The Stovebolt Site Forums The Shop Area Driveline 1948-1949 3 Speed GearBox

I’ve just purchased a 3 speed gearbox that was removed from a 1949 3100 Chevy truck. Its condition was reported to be operational when it was removed months ago. In the process of basic external degreasing and cleaning I removed the three plugs for servicing it. The shape of the rear housing plug used for replenishment of oil looked different than the servicing plug on our 1948 tranny, see attached photo. Can anyone one tell me if this the correct form shape of the plug?

The pin on that rear plug retains the rear bushing assembly. It's normal. If your 1948 tranny didn't have that plug with the pin on it, it's the wrong one that someone has likely changed.

In his picture it is correct.
The drive U joint is still in, which means the round bushing is still intack.. That pin holds the round rear bushing.
Only Two service plugs. Drain & fill level.
I'm in the process of rebuilding my 1950 3 speed.
Todd

See #12 & 18 on schematic

That plug should be used to put 1 pint of gear oil in the u-joint housing after it's buttoned up.

Guess that I’ll use this plug on the gearbox that is presently installed. As said probably years ago someone used a regular plug instead of the one in my picture.
So far the presently installed gearbox is doing fine.

It does take a bit of force to get that bushing assembly out of the transmission case, so that pin on the plug is likely GM's belt and suspenders approach to get it to stay in place, or maybe to keep it from rotating within the housing if the u-joint yoke were to seize on the bushing.

Kevin and Todd:
I'm set with the servicing plugs. Could you set me straight on amount of oil required. The 1948-1953 service manual calls to add 1.5 pts (24 oz) to the tranny through the side filler port, ok. And past servicing videos to add 1 pt (as Kevin states) through the top u-joint port, ok. I've removed these two plugs and the speedometer drive adapter and (with tranny on the bench) and looking from the u-joint housing forward I see that the housing all the way from the u-joint housing to the gear housing reservoir is hollow. To me adding 2.5 pts of oil through the speedometer port would fill the gearbox and lubricate all the gears, bearings and the u-joint.

If you fill the tranny through the rear plug then all that 90wt has to pass through the ball bearing to get to the gearbox. Do your tranny a favor...
Fill according to the specs. 90wt Mineral oil in the main fill port and then add some to the rear port, or through the speedometer port. Then you will have properly lubricated the gearbox and the rear bushing/cross joint.

Also a good idea, when you check the lube level in the trans and need to add some, check the rear end level and see if it is higher than expected, a sign of oakie bushing seal failure.

New question: Can any of you decipher for me the numbers on attached casting and serial number? For picture See pictures in initial submittal.

Texas - Deciphering from your pictures, the main case casting number is "591615" which corresponds to 1948-54 Comm truck 3 speeds. The main case casting date code "K83" = November 8th, 1953. The stamped serial number "MIII" is decoded as M = Muncie plant, II = 11th day of the month and II = the eleventh month of the year (November) which coincides with the case casting date code.

The side cover casting# "3686832" is the same as the GM assigned part number and used on ALL Chevrolet 3 speeds (other than FK & FJ models) from 1948-54.