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Transmission pan gasket tips

Any mechanics out there have any tips on ensuring a leak free transmission pan seal? I’m changing the fluid and filter in my GMC Sierra. I dropped the pan after work today. I’ve changed trans fluid…I dunno, 6 or 8 times in different vehicles - but this one’s a pain. Difficult to get the pan out since the front driveshaft, exhaust, shift linkage, and a heat shield seem to all be in the way. So…I do NOT intend to do this chore again. And I don’t want any seeps.

I’m getting a flush next time! I think I’m getting to old to lie on my back in transmission fluid.

I ordered an AC Delco filter and gasket. The gasket is the rubber type, with no metal backing/reinforcement. I was hoping the transmission had a reusable gasket when I pulled the pan, but alas, it appears to be cork. The transmission on this truck was replaced previously.

So, I will use the AC Delco floppy rubber gasket. I will clean mating surfaces. I will torque to spec in a star / crossing pattern. Any other tips from the pros?

This might be of help.

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If you will accept a tip from a ham fisted amateur, what I have done with long floppy gaskets in tight places is first remove any dimpling from the pan rails then use light sewing thread to tie the gasket to the pan. after you gt the pan in place, start the bolts and then slice the threads with a razor blade and pull them out with tweezers.

I used to buy old beaters for a work car for myself and my 4 kids, anything with a slant six in it and I was the last stop for these cars.cars before the junkyard.

Most of these aluminum case Torqueflights came with at least 3 stripped pan bolts. I would cut the threaded part off longer bolts and put the cut part up into the transmission with some JB Weld in the hole and let dry overnight. If the hole had been hogged out to big for the bolt to grab at all, I woulf twist some steel wool into a rope to put in alongside the bolt first. Add some nuts to hold the pan and I never had a leak.

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Thanks, guys. All advice welcome. I’m not so much worried about the gasket slipping out of place. Usually, I’ll put the gasket on the pan and start a few bolts on the corners. Most times, the gasket is tight around the bolts. The gasket holds the bolts in place, and the bolts keep the gasket aligned. I’m just a little concerned with possible leaks (minor) after everything’s buttoned up.

I guess I was expecting the factory gasket to be on there like it was on my Buick when I dropped the pan on it. It was a nice rubber gasket with ridges to help the seal. The entire thing was “impregnated”, if you will, with a steel reinforcement. I had no worries that thing would seal. This flimsy rubber gasket may be kinda hit or miss on how well it seals. I wasn’t planning on using any additional sealant, but if that’s what’s recommended I can.

Actually planned on finishing the job tonight, but a stomach virus has been going around and I seem to be a victim. Lying on your back under a truck with Dexron 3 in your hair is not the best time to find out you’re not “regular”…

Just did my 99 Buick LeSabre, I too hated the flimsy rubber gaskets. However I did not spill a drop of tranny fluid by lifting the front of the car and loosening lower and bolts until I had a drip then loosen more until I had a good amount coming out and left it sit for an hour in a big pan. That was the first time the second time I went to Harbor Freight and purchased a little pump and pump the gallon out through the dipstick hole and that work real well too. The pump cost $4.99 I will use that little pump from now on before I do anyting in the transmission fluid changing. But I did have problems with the gasket as you mentioned. Just make sure before you tighten all the bolts you see rubber all around the outside and they will seal just fine pain in the butt but they work

The very first time I pull any trans pan without a drain plug, I add one of my own. All but one trans filter I ever changed had anything of consequence in it. And I knew what to expect based on the fluid that came out. The other 99% of the time, a fluid change was all that was really required. So I only drop pans a couple times max in the entire time I own the vehicle. All fluid changes are done with the drain…and I’m more inclined to do fluid changes at 30k mile intervals when it’s so easy to do. YMMV

My Buick has no drain plug, that’s why I purchased a little pump from Harbor Freight it got 90% of the fluid out of the pan, however it still left some in the torque converter. I think it holds 6 quarts and I got five quarts with the pump through the dipstick , using the pump I will do this every 30,000 miles because it’s easy

If you have a torque spec, you could use that, but first run each bolt down to snug. The reason is that if you were to run bolt down all the way to torque spec, it could cantilever the pan and actually be too tight when all the other bolts are torqued down.

