69 firebird 4 speed conversion
 

Hi everybody. Just picked up a m21 for my 69 firebird. It is a 69 350 2 barrel turbo 350 model. I was wondering if you could share any error traps or shortcuts to make this as painless as possible. Is buying a complete conversion kit the best option? Will the turbo 350 driveshaft work? It came with the yoke for trans and a crossmember which I’m not sure is the same. Happy hot rodding!

Nice close ratio trans just thought a 69 F body 350 got a Saginaw and a turbo 350 (THM350) is an automagic. Spline of M21 depends on year.

Maybe we need to start with just what you are trying to do. Going from a THM350 to a M21 takes a Whole Bunch of parts.

I have done a few 4 speed change overs including a 69 bird.Been some years but don’t remember it being as bad as 61-63 big cars.Pretty much all the parts
Are available in combination of new and used.A M21 would not be my first choice for a street driven Pontiac.Tom

Maybe I made a horrible impulse buy. Ha. Paid 800 for the m21 and shifter with a Chevy clutch scattershield and bell housing and miscellaneous crap. I think I can get the bell housing and flywheel cheap. The pedals and all the other stuff I’m hosed.

Tom is that because the 21 is a close ratio and the 455 torque doesn’t need it?

I don’t think it’s good with any Pontiac engine I have street driven.I wound take a M20 over the 21.If starting from scratch and wanted a 4 speed it would be a 2.88 ST-10.BUT if really starting from scratch I would run a Tremic 5 speed which is now what my 69 has.JMHO.Tom

Same here as Tom. I have a 69 bird and converted from Auto to 5spd Tremec. Everything is readily available, OEM 69 manual pedals were the only challenge, one year only and Pontiac specific. The rest was not an issue. Very pleased with the 5spd choice. 3:73 rear gears.

Tim john---

You may not be happy with the M21, in part, due to its 1st gear ratio. The wide ratio M-20 is a better choice because of the better first gear in getting the car rolling.

The wide ratio trans typically used a 3.23 rear gear for the most part, but of course other gears like 3.36, 3.55 were a good choice. The close ratio M21 is better with the 3.90 rear gear.

You did not say what gears you have or if you plan on changing them, but if you have 3.08 or any 2 series rear gears you are not going to like the M21. It can be done, but you can also eat up clutches real fast because you will find yourself slipping the disc a lot more to get the car rolling. If you have taller tires other than the stock 26" tall tires, this will only add to a more difficult start as this can drop the rear gear ratio down to the next lowest set, ie 3.23's down to closer like 3.08's. Your choice of clutch disc material could be important as well as rated clamping force to keep the disc from slipping if you have 3.08's or any of the 2 series gears. More clamping force from the pressure plate may mean a daily trip to the gym to keep your left leg muscles built up - unless you go with a hydraulic throw-out bearing set-up.

I am not a Firebird guy, but you may also want to think about stiffer leaf springs ( believe you have a mono-leaf and may want multi-leaf) and/or traction bars. If I recall correctly, the HO version had a factory type traction bar AND I think the floors had an additional metal plate to add stiffness to the body - but the Firebird guys should be able to confirm this.

What is your rearend gear ratio? If it is 3.90 or lower it will be fine. If it is stock it will not perform very well because of the gear multiplication. An M20 would probably be a better choice with a 350. I had a '68 with a 350 2 bbl and an m20 stock and it worked well with the 3.23 gear.

I am building a 455 with 96 heads and a 66 tri power. I will purchase a custom 9 inch as far as rear end gears its whatever the dr orders.

As this was posted in the street section I ruled out racing.If a race car it could be way different selection of first and rear gears.Tom

As mentioned, the M21 is not street friendly, 1st through 3rd is very tight with a large drop to 4th.
A 3.90 rear would work but as mentioned the Tremec 5 speed would be the best of both worlds.
If your building a street monster and starting from scratch, be best to consider all the options.


Frank

already addressed above ...yeah the wide ratio would be more street friendly as the 2.20 first gear is a bit....slugish without putting some serious gear in the rear.
Moving on....so from automatic to clutch
Obvious parts? Z-bar and linkage stuff...(unless you hydraulic) New driveshaft YOKE (m21 uses a th400 yoke) BEWARE...of the length of NEW yoke as it should be the SAME length as your th350 yoke..there are heavy-duty truck models that are LONGER...which would make YOUR CURRENT driveshaft too long. You will need a different front u-joint accordingly. Don't forget the neutral safety switch and reverse light switch.
It should be pretty straight forward swap and with a little searching I am sure somebody has reference numbers as to WHERE TO CUT THE FLOOR. Also note WHICH flywheel you NEED? 2.75 or 2.5. Your's SHOULD be 2.75 but always worth checking BEFORE you go getting a flywheel. ;)

It should be pretty straight forward swap and with a little searching I am sure somebody has reference numbers as to WHERE TO CUT THE FLOOR. Also note WHICH flywheel you NEED? 2.75 or 2.5. Your's SHOULD be 2.75 but always worth checking BEFORE you go getting a flywheel. ;)[/QUOTE]
2.75 or 2.5? What is this wizardry you speak of? I thought a Pontiac flywheel was a Pontiac flywheel?

I went from a turbo 400 to a tremec tko600& had to have a driveshaft made.
The car also needed an new crossmember which I purchased along with the tranny.
The worst part were the pedals, they are advertised as 67-69 camaro/ Firebird but they don’t fit correctly & they don’t make 1969 Firebird specific pedals . I was lucky & found a complete restored pedal assembly on eBay. It was also very difficult to find a 4 speed console for the 69. As you know most parts for this car are 1 year only & camaro parts don’t fit


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I just ponied up and bought the specific pedals from a board member. Trading the m21 for an m20 tomorrow. So far 275 for pedals 550 for trans and shifter. Another 200 in small bits from Ames . Console what console? Still need a clutch and flywheel resurfaced. The good news is I can run the 350 while I’m building my super badass 455. You guys are right it’s a costly endeavor.