78 Wagoneer - THM 400 Torque Converter Bolt Size / Part Number

[hotrod6657]

Hi all,

Stumbled upon a 78 Wagoneer recently that needs some love after years of searching for one. Wife and I are working our way through the mechanical systems so we can assess what it's going to take to get back on the road.

Ended up needing a starter due to a dead short in the unit and discovered the remnants of 2 bolts wedged inside the nose of the starter, all ground up and beat. The gear on the old starter couldn't move out to engage the flexplate, whoever was driving this the last time it was on the road had a bad day...

A day later with a new starter installed we the 360 to run (at least for a little bit, still have a fuel pump to replace and need to assess what's in the tank) and when we tried to see if the trans would function we got nothing.

Thinking about those bolts in the starter I was fairly sure they came from the flexplate to crank connection (pull the engine) or the torque converter to flexplate connection. Ended up looking under the vehicle and realizing we could see the flexplate through a missing inspection cover so I figured let's bump the engine over and see if we seen any bolts there... Nothing. Not one. So at this point I'm assuming all 3 fell out, one was lost to the road, and 2 ended up in the starter. The holes on the flexplate also look a little distorted so I'm thinking someone used the wrong hardware or didn't torque them.

Now, here's my question... Does anyone know the spec for the missing bolts, or perhaps an ARP or other MFG part number? I've been researching and see that 78 is probably late enough that the THM400 is the AMC pattern case. Summit has 3 different listings for THM400 torque converter bolts, all different sizes, and 1 a different diameter and pitch, but AMC/Jeep isn't listed as an application for any of them.

I'd prefer to know at the onset and order the right thing rather than getting all 3 and having to return, also wouldn't be shocked it's "none of the above" and uses something special that I need to track down.

Thanks in advance for your help. Looking forward to getting the old gal back on the road!

Cheers!

hotrod

[tgreese]

I assume you want the flex plate to convertor bolts.

The parts book says that '74-78 uses a generic bolt G9408973... fortunately for you, it also lists a size (not typical). 3/8"-16 by 1/2". In '79, they went to M10x1.5, I assume the same length.

Since it's a generic part, I'd go to the hardware store and buy some grade 5 or grade 8 bolts that size and try them.

The '74-80 factory parts book is available in paper and on digital media many places, including RockAuto and BJ's.

[hotrod6657]

I assume you want the flex plate to convertor bolts.

The parts book says that '74-78 uses a generic bolt G9408973... fortunately for you, it also lists a size (not typical). 3/8"-16 by 1/2". In '79, they went to M10x1.5, I assume the same length.

Since it's a generic part, I'd go to the hardware store and buy some grade 5 or grade 8 bolts that size and try them.

The '74-80 factory parts book is available in paper and on digital media many places, including RockAuto and BJ's.

Thank you! That's a great start and awesome insight to my situation! I was indeed referring to the torque converter to flexplate bolts. I'm surprised they're potentially generic! I was assuming they would be "special" with at least a really short head. I will see if I can pick up a trio of grade 8 3/8-16 x 1/2 at our local HW store.

Not relevant to my situation really but was it still a THM400 in 79? Prior to this whole rabbit hole I assumed we had a 727 behind the motor but that does not seem to be the case.

Thanks again for the info and the super fast reply! I'll post back when there's an update.

Cheers!

hotrod

[tgreese]

Yes, '79 was the last year for the TH400 in Jeeps. Late 1965 through 1979, the TH400 was the only automatic available in Jeeps. In '80 they switched to Chrysler transmissions (including an AMC-case 727), conforming to the rest of the AMC line. Lots of changes in 1980.

[hotrod6657]

Yes, '79 was the last year for the TH400 in Jeeps. Late 1965 through 1979, the TH400 was the only automatic available in Jeeps. In '80 they switched to Chrysler transmissions (including an AMC-case 727), conforming to the rest of the AMC line. Lots of changes in 1980.

Fascinating, thanks for that info!

[devildog80]

Get an inspection camera up through the inspection port, line up the torque converter to the flywheel mounting points, and snap some pics. This way you can see what thread and approximate size you need.
Did you save the bad chewed up bolts that came out with the starter? Always save these remnants for awhile, so you have an example of what you need/don't need, when looking for replacement parts. Starter might actually still be good too, if you got the jammed bolts out of the gear.

[hotrod6657]

Get an inspection camera up through the inspection port, line up the torque converter to the flywheel mounting points, and snap some pics. This way you can see what thread and approximate size you need.
Did you save the bad chewed up bolts that came out with the starter? Always save these remnants for awhile, so you have an example of what you need/don't need, when looking for replacement parts. Starter might actually still be good too, if you got the jammed bolts out of the gear.

I did save the bolt remnants, they're really chewed up though. Starter seemed like it had a short in the windings, we got a dead short to ground anytime we had it connected and tried to crank, shocked it hadn't melted the old fusible link yet. Local parts store had a reman unit for a pretty good price with the core swap so went that route.

Bolts I found in the starter had a pattern on the head (what was left of it) of a few lines crossing in the center, very similar to what I've seen on chevy torque converter or flexplate bolts. I was initially thinking they had fallen from the flexplate to crank mount which means pulling the trans to fix... Encouraged that it might just be the TC to flexplate instead.

It will probably be a couple days before we get another day to wrench on the Jeep but I'll post back what the results are.

