DIY Build Your Own Transmission Mount

The transmission mount on “Alice”, our 1967 Datsun 411 wagon, needed to be replaced. As most of you know, she has a stroker motor with a fairly lumpy cam and a higher stall speed torque converter, which made the transmission mount failure very obvious. After 45 years of doing its job, the rubber had deteriorated and was allowing too much movement.

Problem: No one has manufactured this part since the 1970’s, and there’s certainly no remaining NOS parts (which would likely be deteriorated as well).
Solution: Make my own!

NOTE: READ THE ENTIRE ARTICLE BEFORE STARTING THIS PROJECT!

You’ll need the following:

1 container of Flexane 94

Propane torch

Wire wheel

Drill

Spray paint

Packaging tape (the clear shiny kind)

An old cardboard box (not corrugated) – I used a shoe box

Aluminum foil

Two deep sockets (I used 16mm)

Scissors

Razor knife

A helper (Thanks Becky!)

NOTE: You may want to use a softer material. I used Flexane 94, which has a Shore Hardness of 97. You can use Flexane 80 or Flexane 60, which have a Shore Hardness of 87 and 65, respectively. Obviously, a softer material will absorb more vibration and allow a little more flexibility.

The first order of business is to take pictures of the part you’re going to recreate. This way, you’ll know what it looked like beforehand and you can get the alignment correct, as well as remembering which was the front / back of the mount. The trans mount on a 411 is similar to most old Datsun mounts – it’s a small cradle inside a larger cradle, and they’re bound together by the rubber. There’s no metal-to-metal link between the trans and the transmission crossmember, the mount isolated the transmission to absorb vibration and allow slight movement without transferring all the movement to the chassis of the car. If the rubber delaminates from the metal, you’ll find yourself with a transmission flopping around under the car on hard acceleration – NOT good!

As you can see, this mount was pretty shagged – You can see the rubber pulling away from the mount, and it was pretty easy to flex the part by hand. Click on the pictures below to see what we were dealing with:

Removing all the rubber from the mounts, even if it’s deteriorating, is still a huge pain. The solution to this?

FIRE.

Yep, fire. Get yourself a small propane torch. Place the mount on top of a metal or concrete (non-flammable) surface, preferably someplace away from people, pets, and nosy onlookers – it’ll make quite a bit of black smoke. Have a water hose handy just in case you need to douse it immediately. Now, fire up your torch, adjust until you get a nice blue pointed flame, and direct the flame from the torch directly at the rubber, moving it around until ALL the rubber in the mount is burning. Mine was pretty well-saturated with ancient trans fluid, so it caught fire pretty quickly. Monitor the burn until it goes out on its own – Do NOT get in a hurry. It’ll take about 20-30 minutes.

Once the part has stopped burning, allow it to cool, and use a set of tongs to quench the metal parts in water. Once they’re cooled, take a coarse wire wheel mounted on a drill and clean off the remains of the rubber (it should flake off very easily). Make SURE to get the entire metal bracket clean, down to the shiny metal. Then, clean the parts (I used acetone) and dry them off.

Now, you’re going to make your “mold” for the urethane out of pieces of your cardboard. Since the mount has some areas where the rubber was exposed (not encased in metal), these are the areas you’ll need to “box in” so the urethane doesn’t just flow out. It helps if you have an eye for engineering, but if you look at the part closely, you can tell where you’ll need to “box in” the mount. Cut the sides of the “box” to support the upper part of the mount and “suspend it” at the original height above the lower part of the mount. Use the packing tape to cover ALL your pieces of cardboard before you assemble the mold (VERY important! Flexane will not stick to the tape, but it WILL stick to exposed cardboard). You want to be able to simply peel away the mold once the urethane dries.

Also, there are two openings in the center of the mount – these are for the bolts that attach the mount to the bottom of the transmission. If your urethane fills in these openings, you won’t be able to install your mount – So, you want to create a perfectly-round “void” in the urethane. I did this by wrapping a piece of aluminum foil, shiny-side-out, around 16mm deep sockets, and setting them down in the holes. Leave the “drive-end” facing up – this way you can use an extension to pull them out once the urethane dries.

Once your mold starts to take shape, use packing tape to pull it all together. You don’t want any open seams or cracks, or your urethane will seep out. Inspect your mold several times, from all angles, to make sure it’s perfect.

Following the instructions in the package, mix your Flexane. The Flexane 94 ratio is 69 parts resin to 31 parts hardener. So, basically, 7 parts resin to 3 parts hardener. You’ll have to GUESS how much total solution you’ll need at the end – I recommend mixing up a little more than you think you’ll need, because you only get one shot at doing this. Regardless of how much you use, DEFINITELY make sure you get your ratios right. I practiced by using water first – I marked off the clear mixing container in 10 increments (with 10 corresponding to the total amount of urethane I thought I’d need to fill my mold). I filled it up to 7 (this would be your clear urethane), then up to 10 (this would be the black hardener). I then poured the water into a small Tupperware container (that was about the size and shape of my mold). Bingo! I guessed it pretty correctly. Dry off your containers thoroughly, practice time is over – It’s time to work with the real deal!

Pour your Flexane resin into your mixing container (up to the 7, remember?) Add the curing agent (black) to the Flexane resin in the container (up to the 10, remember?) and stir gently for 2 minutes. Ensure that the two parts are fully mixed by scraping along the bottom and side of the container. Mix carefully but thoroughly, and keep your mixing tool completely submerged during the mixing process. If not, you will be mixing in large amounts of air and this will create bubbles in the finished product!

Then, pour the Flexane into the mold you built, taking care not to spill any on the sides, or on your sockets that you’re using to block off your bolt holes. It is recommended to have your helper tilt the mold slightly to one side when filling, then level it out as it fills up. Pour slowly to allow any bubbles to escape. Fill the mold until the black Flexane is EXACTLY at the level that the rubber was at originally – Flexane does not expand or contract as it cures, so what you see is what you’re gonna get! Your Flexane mixture will start to set up quickly, so if you need to scrape the last bit of urethane out of your pouring container into the mold, do it quickly! Make sure the part is sitting perfectly level to the ground, and let it dry.

Resist the urge to mess with your part while it sets up. (NOTE: It will give off heat as the resin cures – Don’t touch!) After 30 minutes, touch the surface with your finger – if it feels slick but solid, you did it right! Now, you can go ahead and GENTLY remove your sockets. Let the aluminum foil remain in the holes, you can trim off any extra later – just slip the sockets out (I used an extension for one, twirling it to help extract it from the part. The other one was stuck, so I used a pair of needle-nose pliers to tug it out). Now, let it sit overnight (in a 75-degree or warmer environment).

After 24 hours, remove your mold from the part carefully. Make sure you peel off all the tape and cardboard. Now, if you’ve done it right, your part will look like it was made professionally! You can now complete the curing process by placing the part in an oven at 150 degrees for 24 hours. After this, it’s done curing. If there’s any “flash” (little edges of urethane that creeped out through the seams), you can trim it up using a razor knife. Here’s my completed mount:

Take your new trans mount outside and give it a quick coat of rust-proof paint (remember, the exposed metal is still raw where you hit it with the wire wheel).

For roughly $50, and a couple hours of your time, you have a brand-new transmission mount that can’t be purchased anywhere!

If you have any questions about this project, please feel free to ask them in this thread: Make Your Own Transmission Mount