Saginaw 3-spd Gear Ratio Identification

[jcwren]

I'm having Jim Cook Performance Transmissions & Rear End source me a rebuilt 3-spd Saginaw. I haven't pulled the trans from my truck yet, but from what I read, the inline 6 should have a 2.85:1 first gear, which means there should be no grooves on the spline shaft.

The picture below is from a Saginaw 3-spd off an 1970 inline 6 that I bought about 6 months ago and that I'm using as a core. FWIW, the engine block casting numbers on this engine and the engine in my truck are the same casting numbers, although I can't be sure that either transmission hasn't been swapped at some point.

This one has GM28 / 3925647 / L280 cast on the passenger's side, and the flat machined area has no markings. I'm not sure about mine right now (it's up at the paint shop getting some scratches fixed). My notes says GM51 / NF3899838 and S0R03 (Saginaw, October 3rd 1970). I can't find an 'S' code anywhere on this one.

The picture, to me, looks like it has 3 grooves, making it a 3.50:1 ratio. Am I reading these grooves right? And would they have paired a 3.50:1 with an inline 6?

Any ideas as to what I'm looking at? I have some other pictures if those would help.

[weim55]

I believe the unit you pictured is a two ring unit. I don’t believe that ‘Ring’ closest to the input retainer is one of the identification rings. It’s easy enough to go ahead and just check the ratio By putting the unit in first gear and count the number of turns for the input shaft for one full turn of the output shaft.

3.11 1st.
1.84 2nd
1.00 3rd

Steve weim55 Colorado

[jcwren]

Originally Posted by weim55

I believe the unit you pictured is a two ring unit. I don’t believe that ‘Ring’ closest to the input retainer is one of the identification rings. It’s easy enough to go ahead and just check the ratio By putting the unit in first gear and count the number of turns for the input shaft for one full turn of the output shaft.

3.11 1st.
1.84 2nd
1.00 3rd

Steve weim55 Colorado

Without the truck handy, do you know which positions the selectors need to be in for 1st gear?

[weim55]

Super easy to find first gear: to start put both selectors in neutral which is straight up and down 12-6 position for the selectors on the side cover. To verify hold the output shaft and attempt to turn the input shaft. If the shafts are not locked you are in neutral. Next take the rear selector and twist counterclockwise, that will engage first gear and allow you to verify the ratio by turning the output shaft one full turn and counting the number of turns for the input shaft.

Steve weim55 Colorado

[jcwren]

Originally Posted by weim55

Super easy to find first gear: to start put both selectors in neutral which is straight up and down 12-6 position for the selectors on the side cover. To verify hold the output shaft and attempt to turn the input shaft. If the shafts are not locked you are in neutral. Next take the rear selector and twist counterclockwise, that will engage first gear and allow you to verify the ratio by turning the output shaft one full turn and counting the number of turns for the input shaft.

Steve weim55 Colorado

Excellent. Thank you.