THM400 build up?

[great scott]

Ok I my chev runs a strong 454, SM465, ORD doubler, FF14b with Detroit, D60 with chrome shafts, CTMs and ARB, 1410 u joints on custom drive shafts and 44" Boggers. I am thinking about converting to a THM400 because I like comps and the SM465 is a pain to shift. So what do I need t do to make this thing(THM400) survive?

[KWTMECH]

A well built trans and an excellent cooler and the proper adaptors.

[great scott]

Cooler is on the list along with a performance kit, hd converter, shift kit, maybe even a full manual valve body. Witch adaptor do i need, it would be round 400 to round 203

[onetonbb74]

you need a th400 with super short 203 style 32 spline output shaft (or swap shafts from another tranny) Then you need an 32 spline input gear for the 203. Then you need an adapter for the 400 to the 203. I would find a rare th400/np203 combo stock from a 78 or 79 3/4 or a full time one ton usually found in gmc's over the chevys. Plus all the above mentioned. Have those custom driveshafts re-customized to proper length (longer front, shorter rear)

[KWTMECH]

I have the shafts and factory adaptors if needed.

[ace]

The ORD website said that you can use your factory 400/208 adapter and just redrill and tap the face of the 203.

[bigdummy]

A laundry list of items you might want to throw at that tranny. I would recommend all of them, if you can.

-Trans cooler - for off road use, the bigger the better
-Temp gauge
-Deep oil pan
-34 element intermediate sprag clutch
(When I built my TH400, this was the ONLY internal upgrade that B&M said was necessary to hold up to about 570HP & 500 ft-lb of torque)

Also, if you ever turn more than about 4500 - 5000 rpm in 1st gear, you will need to get the appropriate valve body, or make the appropriate modifications to keep it from automatically upshifting to 2nd. Usually just a modification to the 1-2 shift valve.

[great scott]

The 400 I have is the short 32 spline, the 203 input is also 32 spline, so I think all I need is the adapter? As for the trans I think bigdummy has some good suggestions. Thanks.

[KILLER"B"3]

Originally Posted by bigdummy

A laundry list of items you might want to throw at that tranny. I would recommend all of them, if you can.

-Trans cooler - for off road use, the bigger the better
-Temp gauge
-Deep oil pan
-34 element intermediate sprag clutch
(When I built my TH400, this was the ONLY internal upgrade that B&M said was necessary to hold up to about 570HP & 500 ft-lb of torque)

Also, if you ever turn more than about 4500 - 5000 rpm in 1st gear, you will need to get the appropriate valve body, or make the appropriate modifications to keep it from automatically upshifting to 2nd. Usually just a modification to the 1-2 shift valve.

To run the HD sprag you will need a drum from the 60's to accept the new sprag. Also get a 4L80E intermediate palte and frictions that way you can run 4 clutches, and not 3. Use teflon scarf cut seals, I would reccomend replacing the bushings. Therre are tons of mods you can do. You will be fine with all the above listed in above posts

[great scott]

Just the kinda stuf I was looking for.

[shortsub]

i just had the shifting out of 1st problem with my 400. i bought the hughes manual/auto valve body which is a manual valve body in 1st and 2nd gear and acts normal when placed in third. i like it so far.

[great scott]

Originally Posted by shortsub

i just had the shifting out of 1st problem with my 400. i bought the hughes manual/auto valve body which is a manual valve body in 1st and 2nd gear and acts normal when placed in third. i like it so far.

That sounds like a valve I could use.

[HsOffRoad]

The main things you need (besides a decent tranny cooler) is to make sure line pressure is decent, do the sprag upgrade, and add the extra center support bearing. These make the biggest difference in durability of the tranny.

A manual valve body is a huge asset for off-road use because it allows you to lock the thing into 2nd & 3rd gear low range for various obstacles. Just make sure you retain engine braking or you're most likely gonna miss it. Many of the full-manual valve bodies, especially the reverse pattern ones, eliminate engine braking in at least some gears. The upside of the reverse pattern vavle body is that you can quickly toss it into reverse from first to pull out of a potential roll.

Other things I run in my TH400 are additional clutches in the forward drum, Kevlar clutches & kolene steels, A reworked front pump, hardened input & output shafts, a deep pan, and a 2500 stall converter. These aren't necessary but do help keep the tranny alive in really abusive environments, such as competitive rock crawling.

Another important thing people overlook is the flexplate and the converter bolts. These are not the place to pinch pennies, because for an extra $75 you can get the peace of mind a SFI flexplate and quality converter bolts. Far better than mangling your flexplate or breaking converter bolts on the trail.

You can also swap in the TH475 straight cut gearset to eliminate the gears walking inder load. To me, this is not necessary except for very heavy vehicles with extreme power input.

If it was me building it for recreational and light comp use I'd do a good cooler, deep pan, a full manual valve body with engine breaking, more clutches in the forward drum, a 2000-2500 stall converter, SFI flexplate & good bolts, & the sprag & center bearing upgrades. This should net a tranny good to at least 500hp on a reasonable budget.... and is already overkill for 90% of 4wd enthusiasts. If you manage to smoke that, you're into the realm of big bucks.

Hopefully I'll bump into you competing sometime... growth is always good for the sport.

Hans