Now if you were a mechanical engineering college professor, you would first measure the thickness of the rubber gasket. Then determine the the ideal compression thickness which would would be 30% less than the non compressed thickness. Then determine the pitch of your pan bolts. Then calculate how far to turn each bolt after contact to reach ideal compression.

For example, if the gasket is 2mm thick, you will want to compress it by .6mm to reach the ideal compressed thickness of 1.4mm. If the pan bolts have a thread pitch of .7mm, you would want to turn each bolt 6/7th of a turn after contact. So run each bolt down to contact and then go around in any pattern you chose and tighten each bolt 6/7th of a turn. That’s about 310 degrees.

If you did this and used a beam type torque wrench for the final turn, it will probably be pretty close to the specified torque because that is how they usually figure the spec anyway.

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What about running a bead of Permatex 33694 on both sides of the new gasket? A belt and suspenders approach.

What I’ve experienced is pan gasket leaks (oil or tranny) were caused by not removing the old gasket. Remove it and then scrape any residue and then sand paper it. @oldtimer_11 had a good point to about removing any dimples in the pan around the bolt holes which were caused by the oil pan bolts.

Alright, thanks folks. Finished it up this afternoon. The old cork gasket came off well. I used a razor to remove any residual. Brake clean and a rag to clean the pan and mating surfaces. I didn’t sand anything. Read that a little too late. I snugged all the bolts down with my fingers. Then snugged them a little more with a ratchet in a crossing pattern. Then came back and torqued them to 10ish ft lbs with a click style torque wrench, again in a cross pattern. I say 10ish, because the lowest setting on my torque wrench is ten. I left the dial a little under 10. Service manual says 9. 1A (A1?) Auto video I watched says 15! There were definitely no deformations around the bolt holes of the pan. They had the bolts half a turn past finger tight on that cork gasket when I removed the pan. I felt like I was getting it a little too tight at 10ish ft lb, but the pan isn’t warped from what I can tell. I didn’t use any rtv or additional sealant on the rubber gasket. I’ve read that is a no no. But I don’t know, myself. Fluid back in, shop cleaned up, and will drive it again, recheck level, and look for leaks after while. Or tomorrow. Kinda tired. The little bout I had last night with a virus or mild food poisoning kinda drained me.

Pretty much did it the same way I’ve always done. Never had anything more than a seep, at most. Hopefully it’ll stay 100% dry. If not…I’ll blame it on you guys. :lying_face:

Again, thanks for the tips. Wish me luck!

Oh, and the mechanical engineering approach sounds great. But with my tools on hand, I’d be off enough on measuring the gasket thickness and pitch of the threads to screw the rest of that up royally. Plus, I assume the ideal compression amount would depend on the gasket material used. Maybe? Not sure.

Sounds good, though. If I knew how to legitimately do all those calculations, I’d probably be in a position where I’d just rather pay someone else to do it. Or just buy a new truck :stuck_out_tongue_winking_eye:

I believe you would be right, however the replacement gasket is rubber. Is 30% the ideal compression for rubber? At my last job, before I retired I asked that very question. I worked at a transformer manufacturer. We had a customer that wasn’t happy with the way the bushings (high voltage terminals) were being installed.

We were installing them per the bushing manufacturer instructions. The bushings have a rubber gasket, they go through a hole in the tank and the gasket is between the lip of the bushing and the outside rim of the hole. It is held with a triangular shaped clamp that goes over the bushing on top of the lip and held by three nuts on the studs welded to the tank.

The problem with this arrangement is that the clamp is cantilevered. First you snug down all three nuts to contact, then put 50 inch pounds of torque on the nuts. A test of the gap between the tank and the lip of the bushing showed that the gap under the first nut torqued was small and under the last nut torqued was large. It had been adequate in the field but the customer wanted the bushing even.

