative is to pull over and start again, as mentioned above.

Bill

Altho I'm not real familiar with the older SM 420, some transmissions out there have no syncros, others fully are syncronized, generally speaking.

Bill

The SM420 should be syncro in all forward gears but first... the SM is for Syncromesh (sp?)

No, No, No, no need to double clutch your SM420 Wooody. You have it correct, all but 1st. is sycronized. If it's grinding when your down shifting into #2 it just means that the syncro rings are shot. Ya have to bring the gears up to the same speed or in syncronization with each other for them to slip to geather. If for instance top speed is 15 mph for your second gear and your pulling it down at 25 or 30 your really winging the speed up on that gear and if there is to much slipage on the cones then it won't be able to pull the gear up to speed.
Your were talking about rebuilding it and your kit from Novak I know will have new syncro rings in it.
You gonna be so happy with the way it shifts after you rebuild it, that ya won't be able to wipe that smile off 'n yo face!

Denny Graham
Sandwich, IL

Yea Denny, this thing grinds unless I come to a complete stop and now with my new 3:55 R&P (with completely rebuilt rear end ) I'm needing that 2nd gear if I'm still rolling and barely stopped, whereas before I could use 3rd without lugging the engine.

It did seem odd to me that a tranny that stayed in production as long as it did would have such a fundamental shortcoming. Luckily, this is an easy rebuild.

Double clutching will make the synchronizers last a very long time, because by double clutching you reduce or eliminate the work they have to do.

An easy way to tell if you are double clutching properly, is to watch the tachometer. If when you finally complete the shift and let out the clutch the engine RPM should not fluctuate any. In other words you want Exactly the same RPM just before and after you release the clutch with the trans finally in the chosen gear.

The goal is to use the throttle to set the engine RPM to exactly what it needs to be for the road speed and gear (double clutching as needed so the transmission is thinking the same thing), then let the clutch out and continue driving.

Grigg

I understand the principal because back in the 60s I drove a Chevy pick-up as part of my job and I used to see how well I could shift gears without the clutch. I don't recall driving all over town without using the clutch but I do remember getting to the point where I could do it without much problem.

I think I'll just rebuild my transmission... at this stage of my life that's an idea I can live with.

Thanks for the info. I had to double-clutch on some Army trucks (up and down) because they wern't synchronized, but I didn't understand what was happening inside.

Woody, not trying to hijack the thread but you mention your new 3.55 setup. How's that working? I've ordered a set, have yet to get it.

I don't mean to hijack the thread either but I believe after much practice and many a grinding gears that maybe I should bite the bullet and rebuild my tranny too. How difficult is this and where is the best place to get parts?

Bill, do you have an SM420 in your truck? They were not available until late 47... your truck should have an unsynchronized 4 speed I thought?

I have heard that Novak is the place to go for rebuild kits, but have no personal experience with them.
http://www.novak-adapt.com/catalog/sm420_package.htm

Grigg

Yep, your right, I'll shut up now. I see a post that has to do with trouble shifting and I'm in, seeing as I still can't drive mine real well. Still may need a rebuild though.
Sorry for the intrusion.

While you're in there replacing the brass blocker rings, do a close inspection of the splines where the synchro collar meshes with the gears. I like to use a magnifying glass to inspect the ones I repair. If there's any wear or chipping evident on either the internal splines on the collar, or the external teeth on the part of the gear that meshes with thw shift collar, the blocker ring swap won't help much. If it's been grinding very long, there's almost ceertainly a little gear wear or damage to deal with.
Jerry

Bill, you can update to an SM420 if you want something synchronized. It may involve a bell housing swap too? Not real clear on what interchanges with the older trucks?

Grigg

this old guy i knew got his p. licence signed by the wright brothers told me dont down shift unless you realy have to and your tranny will last twice as long so when im coming up to a stop i just pop it in neutral till i stop and start out in first only time i do down shift is for hill but i think he was right i think it keeps a lot of ware out of the tranny also some stp. mixed in the lube will make a gringing tranny shift smoother

I didn't do the work myself... I'm a sheetmetal guy and know when to turn the heavy stuff over to a professional. I'm VERY lucky to have a guy (genius, actually) 5 miles away who does a lot of work on classics and modifications. I made it a point not to pester him but I did stop by one day to see how it was going and the whole rear end was in pieces, I guess because I had him rebuild it while he was in there. He charged me $350 for the labor. I bought the parts from Patrick's.

I can't tell you how difficult it was but I can tell you he did the whole job without opening the manual or referring to the skimpy directions and vague photos that Patrick's supplies. The installation seems perfect... the rear end is now silent. Whatever wear was there is gone and the gears are perfectly set.

The gears make an immediate impact on driving. My 216 still isn't happy over 45 but I'm going to swap it out for rebuilt 235 so that's a moot point. For low speed driving around town, the taller gear means a change in driving habits. Whereas before I could drop down into 3rd when almost stopped and pull away with little effort, I now need 2nd gear which, in my SM420, needs a new synchro. So for now I'm trying to anticipate so I don't get to a near complete stop situation.

First gear is still pretty much useless without a heavy load, but 2nd is still very comfortable for pulling away from a stop. I'd have to say that the SM420 and the 3:55 make a pretty nice combination... Pleasant 3-speed for general driving with an "extra" 1st when she's loaded up with cargo... nice pull-away power without the instant redline that you get with the 4:11.

