The Stovebolt Site Forums The Shop Area Driveline Key Part Missing to Finish a SM420 Rebuild

So here’s my dilemma, the Chevrolet booklet that supplements the film strip for the Four-Speed Synchro-Mesh (SM420) Transmission Overhaul, gives this instruction on page 8:
http://www.pbase.com/dennygraham/image/137384964/large

The “spacer” in this transmission is a special cup shaped freeze plug with an OD of 2 7/16” which doesn‘t match anything that I can find under 4.426 in any of the parts books.
Here are a couple of actual pictures of the SM420 out of my 1950, 3600. This is the original transmission that it came from the factory with and I got that from one who was there when it was new and helped maintain it over the years. http://www.pbase.com/dennygraham/image/137384971/large
http://www.pbase.com/dennygraham/image/137384973/original

Now punching a hole in it and prying it out was all fine and dandy back in 1950 when the mechanic could walk across the center isle to the parts department and have Andy Anderson that parts man pull a new one out of the parts bin.
But Old DG taint got that luxury and it’s really dragged me down to a halt on this project. The kit I got from Novak, which I thought was supposed to be complete, does not have one of these in it, in fact unlike the Chevrolet booklet and film strip, they don’t even make mention of it in their disassembly/assembly instructions for the SM420 kit. http://www.pbase.com/dennygraham/image/137384975/large

Anyone run into this during your SM420 rebuild?

Denny Graham
Sandwich, IL

Looks like an expansion plug,could that be part of the bearing ?Did you look at all of the bearings and see if it's in there?
novaks
The parts list at novak show two expansion plugs available part#1 and part#14,could these be what you need ?


Tim

Thanks for the come back Tim, yes I did open all of the bearing boxes, that was my first thought, but there are none in there. There are no expansion plugs at all with the kit.

It appears that at some point in time, I'm assuming later on the later SM420 transmissions, they went with a smaller bore for the front countershaft bearing. And those use a 1.875" closed caged needle bearing in the front. I have one transmission here that I’m not sure what year it is and it’s like that. This caged bearing is in pictured in the kit at the left lower corner of the box.

I was unaware that Novak listed any in the parts lists. I just assumed for the cost of a Master Rebuild Kit that it would have every thing in there to complete the job.

I really planned on having the trans rebuilt and back in the truck this coming week but since I'm not sure about the availability of the spacer/plug at this time I'm not going to go punching a hole in it and destroying it quit yet. And I can't clean up the case to my satisfaction till I get the countershaft out, so as I said, I'm dead in the water on this project for a while till I resolve this.

Denny Graham
Sandwich, IL

Just looked at Novak's product page for the SM420 and I don't see any expansion plugs listed. Where did you see them???

You could always drill a small hole, screw a sheet metal screw for a slide hammer in, and weld up the hole and re-use the plug for reassembly.

Last resort, just turn one out on a lathe with a magnetic chuck. It would also be possible to set the case up in a Bridgeport mill and counterbore the hole a little to create a step for a flat dish-type expansion plug.
Jerry

Well I'm really surprised someone saw this over here Jerry, you‘re probably the only one that has seen it. Good old Charlie shoved me off into some obscure corner second time this week. Been happening about every week, I honestly think the guy has something against me.
I was gonna do a nice picture write up on this rebuild for the less experienced members. I know there are a lot of guys that are scared off by the thought of rebuilding a trans. But I'm not gonna go thru all the trouble of photographing, editing and posting to my album and the forum just to have it pushed off to where no one will ever see it. Something like being an auto shop teacher and spending a few weeks prepping for your class and then they assigning you just baby sit the home room classes.

So, drill a small hole and weld it up? Yeah, that's one idea. I could turn one on the lathe, wouldn't be all that much of a problem but that would be a last resort. I'd just turn it from solid stock and then use a cut off tool to slab it off. Come on, you know me by now, I wouldn't think of modifying the original case. I'd have reproduce the original plug.

