The Stovebolt Site Forums The Shop Area Driveline Need Clarity SM318 1950

Ok, When I removed my 1950 3 speed clutch gear I had 14 short thick needle bearings & 24 thin needle bearings ALL come out of the Cluch Gear Inside where the Main shaft slides inside it.
My Chevrolet truck shop manual 1948-1953 page 7-7 shows ONLY one set of needle bearings.

Today I take apart another (got one local the other day, unknown history)1950 3speed and in the Clutch Gear there has Only the short 14 needle bearings. The Later Schematic does show TWO sets of needle bearings. Looking at the inside (where needle bearings go) my original is more open & deeper compared to the one I just took apart?
So does it depend on the Clutch Gear I use? I am assuming Different year Clutch gears but are the same size/Part ?
Thank you for any input on this.
Slight note the Shaft Splines on the one I just got is longer compared to my original but same teeth count.
See picture.
Todd

Todd - Did we ever decode the casting numbers/date codes on your existing transmission? I know we did for the later 1955 SM318 that doesn't support your torque tube drive.

Reason I ask is there was a design change of the internals of the SM318, from what I can tell searching through the Chevrolet Data files. In 1953 the SM318 3 speeds switched from the 1948-52 single row of roller bearings design between the clutch gear and main shaft, to the double row (14 thick and 24 thin) roller bearing design setup. There were other internal part changes, like the gears which prevent the '53 and later pieces form swapping with the '52 and earlier parts.

Below are screen shots from the GM Heritage center files showing the needle bearing transition.

I would also mention the 1954 Chevy Truck Shop Manual disassembly/reassembly instructions for the 3 speed describe the dual row of needle bearings in the service/repair instructions guidance. They can be found in step "6" of this page from the manual.

Perhaps you can post the two transmission casting numbers details so we can try and decode them and confirm what they actually are?

Ok #1 is my original
#2 is the one I just got#2 serial stamp: H0II3328

No visible number on my original (stamped #)

Ok main shafts Different. See picture.
11 3/4+ and 13 1/16
Overall Same when combined *
I have to double check shaft diameter next.

*** Same Diameter shafts.

Here's what I decoded based on pictures 1-4.

1 - Casting Number: 591615 = 1948-54 SM318 3 speed main case - casting date code: D65 = April 6th, 1955 - No stamped serial number = indicates this was likely a service replacement (considering also the date cast was after the end of the 1954 model year. *

2 - Casting Number: 591615 = 1948-54 SM318 3 speed main case - casting date code D280 = April 28th, 1950 - Serial# HO113358 - HO = 1950 3100 series truck - 113358 = 112,358th transmission built at Muncie Plant (all Unit serial#s started at 1001). Serial# info taken from the 1950 Chevrolet Truck Data PDF.

More on the different main shafts in next reply.

* This makes your existing "original" SM318 not the original transmission from your truck, and of the 1953 and later design which requires parts specific to that design to rebuild/repair.

Dan,
Thank you.
Yea, I was confused on the "5" on date...for my original one.

On to some details on some of the differences (not all) of the 1948-52 and 1953-55.1 SM318's.

First is on the "clutch gear" (transmission main drive). The first pic below shows different descriptions and part numbers between the early and later SM318's (prior to 1955.2 Series changes).

Second is on the roller bearings for the clutch gear to main shaft. The 2nd pic below shows the part number and quantities for the different early and late designs. The Early & Late large diameter rollers (14 count) are the same in both versions, but the Late design also uses the skinny diameter rollers (24 count) as we previously discussed.

As mentioned, there are other design differences, including the gears and synchro hubs, etc. primarily due to the difference in main shaft thickness. It might be easier to address those individually as needed for pieces you find you will need to replace because of wear or damage. Will be glad to help with that when you get to that stage.

Bottom line is you won't find much to swap between these transmissions to fix up your existing later design one unfortunately.