Help!!! Stripped Transmission Bolts

[cab83_750]

So, this is probably my 10th engine/tranny drop without issues. Now for some reason, I stripped both tranny bolts last night.

I was able to find replacement ones and they torqued correctly.


Here are my questions:

1. How tough is the actual mounting hole? If the bolts are stripped, does that mean that the mounting holes are also stripped?

2. If the holes are stripped, what is the fix? Heli-coil? Make it bigger?


So far, the car is running fine.


Thanks you guys!!!!! And Happy New Year!



P.S.,

I did see a post about the female part being the stripped side. Mine is the male side.

[David]

I was able to re-tap mine and get them to torque. The best option is probably Timecerts.

[1990C4S]

Re-tap and try to torque the bolt. They are fine thread be careful not to cross thread it. If it holds add a bit of blue loctite and you are done.

If you can't torque it go to the next size up and use an imperial fine thread. Search, the info is here. Somewhere. I think Gunther posted it once...

[cab83_750]

Been reading the archives and it looks like timecert is good.

Right now I am not sure if I actually have 2 stripped holes. I don't mind the retapping job; I just don't want to tap if it is not broken.

As previously mentioned both bolts torques correctly. If one bolt falls will I know right away while driving? I am guessing the shifting lever will move.

[Grady Clay]

If the M12 transmission bolts are coming out damaged, there is a problem in the chassis side female threads.

Take the transmission mount off and look up in the hole. What do you see? Thread a perfect bolt in. Does it go in all the way with only fingers? If not, try a ‘thread chaser’ (SnapOn thread restoring tap TRT56 or equivalent). Do not use a cutting tap.

Has there been some previous repair? A HeliCoil or other is a harder part than the bolt or the original Factory threads. An errant repair piece will damage the (softer) bolt.

The choice of which repair part to use is based on the extent of damage to the original hole and if there have been previous repairs. HeliCoil is the least ‘invasive’. If you have a failing HeliCoil, you can simply pull out the coil, chase the threads with the HeliCoil tap tool and install a new HeliCoil with the installation tool. All the tools are in a M12x1.5x19 mm HeliCoil kit (#5406-12) available everywhere.

Keenserts, Timecerts and others take out more of the original metal and limit future options. These may be necessary depending on the extent of damage to the hole. I would never use these as my first choice unless absolutely necessary.

When the transmission mount bolts and chassis threads get damaged it is usually from misalignment and using force (impact wrench) on installation. Always install the engine mount bolts first but leave loose. (If you have a threaded engine mount, I prefer to drill out the threads and use a longer (M12x1.5x92 mm) bolt, lock washer and nut as early cars did.) With patience, you should be able to spin-in the transmission mount bolts with your fingertips. ALWAYS try the bolt in the hole before you install the engine & transmission.

The Factory bolts have a taper ‘lead-in’ end to them. This implies you don’t have to start the bolt several turns with your fingers. Nothing is farther from the truth. This probably worked perfectly on the Factory assembly line but is not for our old cars. Start the threads by fingertip for most of the distance. Use a torque wrench for the final turn or so.

A ‘trick’ is to leave the four M8 nuts between the transmission mount and the transmission nose piece loose a turn or two. This allows you to better align the M12 bolts and thread them in by fingertip.

Unless you are using all new (plated) hardware, I dip the bolt threads in SAE30 oil. Use a new lock washer every time. Torque to specification with a calibrated torque wrench.

Remember to re-check everything, including the M8 nuts.

Installing hardware loose is dangerous. The normal procedure is to install a bolt/nut and torque to spec immediately. This procedure requires great care to never forget to go back and tighten everything.

Best,
Grady

[cab83_750]

Grady,

your post is great. You know, I have always wondered why those bolts are 'upside down' and without a nut at the top.

The car appears to be perfect aT the moment and I am comfortable as using 2 new bolts were torqued correctly.

I will visually check next week for the condition of the mounting holes.

[Gunter]

Good stuff, Grady.

Seems like a lot of people generally over-torque bolts or cross-thread.

To minimize enlarging the holes in the body, I'd opt for going with the next size SAE-NF.
Yes, it's non-Metric..............boohoohoo...........and not OEM....but so is any repair with Helicoil or TimeCerts etc.

In the case of failing transmission mount bolts, the transmission will drop, cross member rests on the swaybar and shifting will feel different.

Problems with motor mounts are mostly people finding out that the bolts are so rusted in the bar that they won't open and actually twist/break the ends of the bar. Someone didn't use NeverSeize.
I'd love to find longer M12 bolts, thread them through the bar and then put a nut on the end but these bolts are a special order here and lots of $$.

