Solid Urethane Mounts
So what if you want to go one step beyond the Urethane_Filled_Mounts discussed in a previous article? Perhaps you want more control over the hardness of the overall mount. Maybe you want to shift your engine/transmission up, down, or angled. This modifications allows you the freedom to do both. Industrial suppliers, like McMaster Carr, offer a variety of urethane casting compounds. They vary in durometer (hardness), as shown below:
Part # Durometer
The 60A is soft, like the stock rubber. This is suitable for the passenger's side engine mount and driver's side transmission mount. The 78A is very firm, but is suitable for a front or rear roll stopper. The 80A is just a little firmer. The 85A and 88A are very hard and make a strong front roll stopper. If you use this on the rear roll stopper, be prepared for a lot of interior vibration.
Get to work
You'll obviously have to remove the motor mount before filling it. This article won't go into that, as it covers any motor mount. You will want to follow the factory shop manual and proper safety procedure to remove the mount.
First, you need to mark the location of the metal pin in your stock mount. They are not always in the center. Use a permanent marker to show the height of the pin. This will help you get the pin back in the right place later. Now you need to remove the stock rubber center. You will likely find that a coping saw blade can be slipped through the air gap in the stock mount. Attach the blade to a saw frame and start cutting. If you previously filled the air gap, you may need to drill a hole to slip the saw blade through. Saw around the outer perimeter of the mount, scraping the metal as you go. A coping saw works well because you can tilt the blade to get the correct angle against the metal body of the mount. Once you have the rubber center freed, pop it out. The metal pin is still captured in the rubber center. Set the metal pin in a vise and use the coping saw to skim the rubber off. Keep cutting at new angles until the pin is nearly clean. If you want, you can use a coarse rasp to strip the rest of the rubber from the pin. Now trace the inside of the clean mount onto a piece of 1/4" plastic (or similar). Use a saw to cut around the trace to make an end plate for your mold. Drill out a hole in the end plate that matches the where you want the metal pin to be. You marked its stock location on the mount. You may choose to change the height of the pin to adjust your engine/transmission height. Measure the width of the motor mount body and the length of the metal pin. You want the pin to stick out equally on both sides of the finished mount. You may need to cut a spacer to fit under the end cap to get the pin to stick out the correct amount.
To prevent the urethane from bonding to your plastic end plate, a mold release should be used. Silicone grease works well, if you don't have the correct release. Modeling clay can be used to seal the plastic end cap to the motor mount body. You will want to set the mount on a table, with the spacer underneath. Insert the metal pin into the end plate, with its end resting on the table surface. Check again that your pin will stick out of the mount equally on both sides.
You had better read the instructions that came with your urethane casting compound. Some can be mixed at room temperature, but some must be heated. You can heat them by dipping their can/bottle in a pot of water, warmed on a stove. The casting compounds listed previously weigh 1 pound each. The instructions will tell you the density, if you want to calculate their approximate volume (if it doesn't say, assume a density like water). That is important if you want to see how many mounts you can get out of a can. The front roll stopper requires about 121 mL or 4.09 oz. The rear roll stopper requires about 133 mL or 4.5 oz. The driver's side transmission mount requires about 238 mL or 8.05 oz (the passenger's side engine mount requires just a little more). If you want to use less than a whole can, you will need to find the mix ratio in the instructions. That ratio will tell you how much of your motor mount volume needs to be resin and how much needs to be hardener. There are different ways to actually get the ratio correct. You can use a small measuring cup (which might read in mL or oz) to fill a large plastic cup with the correct volume of resin. Mark the water level on the side of the cup and dump the water. Wipe all of the water out of the cup. Set the plastic cup on a digital postal type scale that reads in g or tenths of an oz. Zero the scale with the cup on top. Fill the cup with resin to the line you marked. Record that weight and determine what weight of hardener you need to add. Slowly pour in the hardener until you reach the desired weight. Start mixing immediately. After the urethane is thoroughly mixed, start dumping it into the motor mount. Use whatever you were stirring with to work the urethane into the cracks of the mount. Keep filling until you reach the top of the mount.
Let it cure for an hour in a warm place. An oven running at 200F is a decent choice for speeding the curing process. Remove the plastic end plate and modeling clay. Use a saw or file to clean up excess urethane. Try not to use a razor blade as they can dig too deeply. Let the mount rest for 48 hours to gain strength. Don't rush this part or you will ruin your new mount. Follow the factory manual and proper safety procedure to reinstall the mount.
Contributed by Corbin
Cars Modifications Power Motor_Mounts Solid Urethane Mounts
Last modified on 2009-01-03 01:29:57