[robg8r]

I'm ignorant about flexplates. I need to purchase a flexplate for my 1969 Pontiac lemans. I am taking out the 350 and putting in a newer 400 from a grandprix (1975 or 1976) with 6X heads. The transmission is the original TH400.

Where can i go to purchase this and what kind of search criteria should i use?

Thanks for your help
Rob

sorry about posting in the Lobby, it's been awhile since i came by

[bhill86]

Not sure if you’re hoping for new or used but if you decide to go with used I have one that came off my ‘69 400 that’s in my 77 T/A. Was mated to a TH350.

[PAUL K]

Not ignorant at all. Around 1975 Pontiac changed to a 2.5 flywheel register (hub). The earlier cranks and all 455's used a 2.75. Measure that first. If it's 2.5 you'll need to find a flexplate for a later 350/400. Also make sure the flexplate will bolt to your converter. Some flexplates come with dual pattern (Th350 & Th400) torque converter bolt patterns. If you get a TH350 only flex plate I believe two of the TH400 bolt holes will be there already and the third can be drilled.... Going from memory.

IMO.... If you can find a good original Pontiac flex plate I think you'd be better off than buying one from an "auto parts" store.

[Big Bear]

The crankshaft register was changed from 2.760" to 2.600" late in 1976. Better measure dia. to get the correct size hole in the flexplate.

[Big Bear]

Paul treed me LOL

[robg8r]

Thank you Gentlemen!

[FrankieT/A]

Pioneer sells them

[Mr Anonymous]

WHY, after all the years of production, would they change that? Makes no sense to me at all.

[25stevem]

It makes no sense because your not thinking a long the lines of factory production cost !

Pontiac switched over to that smaller flywheel register size in 75 when they where in development of the 301 motor for release in 77.
They wanted to standardize on tooling and machining procedures and not have two different tooling setups to operate and maintain.

[Mr Anonymous]

Originally Posted by 25stevem

It makes no sense because your not thinking a long the lines of factory production cost !

Pontiac switched over to that smaller flywheel register size in 75 when they where in development of the 301 motor for release in 77.
They wanted to standardize on tooling and machining procedures and not have two different tooling setups to operate and maintain.

Using that logic, the 301 should have had the larger register since the production line already used that tooling.

[PAUL K]

Originally Posted by Mr Anonymous

WHY, after all the years of production, would they change that? Makes no sense to me at all.

Originally Posted by Mr Anonymous

Using that logic, the 301 should have had the larger register since the production line already used that tooling.

I agree. Engineers do things for a reason... I'm not sure we will ever know that reason. The register change happened around 1975 so they were using two flanges for more than a year. I always thought the change happened when they went with the light weight crank castings. I was blown away when we went to balance a 452 (light weight) crank and it had the large register.

[78w72]

maybe the register size change had to do with the power levels of the engines? 75 was when they changed to the thinner 500557 400 blocks too. but, the 78-79 W72 481988 blocks still used the bigger earlier year crank register size. doesnt make much sense to change all the tooling for the blocks either that late in the run when they probably knew the pontiac V8 was doomed in 3-4 years, coulda/shoulda just left it alone, the weight saving of the 577 blocks didnt amount to much if any savings on MPG.

i remember about 10 years ago when i was rebuilding my 78 W72, tried to buy a new flywheel from local parts stores, they all carried only the smaller register size, i had to buy a hays billet flywheel from summit to get the right size for the crank.

[25stevem]

Don’t think for one second that the factory lightened up on the blocks for MPG reasons, they did it for production cost.

[mchell]

It’s called “value” engineering…. …. Just because engineers “ do things for a reason” doesn’t mean that it will always benefit the end user….MONEY always gets in the way of quality…lol

[robg8r]

Thanks again. I have just purchased one from Pioneer that appears to fit. Appreciate your insight as usual!

[Tom Vaught]

Originally Posted by mchell

It’s called “value” engineering…. …. Just because engineers “ do things for a reason” doesn’t mean that it will always benefit the end user….MONEY always gets in the way of quality…lol

The Bean Counters, be it on the Engineering/Designer Floor, or in the plants will always strive to lower the
average cost of something made.

Wonder why they were not around when that Pontiac FEAD (Front End Accessory Drive) Engineer changed
designs/parts every year on the Pontiac Engines. He should have got it right on the first/second attempt,
not over 12 years of changes. There are Good Engineers and not so good Engineers and a few
EXCELLENT ENGINEERS: Mackellar, Adams, Nell, for example.

Tom V.

[Mr Anonymous]

Yes engineers do stuff for reasons - like they're dumb and make bad decisions. crank registers, brackets, the examples are everywhere.

[MarkS57]

Originally Posted by Tom Vaught

The Bean Counters, be it on the Engineering/Designer Floor, or in the plants will always strive to lower the
average cost of something made.

Wonder why they were not around when that Pontiac FEAD (Front End Accessory Drive) Engineer changed
designs/parts every year on the Pontiac Engines. He should have got it right on the first/second attempt,
not over 12 years of changes. There are Good Engineers and not so good Engineers and a few
EXCELLENT ENGINEERS: Mackellar, Adams, Nell, for example.

Tom V.

I always thought the 68- 69 set up was nice compact design. Was thinking the change in later years may have been done to eliminate the cast aluminum mount of the 68-69 as the stamped steel replacements were probably cheaper to make.