The Stovebolt Site Forums The Shop Area Engine Shop sm420 trans leak

My transmission is leaking from the front end. I don't recall a seal on the front side. Any experiance or advise on this would be helpful as I plan to pull it down next week.

Virginiafiretruck,

To my knowledge there is no actual seal where the input shaft (clutch gear) enters the case. There is a slinger to keep oil from running out but no mechanical seal. The retainer has a thin paper gasket under it, but I think that's it.

If the transmission is over-filled and the truck is parked nose down on a slope it could leak. It would probably leak out of the bellhousing area...the pan that covers the clutch/pressure plate.

I think this is how it works.

Stuart

The later year models of sm420 actually have a seal I think it was 64ish and up??? Earlier, no seal, just a beveled out section from the main input cover to let the oil drain back into the bearing.

Thanks. It is leaking out of the bellhousing area.

If you can read all the casting numbers on the side of the case we can figure out what year yours is and if it has the seal.

Another thing to consider is when I rebuilt mine (which came out of a 63-no seal) it wasnt until it was derusted and primed that I saw a hairline crack all the way down the shaft housing and into the oil galley part. Just something to consider.

If it is a newer model I think I have a seal you can have for the cost of postage.

Jeff

Thanks. I'm out of town until Thursday (next week) but I'll take a look when I get home and get back to you.

I looked up the serial numbers and it is a 1951 transmission. I guess no seal.

Is there anyone who offers a kit where a seal can be retrofitted?

Are ye shure it's not the rear main seal leaking, bout every one I've ever seen is leaking and ya can't keep the bellhousing dry inside to save your soul. If I remember corectly your's was recently rebuilt but it could still have started leaking. I think they were made that way to keep the clutch lubricated, ha!
Denny Graham
Sandwich, IL

Denny,
I pulled the tranny to replace the clutch and the gear fluid was leaking from the front, not very much but enough to be noticed.
I'm getting another tranny. It works but leaks from both front and back. I plan on rebuilding it to replace the one I have.

I spent the afternoon with the firetruck. It is leaking from the drive shaft seal area(only leaks after driving) AND it looks like the rear main seal(very small leak). I guess I'm going to have to drop the drive shaft and replace that rear seal. Any sources for a sm420 2-ton tranny seal avalable?

Bill, I thought we were addressing a leak at the front into the bellhousing???
Now your saying that your leak is from the drive shaft area. Do you mean from the seal just in front of the center support???
My ’50, 3604 drive line was meticulously rebuilt last summer. No wear at the sealing surfaces in the rear and a NOS U-joint and bushing in the front. When I keep the trans level even with the bottom of the fill hole, the fluid that is washed back to the U-joint seeps back past the bushing and after a drive I have to put a diaper under the center support go catch what is leaking past the seal. I’ve posted about this before but didn’t get a response to it. there is no seal that prevents the trans fluid from leaking past the bushing and pooling in the rear of the short torque tube. Since it slopes rearward there is no way for it to drain back to the transmission. I believe that when enough pools into the tube it leaks past the seal.
So your front leak into the bell housing is from the rear main engine seal???

Denny Graham
Sandwich, IL

There are two. One very small only a couple of drops and a larger one at the place where the drive shave exits the rear of the transmission. I do not have a center support on the fire truck as it has a three part drive shaft. I think the front one is the rear main seal but every "old guy" I've spoken with around the firehouse tells me: "they leaked in 1951 and don't worry about it." It is the larger leak at the rear of the transmission that really concerns me as it seems to leak a lot. I do not like small puddles...

I replaced the one on my 1 Ton. I had a small leak from the gasket and I knew that if I removed the plate, the seal would be done. The one I took out was the old felt type. I think an ordinary parts place would carry the seal.

I believe that BCA/NATIONAL Part # 471341N (Size=2.25" Housing Bore=2.996" Outer Diameter=3.001" Width=0.437") is the seal you need.

The original part number should be 591448 w/ 2 3/8" ID, 3" OD, and 1/2" thickness.

So I guess there were small variances. Hopefully that will get you close.

Good luck, Jeff

Maybe someone that has overhauled one of these can comment here. I get the impression that just taking the plate off can lead to a bit of a disaster. In my case this wasn't a problem. Maybe soemone could confirm or deny this urban myth!

Ive never heard that and cant even see how it could affect anything??? They are such a simple tranny and only go together one way, unless of course you forget to drain the fluid first...

Maybe it's not the trans seal at all. I have seen them leak at the splines (trans output shaft to diveshaft flange). The cure is #2 Aviation Permatex (in a can with a brush) on the washer, under the nut for the driveshaft flange. If you pull the nut and washer off, you will see it wet there. GM made a washer that was coated with a rubber like substance to be used on some trans. Take the nut off and look.

Thanks. I'm getting started on taking it apart this morning. We'll see what comes from the effort.