I've sent an email off to Novak to see what the experts have to say. I'm sure they've run into this before.
Thanks for the suggestions,
Denny Graham
Sandwich, IL

Denny, there are 2 "expansion plugs" on the Novak list of 67 items, #1 and #14, on the link Tim pointed out, and they are noted as NOT part of the master kit, which appears to include about 16 items

as for Cletis having a bug up his whatsit for you, his job is to sort out posts that more properly belong in the forums devoted specifically to them, and hopefully any less experienced members will be better able to find a thread about 'key parts for a SM420 rebuild" in the transmission forum, either by using some common sense or the search engine here which has an 'advanced' version that allows searching appropriate individual forums

Bill

Ok Bill, I see where you guys are getting that from. I was looking for an actual part number rather than an item number. I'll have to wait till morning to contact Novak, as I mentioned I've sent them an email but there's no substitute for talking directly to a person.
There are times when I think of you guys here on the forum as real people but of course your not. And then there are times when I feel like I'm just talking to the wall especially because I‘m such a nit picker.
Well tks for pointing out those item numbers, which I'm sure aren't the part numbers.
So far it appears that no one has run into this particular design before. That seems odd because with all the early '50 SM420's in service for sixty years I would have thought there would be a lot of them that were in need of a rebuild. With all of the experienced mechanics I thought for sure one of them might have had one of these apart.
I drove my '54 with an SM420 home from the next town over last fall and at every stop light when I pushed the clutch in the main shaft front bearing was telling me not to drive it much further before replacing it. That transmission will be on the bench as soon as I get over this hurdle and get the '50 back together.

DG

Could one be made out of a freeze plug?

If I have to George, I would use Jerry's suggestion and turn a new one out on the lathe. It would only take an hour to setup and turn and it would look exactly like the original.
I can only see it from the outside right now but it appears to be just that, a special freeze plug. I won't know till I get it out whether the relief in the center is actually necessary or if it's just there to stiffen up the plug.
I can't see where it actually as its name implies, "spaces" anything at all. Rather it just plugs the bore in the front of the case. The counter shaft is held in position in the center race of the rear bearing via a bearing retainer washer and two bolts. The bearing outer race kept from moving forward by the outside snap ring and the snap ring is trapped by the rear bearing retainer casting.
I’m on hold till I hear from or contact Novak. I sure wish they had a voice on the Bolt.

Denny Graham
Sandwich, IL

Denny,
That spacer is shown in my 1952 parts book 4.422 591392.
Sorry I am out of stock of them.

Hey Bruce, thanks for lookin' back in those parts bins, if any parts room still had one, yours would.
I've also seen it mentioned in the 1954 Truck Service Manual page 7-127 paragraph 14 so it appears it was used in the SM420 up till '54.
DG

Okey dokey, it’s been a long week. Wasted all of last weekend from Friday till Monday afternoon waiting for and answer from Novak’s tech guy. Never did get to talk to him, he was busy rebuilding a transmission I was told. But by the time I got to talk to someone I’d already pretty much figured out how I was going to attack the problems I was encountering. I finally got an email from Novak yesterday evening but by then I was already reassembling the trans.
As usual for me, this dragged out way longer than the average guy would have taken. Today I don’t like to just rush in with a hammer and punch like I had to do when I was a kid back in the day. Back then I needed the car/truck for work the next day so I had to get it done that evening. But now I can take my time and do it right which includes making some “Special Tools” so the next couple will be a breeze.
So here’s some pics of how the process went:
http://www.pbase.com/dennygraham/sm420_rebuild&page=all

Oh, and in answer to the original question, after having gone thru the trans I can now see that you can pull the counter shaft out the back without having to drive it out from the front. That way you can then drive the “spacer” out the front with out destroying it and service the front bearing. On this one before I knew better, I simply drilled a 1/ 8” pilot hole in the spacer and popped it out with a slide hammer. Tig welded the 1/8” hole back up and reused it.

Dropping the SM420 back in after breakfast and that should be the finish of a total restoration of the complete running gear front to rear on this ‘50, 3604 .

Denny Graham
Sandwich, IL

Thank you Denny for this invaluable information/picture album on rebuilding the SM420. People like you make this place a wealth of knowledge and help out guys like me, saving time and money.
Steve

Denny, The FRONT .........cover / spacer on the SM 420

is GM part # 591392......SUPERSEDED to: GM part # 3795505

see PIC below

Dan's GM Obsolete
Chinook, Montana
phone: 406 - 357 - 2615

Dan's has ( 10 ) GM # 3795505 in stock


http://i669.photobucket.com/albums/...M%20420%20COVER%20SPACER_zpsldrcszue.jpg


George
'54 3600