Replacing THM400 pan gasket
 

I'm new to the group and have already learned more in two weeks from the member posts than I could ever imagine...THANKS!

I have a 1970 C10, 402 with THM-400 Turbo-Hydro and need to replace the pan gasket. I have read a few of the previous threads regarding the best gasket and I purchased a FeltPro with RTV adhesive. My question is hopefully a simple one - once I remove all of the pan bolts, will the pan drop down easily? I see there is a cross member across and under the rear of the pan, near the drain plug. I have never dropped a THM-400 pan before so I'd like to get some feedback in anticipation of the job. Any feedback would be appreciated.

Thanks!

Re: Replacing THM400 pan gasket
 

when you get the pan off and cleaned up to install the new gasket first check that the pan is straight and that the holes are in good shape// overtighting is the worst thing on tranny pans as well as valvecover gaskets sometimes you can tap them straight a lot of times a new replacement is better for the tin pans

Re: Replacing THM400 pan gasket
 

Yah, everything I read suggested to get a new pan. Any recommendations? I usually buy from LMC. They have a "finned" pan @ $29 but I have no idea of the quality.

So the pan will drop down even with that cross member across the rear of the pan?

Re: Replacing THM400 pan gasket
 

I always use a dry fiber or cork gasket. Clean a drip pan to reuse the transmission fluid if it's still good. I loosen the pan bolts slowly while leaving the front bolts finger tight. The pan will start raining fluid. Be patient keep loosening until one bolt remains at the rear and the two front remain. Hold the pan at the rear and remove the last rear bolt. Once the fluid stops remove the front two bolts and drop the pan. Clean the old gasket off both the pan and the transmission. After cleaning the pan, flip it over and make sure it is flat on the gasket surface (very important). Put 4-6 bolts thru the pan and gasket and install on trans. Use a 1/4 drive socket wrench to make sure you do not over tighten the pan bolts. Refill fluid and check for leaks.

Re: Replacing THM400 pan gasket
 

The cross member is a bit of a pain, but the pan is removable. Drain as explained by SoCoC10 described. Just be prepared for a mess from the fluid. The drain plugs and stiffer lip on the aluminum pans make them desirable from my perspective.

EDIT: I see now that you have a drain plug. That'll help a lot!

Re: Replacing THM400 pan gasket
 

agree the biggest thing in not to over tighten the bolts, I always use a 1/4 drive to do this 3/8 drive makes for trouble with this job.

You said you have a drain plug did you mean in the trans pan if so then I would think the pan has already been changed at some point , all the stock400 and 350 pans I have ever seen don't have drain plugs ,but could be wrong.

drain plugs are a +

Re: Replacing THM400 pan gasket
 

Yes, there is a tranny drain plug and you are correct, it is not the original pan.

Re: Replacing THM400 pan gasket
 

I would not recommend a stamped steel pan unless you found an OEM version, or at least something of the same gauge thickness. The aluminum pans are more expensive, but they don't flex under torque.

Re: Replacing THM400 pan gasket
 

This is the aluminum pan I bought. I'm pleased with the quality and finish, and it sealed up great. It's reasonably priced in my opinion also.

http://www.ebay.com/itm/GM-TURBO-400...T7d6fw&vxp=mtr

Re: Replacing THM400 pan gasket
 

If the pan doesn't come out easily from the edge that may be trapped by the cross member - what helped me was to loosen my cross member trans mount bolts and then to lift the tail end of the trans up about 0.25" or so to get the pan out without galling it's gasket surface all up..

Same ting when reinstalling.

Just be aware that when you lift the tail end of the trans up you are flexing your front motor mount rubber - (assuming you have a V8) - mine could easily take a quarter inch of flex at the trans tail end but don't crank it too far or you risk separating where the original motor mount rubber is vulcanized to the metal part of the mounts...the mounts are only an $8 South Korea part so not the end of the world but if your mounts are good you don't want to buy new ones.. but if you do bugger them you you definitely do not want to run them that way - all kinds of havoc could ensue; including jammed stuck full on accelerator linkage..

Re: Replacing THM400 pan gasket
 

I replaced the gasket on my pan after the 250 to 402 conversion, and the trans crossmember was not an issue. You should have plenty of room.

Re: Replacing THM400 pan gasket
 

I've had pretty good luck with aftermarket pans made by Derale. They've got tubes for cooling ran through pan. And most are tapped for temp sensor and a drain plug. They are substantially thicker than oem

Re: Replacing THM400 pan gasket
 

Thanks to all of your responses.

Re: Replacing THM400 pan gasket
 

I have a pan for you - if you are interested.
$25 + shipping It's an in the box, brand new TH-400 pan similar to the one shown (aluminum, finned, with drain plug, etc.). Aluminum is nice for heat dissipation, no rail deflection from bolt tightening and of course the drain plug.
NOTE: Expect a LOT of transmission fluid to come out...you won't get all out of the trans and virtually none out of the torque converter with a static drain, but the trans holds around around 6 qts. and the torque converter 4+.

Re: Replacing THM400 pan gasket
 

Half step - I'm going to pull off the pan this week and see condition of the pan. I'm hoping it's ok and just needs a ew gasket. If not, I'll let you know about your pan. Is yours new or used? Thanks for reply.

Re: Replacing THM400 pan gasket
 

Half step, disregard question, if I would have read your reply more closely I would have answered my "new" question! Doh!

Re: Replacing THM400 pan gasket
 

Hopefully you'll be fine.
Of course you knew and others did emphasize cleaning the pan rail thoroughly. I'd also recommend cleaning the trans and surrounding area as best as you can before you drop the pan - better working conditions for sure, but you may find other potential leak sources - speedometer input, fill tube, cooler lines, etc. Fluids have a tendency to travel far from their sources...

Re: Replacing THM400 pan gasket
 

Good input, I would not have thought to consider these additional areas for leaks.