I’d made a calculation error. I couldn’t believe I’d done it. How could I have missed something as fundamental as this? Well it wasn’t until it was all together did I notice my folly. Prior to my engine rebuild I had found a need to make spacers to lift the strut tower bar a ¼ inch in order to clear the turbo compressor outlet. The problem now existed that after installation of the GZ aluminum oil pan, which has a ¼ thick flange verses the thin 1/16 inch stamped steel flange of the OEM oil pan, and through which the motor mount brackets bolt to the engine block, my engine sat up about a half inch higher than it originally did. This made it impossible to install the strut tower bar given the new location of the turbo compressor outlet! I had to figure out a way to drop the engine back down to it’s original height. 1) Lower the subframe with spacers? 2) Machine the motor mounting brackets? 3) Use shorter motor mounts? Well number 1) was not an option as it would hose up too many items some associated with suspension components, number 2) I don’t think I could remove enough material to make it work, and 3) I had just installed brand new Noltec motor mounts which were fantastic – stiffer and with NO APPRECIATE INCREASE IN NVH.
Picking the lesser of three evils I decided to go with some shorter motor mounts. After a long search I came up empty including consideration of second generation mounts. I couldn’t see anything that would work. The only solution would be to make my own. How hard could it be to make a set of “low-profile” motor mounts? Turns out, not so hard. Aftermarket motor mounts are after all just a chunk of urethane cut to length – so I could cut them to whatever length/height I wanted. So I bought several aftermarket mounts to use for some reverse engineering and did my research to source the raw materials and to determine the proper hardness and durability specifications and dimensions for the urethane not to mention the hardware necessary. And I figured if I was going to spend the time to make a set for myself I might as well make a few extra for friends and maybe to sell. So I sourced 3 feet of 3” diameter urethane rod with a durometer hardness of 92. I also bought a foot of 1” diameter rod of the same spec. A box full of Stainless steel 3” washers and 1” washers, and a bunch of SS bolts and lock nuts for holding it all together. All was purchased through McMaster.
After looking at the other aftermarket mounts available the best design I could make on my own, without the need for overly complex fabrication processes, was a solid puck of urethane with a through bolt. However, an additional small puck of urethane on the backside of the sub-frame was added to help minimize vibration. However anytime you use a through bolt design you do essentially have a direct path of vibration from the engine to the sub-frame but it is the best compromise available short of pouring your own molded urethane – I may try that later.
The biggest issue most folks encounter is how to cut the urethane rod. Ever try cutting a piece of rubber? Ever try cutting rubber in a straight line? It’s not easy. The best way to get a dimensionally correct “puck” for use as a motor mount is to machine it on a lathe. So I used a radial arm saw to cut the 3” rod down to the approximate length for each puck and then popped them in a small hobbiests’ lathe I borrowed from a friend. But it wasn’t that easy! Getting the cutting bit at the right angle was tough and trying to cut too much material at one time resulted in the cutting bit grabbing the puck, yanking it out of the chuck, and hurling it and hypersonic speed across the garage! So there was a fine line to be found in order to get a nice clean cut of the soft spongy urethane. That would be somewhere between 1/32 and 1/64 of an inch….so you can imagine it was slow going. After each puck (both the 3” and the smaller 1”) was cut to length I drilled the center holes… on the lathe also. One aspect I did incorporate into my design was the use of a center sleeve for the puck. I cut a length of aluminum tubing and pressed it into the center hole. It was cut slightly short to allow for some compression of the puck when bolted down. The center sleeve would also add some lateral stability to the mount where the bolt passes through the puck.
Apart from that all the pieces are stacked together and installed. I made my mounts about 10mm shorter in height than a standard mount and there result was near perfect clearance of the turbo compressor outlet to the strut bar. The NVH is slightly higher than with the Noltecs and is noticeable but like I said above…it was the lesser of three evils.
OUR "LOCAL7" COMMUNITY | THE WORLD OF THE RX-7
This page last updated July 7, 2012