Need help with eng/trans combo - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 07-21-2015, 10:26 PM Thread Starter
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Need help with eng/trans combo

Just when I thought I was learning something, now I can't seem to find the answer on the forum.

I'm looking for a 400 or 455 for my '67 Tempest convertible. It had a 326 with a TH 400 that someone added. I found a WT block engine 400 locally, but when I check the various sites with Pontiac engine codes, they say WT are either 3 sp or 4 sp engines.

Can I bolt up a TH400 to it or is there some crankshaft or block difference that would keep me from doing that?

I've never heard of any manufacturer making engines specifically built for manual transmissions and another block configuration for automatics, but thought I'd ask the experts before I spent a couple grand on an engine.

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post #2 of 4 (permalink) Old 07-22-2015, 12:09 PM
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All Pontiac V8 blocks have the same bell-housing bolt pattern so you will be fine to use the TH400 behind this block.
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post #3 of 4 (permalink) Old 07-22-2015, 06:59 PM
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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.
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post #4 of 4 (permalink) Old 07-22-2015, 09:23 PM Thread Starter
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What a fantastic resource this site is. THANK YOU!

I bought the car from a kid that worked in a transmission shop. He overhauled the TH400 and put a shift kit in it. Very nice, firm shifts. The problem is he put a Holley carb, HEI and Edlelbrock intake in place of the 2 barrel on a low performance 326 with no other mods. Obviously not a good combination.

I'm researching the history of the 400 WT as it was supposedly recently rebuilt. We'll see if the seller has receipts and how many miles since rebuild. It has Edelbrock heads and intake and a Holley (Too bad about the Holley, I love QJs).

If this doesn't pan out, maybe somebody knows of a good 400 or 455 in the greater Chicago area. I do get concerned abut the guys with the crate engines, not really knowing what kind of stuff they put into the rebuild.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

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