Welcome Guest [Log In] [Register]
Viewing Single Post From: How to Determine Your Transmission Final Drive Ratio - In the Car or on the Bench

Here's how to determine the final drive ratio if the transmission is out of the car already. (See page 1 of this thread if it's still in the car).

Johnny Mullet had the transmission from Blue Jay laying on the garage floor so I went up to his place and he helped me run through this and take pictures. My thanks to him.

Also, pacapo deserves a lot of credit for this whole concept because it was his original post that put me onto the idea of turning the engine over so many turns and counting wheel revolutions. I neglected to mention him in my original post because I couldn't remember where I saw it in the first place. It was his post that started all this.

While working out this procedure I tried to make it generic enough that if you unexpectedly stumbled onto a transmission somewhere, you could, with a couple borrowed hand tools and about 10 minutes time, figure out what ratio it is.

First, the stuff you'll need to do this (assuming the trans is already out and on the bench).

- Something to rotate the input shaft. A clutch disk would be ideal, a pair of vise grips works fine, and an 18mm deepwell socket slides right onto the spline (ratchet & short extension too)
- An ordinary business card or a similar size piece of thin cardboard or thin plastic
- (Optional) A pair of scissors or a sharp knife. (you could actually tear the pointer material instead)
- (Optional) Something to make it easier to keep the left axle socket from turning - 7/8" dowel, piece of broomstick, rag on the handle of a ratchet, a pair of needle nose pliers, etc. (again, optional)

Here's the procedure:

Make a pointer from the business card (if you're going to do this often, I'd make one out of thin plastic)

Posted Image

Bend the 1" x 3" tongue slightly lengthways and insert it into the splines of the RIGHT (passenger's side) (must be in the right side) axle socket so the pointer almost touches the case

Posted Image

Posted Image

Now you have to put the transmission into 5th gear. This was much easier than I expected. On this transmission, which still had a big section of the shifter linkage attached, we were able to do this with no tools whatsoever just by grabbing the linkage shown in the photo and pulling and twisting by hand. For a transmission without the linkage you may have to use vise grips or a screwdriver slid through the holes.

First, slide the linkage in and out to find neutral (there are three distinct positions, just like 1st, neutral, 2nd in the car).

Then rotate the linkage clockwise (it will rotate easily in neutral). Once it hits the "stop", twist it hard clockwise (you'll feel the spring resisting just like when you go for 5th or reverse in the car) until you hit the second hard stop.

While holding it tight against that second stop, pull it towards you and you'll feel it click into 5th gear.

I really thought this was going to be the hardest part of this job and it was a piece of cake - really went smoothly.

Posted Image

Next, VERY, VERY IMPORTANT!. You have to keep the LEFT axle from turning when you rotate the input shaft. An axle stub would be ideal for this purpose but if you don't have one (I know I don't), you can use your finger. I really didn't think I could hold it in place without an axle or a dowel rod or something but, with some effort, it can be done. You DO have to be sure it doesn't turn because the results could be very misleading if it turns, even slightly. So if you have ANY doubts, use an axle, a broomstick, a dowel rod, a pair of needle nose pliers held wide open or some other object that will prevent rotation without damaging the splines or the seal.

Posted Image

Next, you have to attach something to the transmission input shaft to turn it (and to serve as an indicator so you can get exactly 3 revolutions).

- A clutch plate with a piece of tape on it would be ideal but not everyone has one laying around .
- Vise Grips clamped onto the shaft would work fine, with a rag to protect the pilot shaft or spline.
- An 18mm six point socket slides onto the input spline as if it was a bolt head. A deepwell is best because on a shallow socket the pilot shaft extends into the square drive hole. We had two shallow sockets between us and only one would work, but even then we couldn't get the extension all the way into the socket and had to hold it in place as we turned the ratchet. That would be pretty difficult if you were working by yourself and trying to hold the left axle socket at the same time. My advice? Use a deepwell. You'll also need a short extension and a ratchet (you can see the shallow socket installed on the shaft in photos 2 & 3 above).

The stud that sticks out of the case just above the right axle opening makes a convenient marker for counting revolutions.

You are now, finally, ready to do something! Get everything setup as shown in the picture below. Align your rotating tool with the stud as shown. Align the "pointer" so it's at BDC (6 o'clock). Hold the left axle socket to keep it from turning. Transmission in 5th gear. You're ready to go.

Posted Image

Now, rotate the input shaft exactly 3 revolutions CLOCKWISE and look at the position of the pointer. If you're working on a 4.39 transmission, like we were, you should see this:

Posted Image

Here's a summary view of where the pointer will be for the different final drive ratios.

Posted Image

If your results don't make sense compared to this last photo, one of four things probably went wrong:

1. You didn't get the transmission into 5th gear - you probably didn't keep it twisted when you pulled back on the shaft and ended up in 3rd gear instead of 5th. (don't ask how I know this!)

2. You allowed the left axle socket to rotate, or maybe bumped it when removing your finger or dowel.

3. The pointer wasn't tight enough in the spline and/or it didn't turn freely with the socket.

4. You inserted the pointer into the left spline instead of the right. This will produce backwards results: 3.52 will be 3 o'clock, 3.79 will be 5 o'clock, 4.10 will be 7 o'clock, etc.)

If the results don't make sense, start over. It's easier the second time and it really does work.

The only things that I can imagine that would cause this to not work would be internal transmission problems, like gears being bad or something binding up in the transmission. (You don't want that one anyways so who cares what ratio it is, right?)

So next time you go to the junkyard or on a "road trip", make sure you take a business card, a sharp knife and your Vise Grips!
Edited by Wobblybob, May 3 2011, 08:43 PM.
Offline Profile Quote Post
How to Determine Your Transmission Final Drive Ratio - In the Car or on the Bench · Transmission/Clutch/Axles