The Stovebolt Site Forums The Shop Area Driveline stumped

Maybe a stupid question: I'm working on my 1950 GMC 150, with four-speed transmission, all original. I'm trying to change the transmission fluid. Easy enough to find the drain plug, but for my life--and after poring over maintenance and shop manuals--I can't figure out where to put new fluid in.

Is the transmission a sm420 like the chevrolet trucks?

see here if it is the same as the chevrolet:

https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/700011/1.html


Fill is on the passenger side half way up the case. Or take off the shift rod and fill there but you probably still need to take out the side plug so you fill it to the correct level (bottom of the fill plug hole)

Same square head plug as the drain plug. It's located, as said, about half way up on pass side. Fill till it overflows, wait till it stops overflowing and put plug in. Vehicle level, of course. You need a lube jug with a pump and plastic hose. Sold at most FLAPS. 80w90.

Filling to that fill-plug hole will work fine. However, when the oil get warm it will expand and rise above the hole opening, possibly dripping out a little.

Manuals used to say to refill until the level reached about 1/2" below the filler-plug opening, but to top-up a warm transmission/fluid to the bottom of the opening (until it drips out while filling).

" . . . at operating temperature, lubricant should be level with bottom of filler plug hole."

If the fill pipe plug leaks, the drain plug would leak. Same threads, same plug. Total immersion. The pinky test/1/2" below is what manual says so do that.

bartamos,

I do not understand your reply/post:
"If the fill pipe plug leaks, the drain plug would leak. Same threads, same plug."

The drain plug is on the bottom?
The fill plug is high up on passenger side?

The plugs themselves might be the same, but, the two plugs might not be leaking at the time.

Also: another filling topic: if you have an enclosed drive shaft, when you fill an emptied transmission or u-joint housing the gear oil levels out with the u-joint housing. It is a good idea to recheck a transmission oil-level after driving it, and after letting the gear oil "settle down".

Maybe I misunderstood. When you said "possibly dripping out a little"............where would it drip out? My point was: if the bottom plug does not leak, why would the top plug leak. Same exact "sealing" method. Pipe plug. If oil rises above the fill hole plug, why would it drip out? The bottom plug is immersed all the time and no leak.

sorry, bartamos, I think we are posting about the same thing

jameswm1663 was speaking about the fill plug, but the other plug (drain plug) entered the conversation
"If the fill pipe plug leaks, the drain plug would leak."

Yes, both plugs could leak, or maybe not.

I think on the older 4 spds there is a weep hole so if it is overfilled a little it will drip out of the weep hole could get into the clutch.dont know exactly were the weep hole is. was surfing the web as i was just given a old sm420 with a torque tube universal joint and was looking on a site that shows years i beleave they started using them in 47 to 67. They seemed to be popular with the 4x4,rock crawler.guys for the granny gear and the durability.there was mention of the older ones having a weep hole as one of the guys posting had oil on his clutch s they are often climbing up steep terrain .

Just took a look on the novaks guide to the sm420 and the older 47 to 53 tranies have the weep hole in the6 o clock position of the bearing retainer.

Pressure vent hole?