TH400 Build Question

[85 c-10]

I am in the process of building a 400 to replace the 350 that decided it had enough. I have a rebuild kit with Alto Clutches, Kolene Steels, and a 34 sprag and drum this is to fir 6 clutches. I have a late 70's truck trans, and a 69 GTO trans. I have used the output of the GTO so I can run a 1pc. D/S. My issue is this: The direct drum is from CK Performance. It is suppose to be a 6 clutch unit. I have found that the direct piston has more say so on how many clutches fit. The piston from the car trans is aluminum and had the 2 seals, plus the 1 on the drum itself. The truck trans had a steel piston with 2 seals, but none on the drim. Does anyone know if one piston is prefered over the other? What about the drum seal? Will this just depend on the piston I use? Next question is the springs for the piston. The aluminum piston has longer springs than the steel one. I think these help control the harsh reverse engagement but which ones make it softer? I can call CK tomorrow, but I can get more done if I can find out sooner.

Thanks,
Josh

[James McClure]

Josh, morning. The drum to which you refer, is the direct or high reverse clutch. Reverse is the same ratio as second gear, hense the need for higher pressure to make it hold. With that said we can move forward. The drum itself, no matter the maker has nothing to do with the number of clutches that will fit. You are correct in your assumption that the apply piston governs the number of clutches and more is better in this case. The piston and drum have to match. If you use the piston that uses a 3 seal setup, you have to use the drum that was 3 seal. Same goes for the 2 seal setup. Now, if your stuck in the position where the piston\drum assembly you want to use will not accomadate 6 clutches you will have to mill the apply ring or step on the piston down a distance equal to the thickness of 1 steel and 1 fiber. When you have this all done, assemble the drum without the seals and check the clearence between the very top heavy steel ring and the first fibre plate, it should be between .040 and .060 inches. If it's good reassemble drum with the matching spring set. If it's to tight mill more off the piston, if it's to loose, you can add a steel to the bottom of the stack and try again. Be VERY careful that the first plate in (steel) that it will not slip into (rotate) the cut at the bottom of the drum just above the piston bore!!! If this happens you will have no application in 3rd or reverse. If the drum used a beleville spring under the first steel make sure you reuse it as this will help eliminate a steel getting into the cut in the drum. There is no practicle way to fill that cut. Iv'e seen guys put a very narrow snap ring in there but with mixed results, so tread lightly here. Also check the thicknes of the of the top plate. Those can be milled thinner but no thinner than the tab lands on the plate. You can get the plates from GM in different thicknesses. If you split the amount of an extra clutch and steel between the plate and the piston you should be ok, but remember "Murphy's law" applies here. If you need more info or talk about it, E-Mail me your phone number and I'll call you. My e-mail address is
jmcclur2@nycap.rr.com. I'll try to help you get thru this.Jim

[Thumpin455]

JM is correct. Good advice right there.

A little more about the seals, the one that fits the drum is a cushion, it slows the apply a tad so the shift isnt so harsh. When I build them for my cars and trucks I tend to leave the cushion seal out and only use the seals that hold pressure around the piston.

There were a few TH400s that had 6 disc clutches, tons more that had 5 and some with only 4. If you swap the entire drum for the larger type, you dont have to mill anything. Finding one in good shape is the problem. The 6 disc clutches were prevalent in big block trucks and very high tq cars, like Buick Stage 1 and Cadillacs. I have only seen three of them the entire time I have been building transmissions.

[85 c-10]

I spoke with CK Performance today. They told me that leaving out the seal on the hub is done in conjunction with the plugging of the feed hole next to the center support bolt. This is commonly performed to run a dual feed to the direct drum. When I picked up some shims from the tranny shop I have used in the past, they told me that you could either leave out the seal, or the 2nd ring from the top on the center support to run a dual feed. He did say that leaving out the seal would cause a tire chirping reverse to drive shift. The aluminum pistons are prefered as they have more surface area to contact the lowest steel. I am going to use all 3 seals, and leave off 2nd ring. The trans shop also said to use the absolute stiffest springs you can on the piston. The thought is that at high RPM, the piston will be forced to the applied position, thus torching the clutches. I just hope this works. I'm tired of replacing everything!

Josh