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Trans tunnel cracks and underbody paint/coating

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Old 09-16-2018, 08:40 PM
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Trans tunnel cracks and underbody paint/coating


So I have been slowly rebuilding my transmission, and about done with that (had some issues that took some extra parts and time). But before I put it back in I have been cleaning up the underside of the car, and I found some tiny cracks in the metal of the transmission tunnel. I am not sure how to take care of them, I don’t have access to a welder and I was hoping to just get the car on the road before pulling out the interior since it needs some work and I don’t have the money for that now.
So other than these little cracks the metal looks good, just some rust on the outside edges and beteew where the heat shields didn’t cover near the cat. So my question is what would be the best to paint the undercarriage with? I was thinking rustolium with rust converter then covering it with some kind of under carriage paint (any recommendations?), and maybe one with some insulation vale in is since it always seemed like the transmission tunnel had heat radiating off of it on long drives.

Last edited by david86camaro; 09-16-2018 at 08:49 PM.
Old 09-16-2018, 11:21 PM
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Re: Trans tunnel cracks and underbody paint/coating

It looks like that metal may have been hit, which bent them, and cracked. You want to bend them back smooth and drill a 1/8" hole in either end to stop the propagation of the cracks. Find someone to then v-notch them and weld them up. Welding is the only fix for these. You can band-aid them by covering them up but you'll regret it later.
Old 09-16-2018, 11:38 PM
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Re: Trans tunnel cracks and underbody paint/coating

Originally Posted by JoshHefnerX
It looks like that metal may have been hit, which bent them, and cracked. You want to bend them back smooth and drill a 1/8" hole in either end to stop the propagation of the cracks. Find someone to then v-notch them and weld them up. Welding is the only fix for these. You can band-aid them by covering them up but you'll regret it later.
Well I guess I need to find some one local to help me weld it up, and I need to start working on removing some of my carpet to get these fixed!
Old 09-17-2018, 12:13 AM
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Re: Trans tunnel cracks and underbody paint/coating

I’m trying to think what would have coused this, back in high school and college I did have a few accidents but I’m surprised that I have damage there. I’m the second owner and bought it when it was only 7 years old and was not told of any previous accidents so I only have myself to blame!
The acidents were minor front end and one rear ending, the only one that it was hit hard enough would have been when it got backed into (I wasn’t in it), got hit in front of the front wheel pretty hard. The rest were a dog, a coyote and a construction sign, nothing besides fender and bumper replacement!
Old 09-21-2018, 10:04 AM
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Re: Trans tunnel cracks and underbody paint/coating

Yep, needs welded. A good weld will burn right into that sheetmetal, no need to V it out. Just clean the surface with a light grinding and weld it up. Prime and paint both sides.

Hit in the rear huh? I'm curious, are there slight pressure buckles in either outer sail panel?
Old 09-21-2018, 10:16 AM
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Re: Trans tunnel cracks and underbody paint/coating

Originally Posted by 86blackiroc
Hit in the rear huh? I'm curious, are there slight pressure buckles in either outer sail panel?
Not that I have seen, but it was a such a light hit that, the bumper cover was not replaced, just repainted. There is some slight misalignment in the fitting of the bumper, I wish I forced them to fix that, but at 20 I didn’t know better, and was being nice!
Old 09-21-2018, 10:29 AM
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Re: Trans tunnel cracks and underbody paint/coating

Anyone have any suggestions as to what would be the best undercoating/paint? I’m thinking of painting any rust spots with a good rust converter, then paint everything with some rustoliom, and then some sort of rubberized coating for some sound and heat insulation. But want make sure that doesn’t case issues, I have heard that sometimes the rubberized coating can cause issues with trapping water, but that is why I was thinking a good coat of paint first will help prevent that.

Also my rear end looks pretty rusty, it’s only surface rust, so I was thinking of painting that, any suggestions as to what kind of paint? Was thinking just black for the axle, then a gray or red for the cover since I have to pull it off anyway (it started to leak after I cleaned it off, was covered in a pretty thick layer of road grime).
Old 09-21-2018, 10:38 AM
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Re: Trans tunnel cracks and underbody paint/coating

Undercoating doesn't really have any sound deadening or insulating qualities. But it does resist chipping which is nice in wheelwells. It's kind of ugly though, imho.

I always use engine enamel on rear axle housings and such.
Old 09-21-2018, 03:05 PM
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Re: Trans tunnel cracks and underbody paint/coating

Clean the area thoroughly, apply seam sealer, let cure before driving.

To weld those areas, you'll need to get the carpet up off the floor, or it'll melt or worse catch fire. Unless you really feel comfortable removing the seats, console, and a bunch of trim, not to mention finding someone to do the welding... It's probably from being jacked wrong, slipping off a jack/stands/lift, or driving over a foreign object. If it's from structural stress, a weld will probably create a stress point and it'll just crack again anyway. Clean it up and cover the gap/bare edges with seam sealer, it'll close up the hole, keep water out, prevent it from rusting. It'll also flex just enough that it probably won't need further attention unless you decide you dig into it.

Any bare areas that are starting to rust would best be stripped to bare shiny steel, then sealed with something like epoxy primer. The lazy way is to remove as much rust as possible and hit it with some kind of rust converter, or encapsulator, which pretty much just means you're painting over the rust, and hoping it slows the rate of corrosion. No easy answers there...