[tgreese]

Lines on the head is a grade marking.

https://lightningboltandsupply.com/wp-c ... rength.jpg

[hotrod6657]

Lines on the head is a grade marking.

https://lightningboltandsupply.com/wp-c ... rength.jpg

Thank you, yeah, I'm familiar with the SAE and Metric grade markings, the marks on what was left of the heads of these were debossed, cut into the head of it, rather than embossed like I'm used to seeing. I've seen that with older chevy flexplate and flywheel bolts but they are a special low profile head which was what made me assume the torque converter bolts were going to be some type of specialty fastener.

Checked our big blue home improvement store today... They don't have anything graded in a 3/8" shorter than 3/4... Guess I'll head to a more proper hardware store and pick some up. Those big stores never have any of the graded hardware I need!

I'm assuming I can locktite and torque to factory spec? I'm curious if there was a washer of some type there originally.

I have a copy of a TSM for this but mine seems to be missing most of the parts I need... Wiring diagram doesn't have the tailgate power window on it and I couldn't find any reference to the size of or part number for this bolt like you were with the factory parts book. Might be time to look at picking up a copy of that myself as I suspect this won't be the last head scratcher on this wagon haha...

[tgreese]

Parts book does not show a washer.

Parts book: https://www.rockauto.com/en/moreinfo.ph ... 10&jsn=394 Not expensive. Also available in paper, which I have.

They also have the TSMs, if the copy you have is incomplete. I would look at the electrical and manuals sections of same and near years at the Tom Collins site for the tailgate diagram.

Note that the Haynes and Chiltons manuals are not the TSM, if you did not know.

[hotrod6657]

Parts book does not show a washer.

Parts book: https://www.rockauto.com/en/moreinfo.ph ... 10&jsn=394 Not expensive. Also available in paper, which I have.

They also have the TSMs, if the copy you have is incomplete. I would look at the electrical and manuals sections of same and near years at the Tom Collins site for the tailgate diagram.

Note that the Haynes and Chiltons manuals are not the TSM, if you did not know.

Fantastic, I'll take a look. I am aware the Haynes / Chiltons manuals are different. The TSM I have was from Tom Collin's site but the one for '78 seems to be missing at least the wiring diagrams and the diagrams he has for '78 don't have the rear window (that I could find). I'll look around his site some more though and see what I can find in similar years. I've barely scratched the surface of his website and boy is there a wealth of information on there!

[devildog80]

Probably need to get the bolts from auto supply, as they are not general hardware type bolts.
And was thinking the old bolts, you might be able to see thread for size of the bolt, also what thread size they are, metric or standard. But get a camera in there to see it the hole size matches the bolt size. PO might have used the wrong ones, and why they came out?

[tgreese]

Pack of 50 from McMaster-Carr: https://www.mcmaster.com/92620A619/

Yeah, I'd buy an ungraded 3/8"-16 bolt from the big box store to test the thread size. You'll likely need to shorten it to fit in the space available. Knowing the thread size, you can look for the proper part.

The M10 bolt for '79 has an actual part number 420 0302 which probably echoes its GM part number. The G number I gave above indicates a hardware item, not Jeep-specific.

[hotrod6657]

Pack of 50 from McMaster-Carr: https://www.mcmaster.com/92620A619/

Yeah, I'd buy an ungraded 3/8"-16 bolt from the big box store to test the thread size. You'll likely need to shorten it to fit in the space available. Knowing the thread size, you can look for the proper part.

The M10 bolt for '79 has an actual part number 420 0302 which probably echoes its GM part number. The G number I gave above indicates a hardware item, not Jeep-specific.

I probably have a few misc. 3/8-16 bolts laying around. I've got the metric ones out of a 700r4 that are for another project so I could at least see if the thread is correct in the TC. I've also got taps so I'm sure I can chase the threads in the TC to confirm. Just hoping to have the new parts on hand before I get back into it so it's not getting dragged out.

I'm wondering if the previous owner lost the "original" bolts and ended up putting in something longer than spec because that's what they could find, not getting it tightened properly, and that's what set this whole failure in motion...

Hilariously, that 50 pack from Mcmaster doesn't cost much more than the 3 pack from ARP haha...

[hotrod6657]

Probably need to get the bolts from auto supply, as they are not general hardware type bolts.
And was thinking the old bolts, you might be able to see thread for size of the bolt, also what thread size they are, metric or standard. But get a camera in there to see it the hole size matches the bolt size. PO might have used the wrong ones, and why they came out?

I'm suspecting something along those lines... They needed transmission work, had the work done or did it themselves, installed the wrong bolts or forgot to tighten them, went on the test drive... at some point all 3 worked themselves loose, 2 lodging themselves in the starter nose... Vehicle of course stopped driving because nothing was connected anymore... Guy probably tried to crank, heard horrible sounds, kept cranking, engine stopped turning over. He said "Forget this!" and had it dragged home and parked it until we came along and got it.

Gotta love trying to figure out what happened with a vehicle, especially something older like this.

[devildog80]

Love those challenges, and when it was something stupid easy like yours, got it for much less that with it driving down the road. Hope the missing bolts, and starter replacement, is all that's wrong to get it moving again under it's own power. Then move to the next fix needed

[tgreese]

...Hilariously, that 50 pack from Mcmaster doesn't cost much more than the 3 pack from ARP haha...

Yep. Lifetime supply. They ship fast too. "Delivers Tomorrow" at 4PM. You'd likely wait an extra week for the ARP parts.

I'd check that the holes in the flex plate were not wallowed out.