Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Mentioned: 1 Post(s)
Tagged: 1 Thread(s)
Quoted: 404 Post(s)
The Pontiac GTO came standard with the 3-speed manual transmission if the code indicates a manual trans. The 4-speed was actually and option, but most GTO's had the 4-speed versus the 3-speed.
ALKYGTO is correct. The problem can sometimes be the other way around, automatic to a 4-speed conversion because not all crankshafts had the rear of the crank drilled for the input shaft/pilot bearing for the 4-speed.
As I recall, however, the factory flexplate is weighted so it will be balanced correctly for a Pontiac engine. If the flexplate on your present TH400 is good, I'd use it. If you go aftermarket, you want to look into this as often times the aftermarket pieces are "neutral" balanced and need to be balanced with your engine at the time you rebuild it. It would still be a good idea to include your flexplate off your TH400 (providing it is good) along with the new harmonic balance (which you really want to get) when you get your engine balanced as part of the rebuild -and a good machine shop will know this anyway.
Don't know your plans for the TH-400 as whether you plan to use it as is or rebuild it. If it seems good as is, I do suggest adding yourself (if mechanically inclined) or having a trans shop install a shift improver kit. The GTO's from the factory had some changes internally to firm up the shifting which in turn lessens the slippage between shifts and gives longer life to the clutches (my '68 GTO would chirp tires manually shifting from 1st to 2nd on the old skinny 26" bias-ply tires). The kits are made by a number of manufactures and are easy enough to install. Some opt for a valve body swap. 30 years ago or so, I used to simply add the inexpensive shift improver kits, the NAPA brand at the time, and always had good luck with them. B & M was another popular kit, but I was a "poor boy" back then so cheap was good and NAPA got my business.