I determined that the gap should be the size of a 19 gauge wire, which was convenient because we had a lot of that wire around and a scrap piece could be used as a gauge. That yielded exactly 30% compression that the one and only mechanical engineer on staff (the rest are electrical engineers) told me was ideal. I asked him how that was determined. His answer, that’s what they teach in school. He did not know how that was derived or if anyone had tested that theory.

I contacted two of our bushing manufacturers and talked with three mechanical engineers and got the same story.

Rubber formulas have changed over the years, but no one has ever gone back to validate that 30% rule of thumb. BTW apparently 10 to 50% works as a range with 30 being the best.

You mentioned the torque varying with the pitch of the thread, And How!

I worked 17 years at a large trucking company that was notorious for overloading the trucks, The firs year I worked there I saw payloads of over 100,000 pounds go out of that terminal.

We were losing wheels on the road because of the stress, so they started running all outbound loads through a check line after they were hooked up.

They had two torque wrenches in the check line set to 300 ft lb. and the mechanics were told to check 3 lugs on each wheel, if any moved, all lugs on that wheel were checked. All of our fleet was equipped with coarse thread lug nuts on what were nicknamed “manure spreader” wheels and we stopped losing wheels.

Then we got some GMC tractors with Budd wheels with fine thread studs and nuts. To my surprise, the mechanics were using 300 ft lb on these. I went to the shop manager and told him that the torque should be much lower on these. He called me an idiot and said "300 pounds is 300 pounds?

They changed it after pulling some studs out of the hub.

Your manager is correct, 300 lbs is 300 lbs. What changes is the clamping force. 300 lbs on a fine thread will provide a lot more clamping force than 300 lbs on a course thread. And it is the clamping force that pulled out the lugs.

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I can hear the groans from the mechanical engineers here already. I realize that this was said by both an engineer and the supplier but there are so many things not right about this it could take a day to write. I would hope the supplier knew a bit about the application and made that statement based on that knowledge. First off, “rubber” is like saying “aluminum” or “metal” for that matter. There are so many formulations of “rubber” it isn’t funny. Secondly, how it is made matters, for example the cell structure; closed cell versus open cell. Then there is the hardness/durometer/shore value that needs to be considered. Not to mention the application. What is it sealing? Is there pressure to consider? What shape is the gasket and how is it contained? What kind of relaxation and compression set will the chosen material have over time and temperature?

We have gaskets that span the range of 10% compression to more than 70% depending on all of the above factors and more. I suspect somebody originally designed that gasket and parts and took all of this into account. You’ll see cast or machined covers with built in limits to avoid exceeding the clamping force on the gasket. But the sheet metal parts are a bit more challenging to design in prevention measures. Sometimes you’ll see a bushing embedded into the formed gasket or the part that it goes against but this costs money and often left out for that reason.

That was my point exactly, when you change the thread pitch, you have to change the torque settings!

With all the transmission pan gasket talk. I should mention on all my front drive Chrysler products from 78 to 2004 ( the last one I had) Chrysler recommended not using a gasket at all on transmission oil pans. They wanted you to use just a bead of RTV sealer. I will admit that since a gasket came with any new filter, I sometimes used that when I found my tube of RTV dried up. Either way seemed to work just fine.

I’m not a big fan of RTV gaskets in service environment, especially in-situ. Great when brand new and manufacturing the product. Much harder to do in practice with inverted sealing surfaces and residual oils continuously running down. For the DIY trans pan; no lift, on your back, with minimal room to position the parts prior to engagement…arggh!

Perhaps I should have mentioned, I have a very large supply od used bolts from all the cars I have junked and use 2 or 3 with the heads cut off and just lightly threaded into the case to guide the pan into place. I put the RTV on the pan on my workbench. Never had a leak.

I am far too slow to make any money working on other peoples cars but I still enjoy working on mine. My grandchildren don’t ask me to work on their cars much anymore, they think I am too old.