Torque Converter & Shift Kit

Because of the cam in the engine, I needed a stall converter. I did a wack of research online. For an "off-the-shelf" converter, Hughes is frequently well regarded. I had contacted a number of manufacturers, and found Hughes to be the most personable and informative. Hughes recommended their GM25 – 2500 stall torque converter for my application and intended useage. The converter was roughly $300 delivered to my door. I also added a transmission oil cooler.

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At the same time, I installed a B&M Shift Improver Kit. Again, marginally happy people online about this kit. Apparently it shifts very hard, and breaks things. Like the transmission. I did more research, and installed the kit slightly different than the instructions:

According to the instructions:

    • I installed the Dual-Feed Direct-Drum plate
    • I installed the gold B&M Separator Plate (blocks 2-3 accumulator)

Against the instructions:

    • I kept all check balls
    • I retained 1-2 accumulator operation
    • I drilled the 2nd feed and 3rd feed holes 0.125″ (not 0.188″. If I were doing this again, I would drill 3rd feed to 0.140″)

Excellent resource: TH350 Rebuild Tech – Crankshaft Coalition

With the converter and these mods, the truck drives very fine under light throttle. The "shift kit" is pretty mild, and largely unnoticeable in my opinion. Under full throttle it certainly shifts firmer, but not neck-snapping tire-chirping shifts at all. This is my first experience with a stall converter. Pulling away from a light, the truck seems completely normal. Put the hammer down, and the engine flashes right into the cam's power range and the truck pulls away. Fun. I might want some deeper gears in the future.

I also installed a B&M Drain Plug kit, which was stupid – I should have just brazed on a nut that matched a typical GM oil drain plug (I did so on the V8 Firefly).

When we first got the truck at the school, it had an ever-growing pool of ATF under it. Over the course of the year it was in the shop, we tried to hunt down the source of the leak.

While I probably should have just run some dye through the transmission and inspected with black light, instead I threw parts at it. All the parts. In order of appearance:

  • Output shaft seal
  • Extension housing seal
  • Pan gasket
  • Shifter shaft seal
  • Speedometer seal (this was a pain. NLA from GM, NA from LordCo. Picked up one from a local trans shop, then machined the bullet deeper and ran both the original and the new all rubber seals stacked together.
  • Torque Converter seal (front pump seal?)
  • Dipstick tube O-ring (two actually, a larger diameter one to seal the tube just under the flange where it meets the case)
  • Kickdown cable seal
  • Vacuum Modulator O-ring (stacked 3 O-rings in a row while installing an adjustable unit).

But you know? This truck is SO EASY to work on. Wow. It's GIDDY how easy it is to work on.

And Then I Broke The Flexplate….

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That can't be good….

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Nope. Not good.

Swapped in an SFI-rated B&M flexplate in 2 hours.  And all that noisy lifter noise went away – go figure!

The aftermarket tach the truck came with didn't work properly after I installed the MSD box. I finally bought the MSD Tach adapter (8920), and discovered WOT shifts were occurring at about 3800rpm. I knew it was shifting early, but wow. The cam should be good to 6000rpm (231°@050″, 0.470″, 108°LSA, 104°ICL, 10.9:1). Gotta change the governor weights.

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OEM Weights are 0.062″ thick.

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I removed the governor weights and marked them 1/2″ from the end, and 1/16″ off each side. Ron Sessions' TH350 book says grind no more than 1/2″ down, and 3/16″ in. I figured I'd do this in stages – it's harder to put metal back.

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Then ground them down, and de-burred.

I didn't even install them yet. I decided to get more aggressive.

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This is now 5/8″ off the end, and 3/16″ off the sides.

This brought the shift point to about 4800rpm. More grinding is in order.

And Then It Popped

One day, it didn't shift into 3rd.

Gingerly drove home, and took the Lethal Locost to Kelowna Transmission (same place I got parts to rebuild the Hideous Hardbody transmission)and ordered a full rebuild kit, bushings, extra sun gear bushings (you double them up), extra direct clutch and steel (direct piston is machined down the extra thickness plus 0.010″), hardened intermediate sprag race, and new sprags including a wide case-saver late model 700R4 sprag for low/reverse.  And a TransGo Reprogramming kit which fixes problems the B&M kit makes worse.  Ivor was very nice to me.

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That night, I had the trans out and strewn across the shop.

Third had lost one clutch completely, and most of another.

Second had nice clutches, but the steels were all wavy.

Everything else looked good.

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Had some challenges on assembly, but it all went together well, and works awesome!  Good for another 39 years!

UPDATE: A Year And A Half Later….

Noise developing DEEP in the bowels of the transmission. Sounds like a bearing. Cover me, I'm going in.

Low-Roller clutch:

Also had the driveshaft straightened by Truckworks in Kelowna – it was 0.020″ out, which helped reduce a bit of a shimmy vibration in the back.

UPDATE: Another Three Years Later….

Death growl was back, so I took it apart again, but compared parts with another "good" short shaft TH350 I had on a shelf. The rear ring gear had a LOT of play on the output shaft, so I swapped ring gears. I also replaced ALL the thrust washers inside, I changed a planet bushing, and the Direct Drum bushing looked trashed.

I also removed the TransGo separator plate and dual-fed internally.

One of the direct frictions was taco'd and had some bent tangs (?!), so I swapped it out with a used friction. Also scraped down a low-reverse friction from the old trans and put it behind the Intermediate piston to tighten up the clearance.

I also changed cases, because…. long story. But in doing so, and in changing all the thrust bushings, the total end play with no shims was HUGE!  I tightened it up to 0.005″ by putting a 0.030″ shim behind the front planet bushing and a 0.015″ under the pump bushing.