I have read that the engine mount cushions can or should be periodically rotated 180 degrees to reverse any “sag”, thereby extending their useful life. Is this true?
This can only be done on applications that use symmetrical mounts, most commonly having a single-layer bonded cushion, on lower-powered installations.
It usually cannot done on the Model 23-24 and 24 airframes that have the IO360 engine. Beech set a design goal for this Lycoming 4-cylinder IO360 implementation to be as smooth or smoother than the Continental 6-cylinder IO360 installations used in some other airframes. Lord worked with Beech to develop a special mounting kit. It uses two different laminated cushion packs, and two different cushion spacers (one rubber-coated and the other with a gel cushion). They have to be properly installed, with the different laminations in the correct positions and the gel spacers in the correct positions (upper left and lower right). If you read the shop manual you will see what I mean. If the cushions are improperly installed, you will feel the engine contacting the mount during start-up or shut-down, with the potential for a damaged vacuum pump and cracked engine mount, along with cowling damage from the muffler.
Because of the unique cushion design and placement, and a molded-in pre-set in these cushions, they have a locating roll pin that mates with a hole in the cushion backplate. I have found these mounts damaged because he installer failed to align the pin with the hole. And because of this alignment pin, the mount cushion cannot be rotated to reposition the usual “sag”. Lower-powered engines that are mounted with single-layer cushions having no locating pin can be rotated. If you choose to try it, make certain that the bonding is sound between the rubber and backing plates. If it has begin to crack, rotating the cushion can lead to early separation, as it increases the shear load right at the crack-line.