oldsmosteel

i own a 68 442 Turnpike w/ the stock th400. i found a fully rebuilt 70's bop th400 (short shaft) for a real good price. my questions are...

1) do all stock th400's have standard or same shifts/gearing? i'm running the stock 400bb, 2bbl carb & 2.56 differential gearing. i'm surprised how decent on gas the car is on the freeway which is where i do most of my driving & i'd like to keep gas mileage where it is.

2) i know the 67-68 turnpikes were made to be more economical than the standard 442's.. does anyone know what the factory shifts/gearing is for a turnpike th400? or like i said above would it be the same or close to the same as any th400?

3) what determines the shifts/shift points on a th400? are the shifts adjustable & can they be changed higher or lower by swapping parts?

4) is there anyway to determine shifts/gearing ratios of a th400 dissected on the bench out of the car?

i apologize if my transmission verbage is incorrect but as always thanks in advance.

Marty

joe_padavano

Unless it's been rebuilt with expensive aftermarket gear sets, every TH400 made has the same gear ratios, 2.48:1, 1.48:1, and 1:1. Shift points are customized to each factory application by changing the governor, the plate between the valve body and the case, and the valve body itself. There are also installation-specific changes to the clutch packs, including the number of plates in each and the composition of the friction material. It is not particularly easy to simply take one apart an tell what calibrations it has by looking at it. The two letter code on the outside of the case will provide an indication of the original application and thus calibration if you had a cross-reference, but even that assumes nothing has been changed (like a shift kit added). Also be aware of the TH375, which is a light-duty TH400 built in the late 1960s (before the TH350 was released) that used the smaller TH350-size output shaft and lighter duty clutch packs.

oldsmosteel

Good information! Thanks Joe

Mikes442

Hi Marty and Joe
I'm the one changing my M21 to a T400.

I couldn't get to my T400's so I bought a short shaft T400 at a swap meet. While I was looking it over found out it was stamped CK78 which told me it was a Chevy Trans. I asked the guy and he said it came out of a Diesel pickup(Olds diesel). $30, no brainer!

When I had it rebuilt with a shift kit my guy told me that these T400's has either (Can't remember) 2 or 3 extra clutches and a more heavy duty output shaft.

I didn't know that, but it makes sense.

Mike

MDchanic

I can't say when the TH375 started production, but they were definitely still used on small block B-bodies in 1973, and I think '74 as well.

- Eric

joe_padavano

That's correct. There are a number of different versions of the TH400, with different size clutch packs. Years ago I rebuilt the TH400 in my 69 H/O using a B&M kit and I had to buy one extra clutch plate because the kit came with five plates for one of the clutch packs and the H/O trans had a clutch pack with six plates.

MDchanic

Also, for the sake of clarity, there was also a TH375-B, which, rather than being a downgraded TH400, was an upgraded TH350, used in some trucks, I believe.

- Eric