Best Option for Cleaning Threads in Aluminum Transmission Case

[Gerrha]

I am rebuilding both the transmission and transfer in my 97. Things are going relatively well, but there are a few main case threads giving me some trouble, specifically the 10-mm and 12-mm bolts both at 1.25-mm pitch. I don’t understand why Toyota went with that fine 1.25-mm pitch anyway, but the aluminum in contact with steel bolts over the 25+ years has corroded. I was able to get the original bolts out, which I guess I should be thankful about, but now what.

At this point, I have; Soaked the holes with various cleaners; Brushed the threads with various brushes; Attempted to use thread chasers, which only thread in about 1/4”; And finally, verified that new bolts will not even start to thread in.

I have a set of taps arriving soon and I think my last option is to clean those threads out with taps. I know that is not ideal, and I am sure others have faced this same problem, so I wanted to ask if there is any other option.

 

[azusa]

Heli-coil, I've had to do it. Not fun taking out those bolts.

 

[Gerrha]

Heli-coil, I've had to do it. Not fun taking out those bolts.

I got the original bolts out (thankfully), but I can't seem to clean out the female threads in the case due to the aluminum oxide scale and possibly thread deformation. A thought did come to me. Since Toyota did use fine pitch threads, I might be able to move to a bolt with a pitch of 1.75-mm. I would then need to re-tap the holes in the case to 1.75-mm. This would be a real big pain, but it might work.

 

[jpoole]

Keep in mind that there are taps and thread chasers which are different. You may prefer to use a thread chaser in this case to minimize material removal.

 

[azusa]

I would say either go up to the next bolt size standard or metric, or you need to repair the thread. Aluminum thread is not something you can repair with a chaser or tap. If you tap it to 1.75 I'm betting it's going to strip when you go to torque it.

 

[Gerrha]

Keep in mind that there are taps and thread chasers which are different. You may prefer to use a thread chaser in this case to minimize material removal.

Yes, I already tried two types of thread chasers, and they will only go into the holes about 1/4" before freezing up. I hate the idea of using a tap, but in my best scenario, I am hoping that by going very slowly with tapping compound, I can restore those threads. Maybe wishful thinking...

 

[Gerrha]

I have tapped many holes before, but nothing over 1/4". If I need to go to a bigger bolt, am I correct in thinking this would need to be done at a machine shop and not by hand?

 

[jpoole]

My experience is that aluminum threads like these rarely clean up and then hold torque. I've had luck with time-sert and heli coil installs in situations like these. Going to a larger thread size or restoring the og thread size with a heli coil or time-sert/similar are both options I would consider but I'd have to look at the cases, etc. to decide which route I preferred. I actually like moving to the steel insert setup as it can be more durable long term when you anticipate periodic rebuilds, though that may not apply in this situation. Motorcycle engine assemblies is where I've run into this the most but generally Aluminum is soft and delicate and there is galvanic corrosion between steel and alu so this is a common problem.

With care and attention to detail you should be able to drill and tap them yourself even as you size up. I think larger sizes are typically easier and more successful.

 

[mudgudgeon]

Going up a thread size might create a huge headache in sourcing suitable replacement bolts.

Some bolts on these casings have minimal clearances around the head for sockets.

JIS fasteners have a smaller bolt head than the average metric bolt.

Then, there's finding bolts with the correct length, shoulders in the right place etc.

Recutting to coarser thread with the same diameter will leave you with a weak and compromised thread helix.

You could make a thread chaser by cutting a groove along the length of one of the old bolts with a 1mm cut off disc. This would let you put more torque on it to run it in/out without worrying about snapping a chaser or tap.

If you have to repair threads, I'd be inclined to find helicoils etc with the same thread spec as the OEM bolts, and new OEM bolts

 

[Gerrha]

Moving up a bolt size would not only be a headache as you mention, but I already bought new OEM bolts for every location. I might give your idea of using an old bolt modified as a chaser. The thing is, no matter what I now do, those original threads have been compromised by the loss of aluminum through galvanic corrosion, so they will never be as good as new. The helicoil option is likely the best plan overall.

 

[smritte]

I prefer Time certs over helicoils for transmission. Main reason is, helicoils work best if you don't need to remove the bolt regularly. Ive used both for decades and both have their place to shine.
My record is 25 time certs in the pan area of a transmission. The owner was convinced he needed a trans service at 18k miles. The tech used a small impact and stripped all but three threads.

 

[jpoole]

25 time certs in the pan area of a transmission

Impressive!

 

[Gerrha]

I prefer Time certs over helicoils for transmission. Main reason is, helicoils work best if you don't need to remove the bolt regularly. Ive used both for decades and both have their place to shine.
My record is 25 time certs in the pan area of a transmission. The owner was convinced he needed a trans service at 18k miles. The tech used a small impact and stripped all but three threads.

Wow! I guess he couldn't figure it out even after 5-6 failures... Thanks, I will look at those Time certs. I have never heard of them.

 

[smritte]

Wow! I guess he couldn't figure it out even after 5-6 failures

I know, right? Odds are the tech never knew he did it and would respond with "I've been doing these for years and never had a problem".
I normally use a speed handle on these bolts. Some modern air ratchets will strip them too. Transmissions are very soft aluminum.

When you tear it down, take a picture of the worse clutch. I just went through mine and the most worn clutch still had about .004 ish material left. 260k and i tow with it. Not bad at all.

 

[Rifleman]

Another option would be the use of EZ LOK solid wall thread inserts, I've used them to repair aluminum transmission cases many times, They come in both carbon steel and stainless steel, Another big plus to using EZ LOK insert's, if you ever need to remove or replace one it's easy to do unlike staked in place Time sert's,

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[Kernal]

Long shot: if you haven't tried this yet, you could use a weak acid like white vinegar (acetic acid) to dissolve the deposits.

Stuff a wetted Q-tip or three into the thread hole, let it sit awhile, then use a small cylindrical brass brush (like a rifle/pistol barrel brush) followed by a cleaning tap working it gently along with a penetrating oil (like Free All) to see if any of the deposits have loosened up. Rinse with water when done and dry, then take some close up photos to inspect the threads.

Still might not hold depending how much metal has been lost.

Would be interesting to see some close up photos of the threads, before and after trying to clean them up.

 

[Rusty Marlin]

Another option would be the use of EZ LOK solid wall thread inserts, I've used them to repair aluminum transmission cases many times, They come in both carbon steel and stainless steel, Another big plus to using EZ LOK insert's, if you ever need to remove or replace one it's easy to do unlike staked in place Time sert's,

Threaded Inserts | Buy E-Z LOK Thread Inserts for Metal, Plastic & Wood Online at ezlok.com

Shop for E-Z LOK threaded inserts for metal, plastic and wood. E-Z LOK sells through a wide distributor network and offers competitive pricing, low minimums and cost-effective package quantities. Browse our catalog of thread inserts or contact us today!

www.ezlok.com

DO NOT use stainless in contact with aluminum. The galvanic corrosion potetial is signifigantly higher that just carbon steel to aluminum.

I would be very tempted to make a "clean out" tool from a bolt as @mudgudgeon mentioned. Flood it with WD40, the stuff is amazing for a cutting fluid on aluminum.