Which 4spd transmission to use?

[gtblast]

My 1970 GT rear is making noise and I am going to replace it with a 1971 GT spare rear that I have. The 4spd transmission is leaking a lot at the front, so while I have the rear out, I want to take care of the transmission also. The transmission in the car right now is marked with a date of 1975. I have a 1969, 1970, and a 1971 transmission I can use for a replacement. Is there any advantage to using one year over the other? The 69 and 70 I have are dirty and greasy and would need to be cleaned up and gone through. The 71 is in a parts car and is nice and clean as though it has been serviced sometime recently. The 75 in the car now is also very dirty and greasy and leaking a lot. I am thinking of going with the 71. Any insight would be greatly appreciated.

[yellaopelgt]

I might be wrong, but I think the speedo cables might be of different lengths.
The 70 tranny has the 90 degree gear drive while the 71 doesnt. Others might want to chime in to confirm this.......
Joe

[hrcollinsjr]

The early style has an external reverse light switch on the left side with the longer 90 deg. speedo drive. The later style has a reverse light switch that screws into the rear to the right of the output shaft. The two different trannys require different ratio speedos to be correct. I'd probably stick with the earlier style. If you go with the one that you think was manufactured in '75 then it will have the wrong shifter tower or whatever it's called. You should be able to switch them. The GT and Manta/Ascona trannys differ in the angle for the shifter because of the engine tilt. It also effects what the full level is on the transmissions too. The GT 1/2" below the fill hole while the Manta/Ascona is level with the fill hole.

HTH,

Harold

[wrench459]

Look at the tail housing is the shift rail a bolt on or cast into the housing?
The later models were cast into the tail housing. The 69-70 models had bolt ons. As far as I know theres atleast three different shifters all with different angles.
My guess would be to use the 71 trans that has good oil seals.
Correct me if I wroung here ..the ratios are all the same.

[gtblast]

Thanks for the replies. My speedo is currently off, so I suspect it is because of the wrong speedo gear in the later transmission currently in the car. If I have read the threads correctly, I can swap out the speedo gear from an earlier transmission and use it with the later ones and my speedometer should be correct.

The transmission currently in the car and the 71 transmission have the backup light switch sticking out the back, and as you say, the earlier ones had the switch on the side. Either way is not a problem as I can make the wires longer or shorter as needed.

[gtblast]

Originally Posted by wrench459 Look at the tail housing is the shift rail a bolt on or cast into the housing? The later models were cast into the tail housing. The 69-70 models had bolt ons. As far as I know theres atleast three different shifters all with different angles. My guess would be to use the 71 trans that has good oil seals. Correct me if I wroung here ..the ratios are all the same.

Wrench, I will have another look at them all tomorrow. I am certain you are correct, that they all have the same ratios. So far, I am going with the 71 also. Man, that lower right hand mounting bolt next to the exhaust pipe is a bear!! I do not have it out yet.

[opelnut10]

Originally Posted by gtblast Wrench, I will have another look at them all tomorrow. I am certain you are correct, that they all have the same ratios. So far, I am going with the 71 also. Man, that lower right hand mounting bolt next to the exhaust pipe is a bear!! I do not have it out yet.

Yes, that one is one of several on a GT that will cause you to lose you religion You need to get or make a boxed end 15mm wrench that is called a "dog leg". You can make one by clamping the wrench in a vise and heating the shank and bending it 90 degrees, also grind down the thickness of the boxed end for easier fit on the bolt. Put it somewhere in your tool box that you know where it is because you will need it again.