[REPU] Board Post Reply: Solid motor mounts

From: forum@mazdarepu.com
Date: Wed Jan 15 2003 - 11:50:21 PST

A New Post has Been made to the thread [Solid motor mounts]
User From: Jeff20B
The Reply was:

Here is a post I wrote on the RX-7 Club Forum...

I dabbled in solid motor mounts for about a month and a half. It was an interesting experience.

In November, I got a bunch of large washers from Boeing Surplus and I think a number 10 stud from somewhere else (if it were a bolt, it would be as strong as our pressure plate bolts with the '10' on them). I measured the height of the motor mounts with and without an engine weighing them down. It was like 30-32mm or something. I cut the stud into two pieces of the correct length. I also used a thin rubber pad made from an old radiator hose on each side between the mounts and the cross plate/cross member/front cover mount (or whatever you call it).

I won't bore you with the details, but I will say that it had marked improvement with a few side effects that could be cured with new stock mounts and a torque brace.

The good: the stickshift didn't jiggle left-right anymore, and launches were more predictable. Small changes in throttle didn't cause a push-pull feeling as much as it used to, due to the engine no longer torquing left-right. The oil pan no longer would torque down and hit the crossmember.

The bad: way more vibrations than before. At certain RPMs, they would enhance themselves, and at others they'd cancel themselves out. I found myself unconciously trying to find them all the time. The noise was also much worse than before (and my exhaust lets you hear the engine pretty well). Sometimes something in my door or the driver's fender would vibrate really loudly while letting out the clutch at low RPM.

The result: I couldn't keep driving it like this. It would eventually crack the front cover; I just new it would. I got a good used motor mount to replace the one that had gotten soaked with oil from a bad upper oil line (I got a good used line at the same time). I replaced the line, then the motor mount. What a difference! It turns out that the oil soaked mount was not the bad one. Infact, the rubber part was in better shape than the passenger side mount. I blame the heavy exhaust system and the torque of the engine for wearing it out and allowing the oil pan to hit the crossmember. I moved the oil soaked one to the passenger side and the new (used) one the the driver's side. This will allow them to wear in the other direction for a while. Kind of like rotating your tires. I used to get a vibration with the old stock motor mounts. This was because the little metal bottom cover was rubbing the top cover (REPUs have the bottom cover; I'm not sure if RX-7s do too). Anyway, that old vibration seems to be gone n
ow. The engine again torques over to the side when the secondaries open and I'm thinking of trying out peejay's cable idea to minimize it. I've got the cable and the clamps mixed in with some old junk I've got to throw away so maybe they'll work. Too bad I don't have a shock tower to use (REPU).

Almost forgot: the studs had dinged threads where the washers had slipped down and started rubbing. I should have used the stud protectors I made out of an old Fiat stainless steel tube. The rubber pads compressed which nearly loosened up the nuts. It needed to be retorqued. Oh well. I might still go with solid motor mounts in my MG project, thanks to this experiment. Or not. Also, the four front cover bolts had a little bit of aluminum gunk on them from all the vibrations. I'm glad I went back to stock motor mounts when I did.


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