# Different torque convertor for Turbo 400



## 60sPontiacs (Jul 14, 2016)

I had my transmission rebuilt a while back, and just now getting ready to install it - noticed the bolts (15mm heads) are different than original (11/16" x 3/8"). The new bolts fit through the flexplate and thicker heads have clearance. With TC pushed all the way back, it's ~ 15/16" from front of bellhousing to mounting pads, and about 7/8" from rear face of block to mounting surface on flexplate - so this should fit (is 1/16" clearance about right?). The "knob" on front of TC is about 1.68" and hole in back of crank about 1.73". I haven't tried to fit it yet because the engine's on a stand in garage and lift is in back of the house.

Will this work? and what do I have for a TC? I've only worked on 60s Pontiacs so never saw metric bolts for TC.
Block is a 72 ZX 400 if it makes any difference. Car is 69 GTO - not sure if transmission is original.

And while here... I've done a few, but what is best way to lock engine so it doesn't turn while I torque the TC/flexplate bolts? 
I've jammed a tire iron from starter pad to ring teeth, but not sure if this is okay (it can slip, maybe damage teeth), and at times may have stuck a screwdriver through holes in flexplate to hold it.


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## 60sPontiacs (Jul 14, 2016)

After finding the code and digging around, I found this is a GM8 stock Turbo 400 Torque Converter - the car originally had a GM3. TC rebuilder (PDQ) says they're the same except the GM8 uses metric bolts. Has "low stall speed" which may be 1350-1500 rpm. It's fine for me as I'm trying to get the car basically stock for now - going for efficiency and decent power. 

Stall speed is low, but I had a stock 70 455 GTO which would have had a GM3 Torque Converter. The biggest problem was non-positraction - the power was there, but the tires wouldn't grab. I need to learn more about stall speeds. Main thing is transmission shifts into gear, drives, and shifts between gears fine - the guy did a great job rebuilding it.

After 14 years of sitting, it's nice to have this car back on the road. It handles and drives like new... Lubed front end - ball joints were new but all boots had stiffened/rotted (one completely gone). Polyurethane stabilizer bar bushings held up great. No squeaks or rattles, which surprised me.

Only "problem" is setting up a 1975 Grand Prix HEI distributor to run with this engine - there is no detonation. Will start another thread about this.

Oh... also learned there's no problem with engine turning while torquing the TC bolts. 
Turned engine with breaker bar on crankshaft bolt (easy with no fan/belts) until could access TC bolts on right side (opposite of starter). Got them all started/finger tight, then torqued - Wallace says 35 ft lbs. If I pulled at a right angle to a line running to crankshaft, while torquing, the engine didn't try to turn. I was thinking of having to lock engine to remove crankshaft pulley bolt (these days I use an electric impact driver).


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## 1968gto421 (Mar 28, 2014)

Sounds like you're making good progress on your GTO.

Here's a link that does a good job of explaining stall speed------ What is Torque Converter Stall - Hughes Performance

Hope this helps, best of luck with your build.


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## Pinion head (Jan 3, 2015)

With metric threads on the lugs of the converter, sounds like the converter was built off a stock diameter converter out of a 4L80E from the late 90's-00's.


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## 60sPontiacs (Jul 14, 2016)

Checking this out made my head spin - I had no idea there were so many torque converters... I think some non-TH400s used this convertor - GM changed splines at some point (fm 30 to 27? - both these are 30) and it couldn't have had lockup... not sure if it was the 4L80E.

Found some info showing GM3 and GM8 interchangeable other than Metric bolts. 
Shows ~1/3 down this page, and that both are okay for 1964 on up (not taking into account SAE/metric change).
Turbo 400 Automatic Transmission Rebuild Kits | Turbo 400 Automatic Transmission - Drivetrain

Other source, TCS catalog
http://www.tcsproducts.com/media/1/Converter_Cat1_1.pdf
shows GM3 (for 1964-79) on pg 125 as interchangeable with GM8 (for '79 on up), and vice-versa for GM8 on pg 127 except for mounting thread.

I called the shop (PDQ) that rebuilt my TC the following Monday and he confirmed GM3 and GM8 had same specs aside from threads (he didn't know offhand).

Major issue, of course, was fit - measurements were iffy, but when hooked up, it had the usual (w/my limited experience) 3/32" to 1/8" between TC and flexplate. I didn't check fit right away because the engine was in garage on a stand and I didn't want to move heavy lift from backyard to garage to check - I'd read a few horror posts from guys who couldn't spin the TC when mated to engine (too tight), bellhousing wouldn't pull close to trans, etc. This was first time I did it by self (no one to watch clearances as lowered in), but all went easy (and slow..) - trick is to raise trans against tunnel so mounts on engine clear towers on frame. That piece of plywood to the left in photo was my 'helper' to stop engine from hitting firewall/ripping off wires as it moved back while lowering into engine compartment.


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