Exchange ST400 for ST300 in '65 GS?

**5GScars** · 06-25-2008, 02:50 PM

I have read that there is a kit to change the transmissions but have not been ble to find it. Specifically it is linkage and shifter position plate for a floor console shifter. Is anyone familiar with this?
The 2-speed has died and a 3-speed ST/SP from a '66 is available. Anyone? THX!

**telriv** · 06-25-2008, 05:11 PM

If your looking for the additional power a 3spd. trans. will make over the 2spd. with variable pitch converter you will be very surprised to find out that the 3spd. won't be that much faster. Maybe about a tenth in the 1/4. You have the added weight. Weight=HP. The added HP loss the bigger trans. eats up. The hassle with shifter, console, driveshaft, crossmember movement, plus other things I can't think of right at this moment. Then you have the added disadvantage of less traction from the lower 1st. gear. Yeah, many have done it, but ask them to tell the truth & many would probably say they wouldn't do it again for all the hassles it brought up.
Just my dimes worth.

Tom T.

P.S. By the time your done your car will have been down for who knows how long while searching for the nec. conversion parts=no drivng time, & it will probably have cost more to convert than the cost of getting YOUR trans. properly rebuilt by a trusted trans. shop.

**5GScars** · 06-26-2008, 09:20 AM

The trans(ST/SP300) is at a VERY WELL TRUSTED shop right now. There is an issue concerning a tool he does not have to remove some part to allow access to the total insides of the tranny. The tranny was shifting strangely to drive at WOT and would be at 3700rpm's after the shift when it was working which seems too high an rpm even with the converter in high stall.
Then it would seem to not rev, like something was holding it back and then, no drive gear at all! So the swap is simply an option as I have an ST/SP400. I would prefer the 300 but so far the problem with it is not yet diagnosed. If repairable then it will be done, if it is beyond repair then finding a very good condition 300 might be difficult, one not several states away that is, LOL. Shipping could cost more than the tranny!
I know about the HP losses with the various GM trannies. The 300 uses 20HP, Turbo 350 uses 35HP and the Turbo 400 uses 45HP all due to the internal weight that has to turn, absorbing power. It's about a .3 quicker with the 3-speed none the less if traction is there! Recently I read that the 300 mshould not be used in vehicles weighing more than 3000lbs. but Buick sure went past that figure! This tranny was supposed to have been rebuilt twice. I did tke the motor to 6000rpm a few times recently, could that have damaged the tranny? Seems not but....
How about rear gear since we have the conversation going Tom. The car has 3.08's and I have a set of 3.23's available, both are posi units. What gear has proven in the Skylark weight with a 401 motor and 22-speed to be the best for all around driving? I don't want a set of 3.90's for 1/4 mile times as that is not the purpose for the car but an all around driver but having some extra get up and go is always nice

Thanks.

**bobc455** · 06-26-2008, 12:18 PM

I did the ST300 to TH400 tranny swap in my '69, mostly because I was upgrading to a 455 and the ST300 was tired.

If you want to change, you should do it with the intention of being a permanent swap. Your driveshaft will have to be shorter, and because of the design of these original driveshafts you'd be better just getting a custom-made shaft from aluminum tube.

You'll also need a TH400 driveshaft yoke.

The shifter linkage is on the same side, so that interchanges easily. At least this was the case on my '69 with a column shift. The linkage needed some adjusting, but works fine now. And you can use your existing ST300 kickdown wiring. And your speedo cable will plug right in. Check your speedo gears in the TH400, though, because they might be the wrong ones for your combo.

I believe the crossmember is identical (it is in the '69), however you'll have to move it further back. Because the TH400 was offered in '69 in the A-body, the holes already exist in the frame but I'm not sure about the '64. The tranny mount is different, but a new one is probably in stock at your local parts store or speed shop.

I think (not 100% positive) the cooling lines are in the same place or very close.

As far as I know the bellhousing pattern is the same, but you should verify this some other way because I think the nailheads had a different pattern.

The ST300 is a nice light transmission, and if you don't need the extra weight of the TH400 then I would be happy with the ST300. So unless there are other factors, I would also stick with the ST300 if possible. If it is impossible to rebuild, then that kinda settles that question...

As far as rearend gear ratios are concerned, that could be argued all day. But unless you do a fair number of highway trips over 15 miles, the 3.23 is probably the better choice. I have 2.56s, but I do a lot of long-haul driving and most people will tolerate 60 MPH for their occasional highway jaunt.

-Bob C.