65oldsguy

I am considering swapping the original Jetaway 2 speed tranny in my '65 Jetstar 88 for later model TH400 short shaft. Does anyone know if I have to change the driveshaft as well (I think I have to change the yoke), or will it fit? I can find lots of threads on these swaps in Cutlasses, but not in the full size 88 series. Thanks.

joe_padavano

Doesn't matter what car line you're talking about, the TH400 is still longer than the Junkaway. You do the math. Also, do you have a short-tail or long-tail TH400? That's an extra 5" on top of the normal 2" difference in length. Bottom line is to install the trans, bottom the front yoke in the trans, pull it out 1", then measure the U-joint center-to-center distance from front yoke to pinon flange and that's the measurement for the new driveshaft.

Also, be aware that your 65 won't have the holes drilled in the frame for the TH400 crossmember location, so you'll need to drill new ones. If you have a convertible with the boxed frame and the welded tabs for the crossmember, you'll need to weld extensions onto the tabs.

65oldsguy

Thank you. It is a short tail (I called it a short shaft by mistake). I have an opportunity to pick up a driveshaft from a 65 Dynamic, which had the TH400, but I don't want to spend the $$ if the driveshafts are the same length. Thanks again. MT

HonestDave

Maybe you can get the other drive shaft cheap. Then you know everything will fit. Don't forget, your gear indicator won't line up any more, and you might have issues with the electric kick-down. I've made so many alterations over the years, and caused so many headaches, I've started to go back to stock whenever possible, just for simplicity. I guess I don't require the same power I did when I was younger. I've morphed into a right lane, speed limit sort of old git !!

DeltaPace77

While not doing the exact thing as you, have swapped a few transmissions. Did a swap on a two speed auto to a TH400 short tail on a 68' Pontiac, and other than changing the yoke, the shaft length was fine. My current Olds now has a 700R4, where a TH350 once resided, and had to shorten the driveshaft a couple inches. Didn't replace the shaft, but sent it out for shortening, and rebalance. Also had to move the crossmember back. An o.d. tranny swap, is really the only one I'd curerently ever do. On my car with 3.73 axle, dropped highway cruising rpm from 3500 to 2500.

copper128

Don't forget the Jetstar and Dynamic had different rear axles, so the pinion length will be different. You may also need a conversion u-joint to mate the D-88 propshaft to the J-88 yoke. Check out all this stuff before you start buying stuff.

joe_padavano

Sorry, but I don't believe that. There is no possible combination of transmissions and tailhousing lengths that would let the same driveshaft be "fine". Do I believe that you got it installed and drove the car? Yeah, sure. Did you ever verify how much engagement you had on the front yoke in the trans? Do you feel lucky?

HonestDave

I thought the Jetstar and Dynamic had the same rear end. I've had quite a few of these mid sixties large Oldsmobiles, including Starfires, and other than gear ratio, the only real difference I noticed was 63 and older. They seemed to be slightly different than 64 and up. (Or was it 63 and up?)
Is it possible you're thinking Jetfire, the smaller turbo car, or have I just been missing something?

copper128

65 Jetstar 88 used the 8.2" BOP 10-bolt rear. All other 65 88's, which were 425 powered, used the 8-7/8" P-type (10-bolt cover/12-bolt ring gear) unit which is physically larger and heavier.

The use of the mid-size 10-bolt differentials in the small-block 88's carried on at least through 1968, probably longer. My '67 Delmont 330 has a 8.2" BOP 10-bolt and my '68 Delmont 350 has a O-type 8.5" (12-bolt cover/10-bolt ring gear).

HonestDave

Interesting to know. Guess I never paid enough attention to it. Most of my big early-mid sixties Rockets had the 394 or 425 in them. Probably the reason most of the diffs were the same in early sixties and again mid to late sixties. Do you remember what year the diff style changed? I know I was going to put a 64 Super 88 posi in my 62 Starfire, but it had a different ratio and was a slightly different style. Wasn't sure that it would even hook up correctly, so I sold it.

copper128

I believe the old style big front-loaded "pumpkin" Olds/Pontiac rear was used through '64, with the exception of the '64 Jetstar 88 series, which got a version of the new 8.2" BOP 10-bolt used in the A-body cars of that year.

DeltaPace77

I'm not one to make things up, and what I mentioned earlier, is exactly what I did. I worked with the tranny builder before doing this, and all worked out fine. Probably would have been fine doing a TH350, but everyone was saying TH400, so what I did. Only things I did, was a shift kit, redrilled the flex plate, and changed the exhaust for the vacuum modulator. Maybe don't believe me, but I did it, so don't need to hear any dimesional theorys. If anything, the TH400 install, had more yoke tranny spline engagement than the original 2 speed. This was a long time ago though, and really can't swear, if the tranny builder did something to the drive shaft.

oldcutlass

Sorry your wrong, it won't work, it would not even been close!

Redrilled flex plate?? Changed the exhaust for the vacuum modulator?? Don't shoot the messengers!

So what basically your saying you did not do it the transmission shop did!

DeltaPace77

O.K. guys, I give in, as it wouldn't have even been close. Fact is, I did it, and except for the tranny end shaft yoke change, was nearly a perfect fit. Didn't even have to change the rear crossmember location. Don't know where you guys are getting your dimensions from, but worked fine. Remember, this was a 68' Pontiac LeMans 2 speed tranny swap, not an Olds Jetaway. Didn't even mess with the console shift linkage, as just got 1st, when jamming the T bar down a little further.

oldcutlass

<cough> <cough>

HonestDave

Maybe the trannys were more compatable than we are led to believe. Example, the last 65 442 I bought was sold with a "rebuilt Turbo 350". When I looked underneath, it was obviously a 400. My point being...

On another note, whilst we have tranny familiar guys watching this thread, is there any reason I couldn't take a Jetaway from my 65 f-85, and plunk it in my 64 Skylark? Would it bolt up to a Buick motor, and everything else?
(sorry, I know shouldn't butt in)

joe_padavano

Sorry, but now I KNOW you're wrong. The only way for this to be true is if you swapped a TH350 in place of the ST300.

I'm getting them from my personal experience having done these swaps with my own hands, not second hand from a trans shop.

Both Pontiac and Olds (as well as Buick) used the Super Turbine 300 two speed AT. The only difference is that Olds incorporated the switch pitch converter. That had no effect on trans length or mounting.

Again, it is NOT physically possible for a TH400 to have been installed as you described. On the other hand, a TH350 would be a bolt-in.

DeltaPace77

Well, maybe one last chime in on the this thread, about my Pontiac ST300 swap. Did some research, and the TH400 short tail w/32 spline slip yoke, vs the ST300 w/28 spline slip yoke, the TH400 is 0.56" longer. Maybe explains why it was tough fitting the slightly longer driveshaft length, into the rear axle female. Believe it or not, the smaller 28 spline slip yoke is actually .125" longer than the 32 spline. Probaly did have to relocate the rear cross member a bit, but know I didn't replace it. Maybe should have, but was a high school kid at the time, and just made it work. Just goes to show, some precise measurments are a good idea, but was probably just more luck for me at the time.

65oldsguy

Well, my original question has been answered - thank you to all have replied, I appreciate it!