Like Grady says:

"A ‘trick’ is to leave the four M8 nuts between the transmission mount and the transmission nose piece loose a turn or two. This allows you to better align the M12 bolts and thread them in by fingertip."

Another trick is to leave the 4 bolts on the engine carrier for the bar snug/loose and let the engine/transmission self-align before tightening everything up.
No stress and an even load on all 4 mounts.

People who do a lot of alignments know to leave respective bolts snug/loose until every bolt is in and then tighten to specs with a calibrated torque wrench.

Helicoil, TimeCert or next-size Imperial; your choice.

[ChrisBennet]

I think these bolts/holes get damaged from cross threading usually.
I've Timeserted several so it's not uncommon.

To avoid cross threading, I loosen the 4 nuts that hold the transmission cradle onto the bottom of the transmission so it's "floppy", then start the bolts in by hand. This reduces the chance that side pressure will be applied to the bolt as you are trying to thread it in.

-Chris

EDIT: Oops, I missed Gunter's post saying the same thing. Happy New Year.

[cab83_750]

How much are these long bolts? I searched the web but no 92 in length.

[Gunter]

92 mm wouldn't do it according to the Bentley SC Manual. Page 101 says M12 x 1.5 x 95 just for regular use.

They need to be longer than 95 mm, say 110 or 120 if you want a nut at the end.
I need to check this again to see if 110 or 120 is enough to let them come through the bar so a nut can be put on for extra safety.

I remember something from a few years ago that they had to be ordered in multitudes like 100 in a package?

If you can find M12 x 1.5 x 120 single, please, let me know.

[cab83_750]

Must we stay with "Steel Grade 10.9?" I am going to assume that grade 8.8 is not strong enough!!!!
Metric hex bolts, Metric standard bolts, Steel grade 8.8 fine thread

[scottb]

McMaster sells 10.9 metric bolts.

McMaster-Carr

[cab83_750]

Originally Posted by scottb

McMaster sells 10.9 metric bolts.

McMaster-Carr

Yeah! But, they are at 1.75 pitch. Perhaps, this is what Grady did: drilled out the female thead, used the 1.75 M12 bolt, and everything is secured by the new nuts and washers.

Metric hex bolts, Metric standard bolts, Steel grade 10.9

[Gunter]

8.8 might do. One would have to figure out the tensile strength.

Consider the total weight of engine/transmission and that each bolt carries only roughly a quarter of the weight.

Example: If the total weight is ~600 lbs., would a 8.8 bolt snap carrying ~ 150 lbs. ?

I don't think so.

However, I'd still prefer to not drill and just find extra long M12 x 1.5 to go right through the bar and add a nut.

[1990C4S]

Grade 8 bolts are 120,000 psi, so it would hold, uhmmm, oh a battleship maybe.

[cab83_750]

Forgive my naiveness, but Grade 8 is ok then?

Grade 8 bolts are 120,000 psi, so it would hold, uhmmm, oh a battleship maybe.

[cab83_750]

I believe for now I am done looking for the 1.5. Out of curiosity I wonder how much a machinist would charge to thread a 'no thread' bolt at 1.5 pitch?

8.8 might do. One would have to figure out the tensile strength.



Consider the total weight of engine/transmission and that each bolt carries only roughly a quarter of the weight.



Example: If the total weight is ~600 lbs., would a 8.8 bolt snap carrying ~ 150 lbs. ?



I don't think so.



However, I'd still prefer to not drill and just find extra long M12 x 1.5 to go right through the bar and add a nut.

[Gunter]

cab83,

You can use a search engine for Tensil Strength like:

Bolt Depot - Bolt Grade Markings and Strength Chart

You'd have to ask a Machine Shop about the cost of making longer M12 x 1.5 with 8.8 quality.

[1990C4S]

A machine shop is not a good option. A cut thread is inferior to a purchased bolt which is a 'rolled thread' IIRC.

Again I doubt it matters, either on is strong enough, I suspect.

[1990C4S]

Rough numbers....Amercanized units:

A 12 mm bolt is about 0.175 square inches.

A grade 8.8 bolt has a yield strength of about 93,000 psi, yield strength is therefore about 16,000 lbs.

A 12.9 bolt would be 160,000 psi yield, or about 28,000 pounds.

If you are losing sleep over this please re-read the numbers. You will pull the threads long before you break the bolt, even with an 8.8.

A 12.9 12 mm bolt will lift about 10 cars.