For heat, the stock cat converter heatshields should be all you need.
Old 09-21-2018, 05:01 PM
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Re: Trans tunnel cracks and underbody paint/coating

Originally Posted by Drew
Clean the area thoroughly, apply seam sealer, let cure before driving.

To weld those areas, you'll need to get the carpet up off the floor, or it'll melt or worse catch fire. Unless you really feel comfortable removing the seats, console, and a bunch of trim, not to mention finding someone to do the welding... It's probably from being jacked wrong, slipping off a jack/stands/lift, or driving over a foreign object. If it's from structural stress, a weld will probably create a stress point and it'll just crack again anyway. Clean it up and cover the gap/bare edges with seam sealer, it'll close up the hole, keep water out, prevent it from rusting. It'll also flex just enough that it probably won't need further attention unless you decide you dig into it.

Any bare areas that are starting to rust would best be stripped to bare shiny steel, then sealed with something like epoxy primer. The lazy way is to remove as much rust as possible and hit it with some kind of rust converter, or encapsulator, which pretty much just means you're painting over the rust, and hoping it slows the rate of corrosion. No easy answers there...

For heat, the stock cat converter heatshields should be all you need.
Hmmm, now you’re making wonder more about what I should do. The car is on jack stands/ramps now while I have been rebuilding the transmission (I pulled it out over a year ago), I’m cleaning things up and seeing what else needs to get done before I put everything back together. I may lift the engine a little before I finish up to replace the oil pan gasket since I had an oil leak back when I was driving it. I was hoping to wait on dealing with the interior until I get it on the road again, but pulling out the center console and the passenger seat is not that much work, then shove a welding blanket under the carpet since they are just behind the shifter opening. If I have to pull out the carpet I will need to replace it since it would likely get ripped up being as dried up as it is, I do plan on replacing it at some point but don’t have the money now. One of my coworkers has a small welder so I can weld it if that is the permanent fix, I’ve had the car 25 years and my kids are saying they would never sell it so I’m in it for the long term!

As far as the paint, there is one part of the floor pan that got cooked (around where the heat shields came together), and is a little corroded and rusty then there is only a little rust at the edges (likely from gravel and rock chips). I was planning on cleaning, wire brushing and a little sanding before any paint, but want to still coat with rust converter to make sure I get it all. It sounds like I may just paint and wait till I replace the carpet to deal with sound deading and insulation, since it sounds like anything underneath would not do much good.

Last edited by david86camaro; 09-23-2018 at 11:40 PM.
Old 09-25-2018, 08:02 PM
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Re: Trans tunnel cracks and underbody paint/coating

welding is permanent and honestly its the right and only way to fix cracks.
Old 09-25-2018, 08:36 PM
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Re: Trans tunnel cracks and underbody paint/coating

Tearing a car down after being down for a year already is how projects spiral out of control, and your car becomes a lawn ornament instead of something you actually drive and enjoy. Put it back together, seal up the cracks and when you're ready to do the carpet next spring, get a welder lined up to fix the cracks. But that's just my perspective.
Old 09-26-2018, 04:31 PM
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Re: Trans tunnel cracks and underbody paint/coating

Originally Posted by Drew
Tearing a car down after being down for a year already is how projects spiral out of control, and your car becomes a lawn ornament instead of something you actually drive and enjoy. Put it back together, seal up the cracks and when you're ready to do the carpet next spring, get a welder lined up to fix the cracks. But that's just my perspective.
unfortunately it’s been a lot more than a year (it’s been 14!) so I’m okay to just do it right, since it maybe a few years before I do the interior, I don’t want things to get worse by waiting. But I do appreciate your sediment and with I could have done that in the first place, but kids and money made it a garage fixture!

The fun I have to deal with, was not leaking until I cleaned off the dirt and grease about a year ago.
Old 11-02-2018, 12:46 PM
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Re: Trans tunnel cracks and underbody paint/coating

Hard to tell where exactly this is located based on the photos, but possibly this could be from a bad transmission mount causing the transmission to slam into the tunnel. This could be especially violent with a manual transmission and a high rpm clutch dump (torque arm pushes the back of the transmission upward during acceleration). Best fix is to hammer/dolly back to shape, drill relief holes at each end of the crack and weld. As Drew said though, if the car were actively being driven, just drill the relief holes, seam seal and drive it. Not worth tearing the interior apart for this, unless you are going to do that anyway.
Old 11-05-2018, 10:22 AM
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Re: Trans tunnel cracks and underbody paint/coating

Originally Posted by TransamGTA350
Hard to tell where exactly this is located based on the photos, but possibly this could be from a bad transmission mount causing the transmission to slam into the tunnel. This could be especially violent with a manual transmission and a high rpm clutch dump (torque arm pushes the back of the transmission upward during acceleration). Best fix is to hammer/dolly back to shape, drill relief holes at each end of the crack and weld. As Drew said though, if the car were actively being driven, just drill the relief holes, seam seal and drive it. Not worth tearing the interior apart for this, unless you are going to do that anyway.
Well it’s not from too much torque, it’s only a V6! But when I pulled the transmission 3 out of 4 of the bolts that held the transmission to the bell housing were missing, it did quite a number on the internals. So the one near the mount could have been from this issue. The one near the shifter opening is surprising. This car is on the slow road to getting back on the road, so I will take the time required to get done right, even though I was hoping to wait on messing with the interior until I could afford to replace the worn out carpet.

Thanks for your input.
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