Disclaimer: The author has been a Mangusta enthusiast since 1969, and the owner of 8MA888 since 2000. He is an architect – not an automotive engineer. The following text describes his efforts to analyze and rectify an inherent Mangusta defect. No guarantee is made as to the performance of this component. Updated information on its ongoing performance will be supplied in the future. 

When I bought my Mangusta the previous owner warned me that its motor mounts were “not good”. This was an understatement. Whenever I accelerated hard, the engine would lift up, and the jackshaft pulley would begin to chew its way into the cabin. I had read articles in old magazines indicating that this was a common problem in Mangustas, so I was resigned to renew the mounts. The following chronicles my experience with a motor mount fix.

As I pulled the engine from my Mangusta, the nature of the motor mount problem became crystal clear. After unbolting the mount castings from the block, I raised the engine…and the castings, together with half of the rubber mount biscuits, fell clanging to the floor. The rubber biscuits had failed completely, and my engine had been sitting there, restrained only by the transaxle beam.

To understand the failure you have to look at the mount design. The Mangusta engine was held in place by cast aluminum mounts that bolt to the block, then to a Metalastik rubber isolator biscuit, (Metalastik #31/350) This is a hockey puck-like piece consisting of two rubber discs, each ½” in thickness and 3” in diameter, sandwiched between three 3” diameter steel plate discs. (See photos #1 & 2) Each outer disc has a carriage bolt style stud projecting from it. The upper disc’s stud fastens to the mount casting, the lower disc to the chassis bracket. The rubber and metal disc “sandwich” is bonded together only by vulcanization. This is the inherent flaw in the design.

Under acceleration loads, particularly in the lower gears, the Mangusta generates substantial uplift loads on the motor mounts. (The engine/transaxle, as a unit, wants to rotate around the half-shafts in a direction opposite the wheel rotation.) The Metalastik sandwich can handle this load only within certain limits. Heat from the adjacent exhaust headers, together with repeated compression/tension loading cycles, eventually breaks down the vulcanization of the sandwich and rips it apart. (See photos #3 & 4) Once that happens, the engine is unrestrained, simply sitting on the rubber blocks. Only the dead weight of the engine provides the leverage to move the rear wheels. At the point when the resistance torque load from the rear wheels exceeds that generated by the dead weight of the engine, the engine/gearbox lifts itself off the mounts, rotating about its attachment point at the transaxle hanger beam.

It seemed to me that the answer to the problem was to revise the mounts to resist this uplift. Seeking the simplest route, I decided to rebuild the Metalastik biscuits to use a through-bolt, then add another rubber cushion under the chassis bracket to absorb the uplift load.

First I cut apart the Metalastik sandwiches, thoroughly removing all the rubber. Then I knocked out the studs from the outer steel discs. Next I drilled a hole in the center of the middle disc of the same diameter as the holes in the outer plates. To replace the rubber discs I purchased a 6”x6”x ½” thick nitrile rubber vibration isolator pad from my local industrial supply house. I cut it into four 3” diameter circles and drilled holes in the center of each to match the steel plates. For the supplemental uplift cushions I used rubber motor mounts for a ’57 Chevy (credit Steve Liebenow here), available from Steele Rubber, item #60-0232-11 (www.steelerubber.com) I used a generic hardware store bolt and washers sized to run through the whole assembly. The bolt was chosen to match the diameter of the hole in the metal discs and is threaded only over the last inch or so. This maximizes the cross sectional area at the point of maximum stress. (I did NOT use a high-strength bolt, as I feared the combined load cycling and secondary bending load would embrittle and break a high-strength fastener.) The finished parts for the reconfigured mount are shown spread out on the floor in photo #5, and assembled in photo #6. I believe I spent around $26 for the whole thing. As I re-installed my engine I slipped in the mount components and they all fastened into place without drama. Photo #7 shows the new mount assembly in place. I had considered fabricating a metal heat shield for the mount, but I don’t think it will be necessary.

As you might imagine, the improvement to my car’s performance was dramatic. I now know how quick my car is! Also, by eliminating the effects of a “squirming” engine on an already “flexible” chassis, the handling has improved noticeably. After 200 miles, the new mounts are holding up perfectly.