TH350 Strength

[impstocker]

I am finishing up my 1965 Impala 396/325 Stocker mocking up the motor with TH350 intalled in car. In previous post I mention the tranny has been in storage since 2002, I originally going to use in the 68 Impala I ran. (ran a stick in that one)

The only thing I remember was I did get a 2.75 first gear plus a manual valve body. I don't even remember who built it except I think it was somebody in North Carolina. I finally did drop the pan last week, everything was clean (tranny has never been run) plus has a ATI valve body. I don't know if it has the 36 element sprag, but having it built they knew it was going into a race car, back then I had more $ so for now I can assume it has it. I remember it was around $900 in 2002.

I like doing my own work plus lots of info, in reading Cliff Ruggles "GM Turbo 350" book he says:
"the TH400 case is stronger where it flares up to the bellhousing area. I have seen a good number of TH350 break at this location, but all of those were being used in drag racing with high HP plus traction. Pinion angle, engine mounts (I have solid mounts) and transmission mount play a huge role in case breakage"

He also talks about a solid cast aluminum converter cover w/ strut arms used in some 4X4

1) do other use a solid cover to strengthen, who has them?
2) solid or poly tranny mount?
3) any other issues I am not looking at?

car weight 3900 - 4100 bout 500 HP

Any other info or input would be appreciated. This forum with Billy Nees, Mark Y ect have really helped me with this build, just want to say thanks again. Don't forget, this not a "dime rocket" , more like a "quarter rocket" so I want to stay away from somebody going through it plus a new tranny for now. ( need to get trailer, ect )

Thanks for any help.
Will Lamprecht Div 1 NHRA 65 impala I/SA 2016 debut!

[GUMP]

I wouldn't be afraid of hurting the case at that HP. Poly transmission mount for sure.

[Jim Kaekel]

Most TH-350 4WD cases used the special cast aluminum dust cover that helped strengthen the case, but you probably don't need it since you don't have a big, heavy transfer case hanging off the back of the trans. I would use a stock rubber trans. mount with stock engine mounts and a torque strap on the left front of the engine. If you want to help prevent case breakage, make or buy a brace that connects the boss on the right side of the bellhousing to the boss at the rear that's right in front of the tailhousing. I broke several bellhousings over the years prior to installing a brace. That's my .02.

[Eman]

Rubber mount for sure. Case will crack near cooler lines if stressed, only case I cracked was with a frt. motor plate and only a trans mount and the case was trimmed of tabs which weakened it. If you pull the pump and the input shaft it's easy to ck for the 36 element sprag if you're concerned. I ran with only a hardened race for years with no failures. Be gentle with 2nd gear and you'll have no problems. Start burnout in 2nd and shift to 3rd quickly. Don't ever come out of burnout in 2nd and shock it by having the tires grab.

[ken robinson]

The GM cases on all the glides & TH are not best castings , broke a few over the years . I run solid engine mount & rubber trans mount .1 bolt cross member to rubber mount . The 2-3 burn out works . Tire shake/spin can destroy parts . We run 350s in both cars .Your car has a full frame like Bob kingswood wagon . You'll be ok. Sometimes you just get a pour case/core . Driveshaft balance-ing not to be over looked too . See you at Lebanon ...

[impstocker]

Want to thank everbody for great advice. I am going to fabricate that brace today. I will continue to use the solid motor mounts with a poly tranny mount with 1 bolt. Yes Ken full frame also solid body mounts with 8 point roll bar welded through to frame plus some other gussets ect. No flex in frame and body.

Believe it or not I never raced a "real race car" with an automatic so the info on the 2-3 burnouts i never knew.

Will Lamprecht 65 imp I/SA

[Alan Roehrich]

Originally Posted by impstocker

I am finishing up my 1965 Impala 396/325 Stocker mocking up the motor with TH350 intalled in car. In previous post I mention the tranny has been in storage since 2002, I originally going to use in the 68 Impala I ran. (ran a stick in that one)

The only thing I remember was I did get a 2.75 first gear plus a manual valve body. I don't even remember who built it except I think it was somebody in North Carolina. I finally did drop the pan last week, everything was clean (tranny has never been run) plus has a ATI valve body. I don't know if it has the 36 element sprag, but having it built they knew it was going into a race car, back then I had more $ so for now I can assume it has it. I remember it was around $900 in 2002.

I like doing my own work plus lots of info, in reading Cliff Ruggles "GM Turbo 350" book he says:
"the TH400 case is stronger where it flares up to the bellhousing area. I have seen a good number of TH350 break at this location, but all of those were being used in drag racing with high HP plus traction. Pinion angle, engine mounts (I have solid mounts) and transmission mount play a huge role in case breakage"

He also talks about a solid cast aluminum converter cover w/ strut arms used in some 4X4

1) do other use a solid cover to strengthen, who has them?
2) solid or poly tranny mount?
3) any other issues I am not looking at?

car weight 3900 - 4100 bout 500 HP

Any other info or input would be appreciated. This forum with Billy Nees, Mark Y ect have really helped me with this build, just want to say thanks again. Don't forget, this not a "dime rocket" , more like a "quarter rocket" so I want to stay away from somebody going through it plus a new tranny for now. ( need to get trailer, ect )

Thanks for any help.
Will Lamprecht Div 1 NHRA 65 impala I/SA 2016 debut!

It is most common to use the intermediate sprag from a heavy duty or high performance TH 400. For many years this upgrade has been priced between $400 and $600 alone, on the stock cast drum. It came down to $350 in the last few years. Even at $350, that would be nearly half what you paid to have that unit built. ATI full manual valve bodies for that transmission have been well over $225 for a long time now. The 2.75:1 low gear set has been over $400 for a long time now, as well. So, if you had a completely new transmission built, even assuming you had a very good core that required no hard parts to be replaced, I'd be wondering about that intermediate sprag, among other things, at that price. Especially since you do not even remember who built it. A basic unit with the upgraded sprag, valve body, and low gear set as you describe has been selling for $1500+ on the customer's good core for quite a while.