All driving under 45 is profoundly different - truck is much more docile with noticeably less RPMs in all situations. I'm sure 55 will be acceptable with the 235 and my 15-inch wheels but I have a feeling that 16-inch wheels are in my future.

So, I can't advise you on the level of difficulty for the installation but I'm very happy with the results and once I get the rest of my drivetrain in order I'll be even happier.

Thanks Jerry, that's excellent information and I'll take it into advisement.

Hey guys, Ya know if your real good at timing a shift is when you can do it without the clutch !!
Not that I'm suggesting that but I've had to do it a few times when a clutch rod fell out. Since then I've practiced it just for the fun of it.
I must say it works easier with a truck trans than car trans. And I wouldn't feel bad if you never learn that double clutch down shift, as I feel it is a inborn talent. My wife hasn't learned it in twenty years, so I gave up on showing her how !! lol

I bought a rebuild kit from Novak and I got excellent service from them when a seal in the kit arrived damaged. However, the rebuild kit does NOT include a synchro for second. That one is built into the large gear and cannot be replaced.

Thanks polarcat, I'm going to contact Novak for a verification of this and to get their advice on how to proceed.

One thing I've learned about the Internet Discussion Boards is that you need a lot of lead time when posting a question... look how long we've been discussing this and it only now comes up that the fundamental issue may have been poorly advised.

This is not to say I'm taking polarcat's word as gospel. I will verify this FIRST and then follow up.

While you are talking to the folks at Novak inquire about parts availability for the newer (starting in 1968 I think) SM465.
You may want to consider swapping to a SM465, it replaced the SM420, and is essentially the same transmission, but not actually the same... confusing right?

Grigg

Yep, looks like he's correct Woody. I'm surprised that we didn't hear from one of our transmission experts on this one. I just naturally thought from past experience with 3speed syncro clutch or syncro drum that the syncro rings would be in a kit. I probably sold 3 or 4 of them a week for the 3 speed trans back in the 60's. The hot rodders were notoriously hard on syncros. Your problem is still in the 2nd speed syncro path somewhere and there must be a way of servicing it.
From what I can make out in the service manual the second speed syncro cone is machined/ground into one side of the 1st & rev slider. Hard to tell what ya got without having it out on the bench to actually look at it. Apparently there is no brass insert in the 1st & rev slider side of the syncro.
The second speed syncro ring looks like the most likely culprit. I would think that there has to be some brass in the path somewhere, but I can't tell by looking at the manual whether it’s brass or has a brass insert in it. I’d be surprised if it were steel to steel but it might be.
Unless someone in the know who's rebuilt one speaks up about the only way we are gonna know is to tear into it and find out what's what.

I haven't been into my 420 yet cuz she's a shiftin' just fine. But I'm gonna do the bearing, bushing, syncro, seal thing in the near future just so I won't have to worry if'n I'm out on the road hundreds of miles from home, like maybe in Raleigh or someplace like that.
I have a spare SM420 in the corner but I have too many projects started already, I just can't start another one like tearing into that trans, till I finish some stuff up.
Good discussion, lots of info coming out of it. Let us know what Novak has to say.
Denny Graham
Sandwich, IL

I'm gonna say it needs rebuilding. I was able to downshift my SM420 into all the gears, and I know mine needs rebuilding.
I think your symptom is a harbinger of more problems to come.

Definitely doing a complete rebuild and whatever else it takes to get it right.

Finally heard back from them... Here's the exchange"

ME:
Apparently the syncro for 2nd gear in my 1953 SM420 is bad because I cannot downshift into this gear, it just grinds. I was hoping that using your rebuild kit would correct this problem but someone has told me that the syncro for second gear is built into the "large gear" and cannot be replaced. Is this true?

NOVAK:
Good question. The syncrho that is built-in is actually first gear and it only operates when upshifting into second. If you are shifting from third down into second, then that ring is easily replaced like any other conventional transmission. Our rebuild kit should have you covered here.

See the previous post with the correct info on this issue.

Now that makes more sence Woody. last time I checked their kit was only around two hunderd bucks. That's a cheap fix in my book assuming all the gears are ok.
Take some pics when you do yours, I'm sure there are a few guys here that would like to follow it, including me.
Denny G

That explains why my SM420 grinds a bit shifting into 2nd from first but works just fine shifting from 3rd to 2nd. I rarely use first anyway and just double clutch going into second from 1st.

???

You always shift from NEUTRAL into second, no matter which gear you were in before. Look at the 48-51 shop manual, pages 7-15 to 7-17. The operation is explained in much detail there.

They are saying that one synchronizer works with a 1st to second shift, and a different one works from 3rd to 2nd. Not sure the significance, but that's how I read it.

Yes, you must be in neutral at some point during any and all gear changes.

Yup, that's how I read it, too. And I think they are quite wrong about it. By the time you're in neutral, the tranny doesn't remember where you came from. Neutral means:

First/reverse gear not meshed,
second speed gear spinning freely on main shaft,
third speed gear spinning freely on main shaft,
input shaft and main shaft not locked to each other.

Now you shift into second, which means you lock the second speed gear to the main shaft. There's only one way to do this, and only one synchronizer to help you.

However, the second speed synchonizer is actuated by a couple of springs. Rather than a worn synchro, it may just be a broken spring.

That's what I thought too...
I also thought an SM420 was NOT synchronized in 1st to second or 2nd to first, both of the ones I have and have driven I had to double clutch to make those shifts, and I never worried about if it even had a synchro or not, as I assumed it did not.

Grigg

Well, one thing is for sure... the box is going under the knife and the results will be posted here.