Tension bolt failures are rare when operating below 8,000 RPM. This is caused by the bolt ringing, or being sympathetic to major frequencies produced by the engine. Early engines were mounted by the front cover and the next support was the transmission mount. So a big bending load was always in play. Under shock loading such as a dropped clutch, the engine twists against the dowels and this is resisted by only the front cover. Later engines were supported by mounts bolted to the center iron. This helped with failures of broken dowels in very high output engines. Later engines were supported only by the rear iron for the same reason. Some early engines would break a chunk out of the rear iron dowel hole. These were drag racing engines at high power settings being shifted with dog ring transmissions that produce brutal shock loads. So, builders started adding more dowels. Works well up to big HP. Then add turbocharging and more broken dowels. Then add more dowels and, or, add oversized bolts. or studs. The stack has to be assembled and holes reamed to be a tight fit on the bolts. Holes drilled out and re threaded. And stupid high torque on the bigger bolts. The next step was to increase the size of the oil pan bolt holes, and then add a thick plate to the engine with a hole in the center under the center iron drain back hole. Bolt the pan to that or just run a dry sump and the plate would have no through hole at all. Airplane engines probably will never make enough torque to have these problems.
Later engines came with a sort of rubber hose molded around the center of each bolt which cured the problem. Or you can add a spiral of silicone (GE 100% pure) tub and tile caulk, the full length of the bolt. Let it gel up for a few minutes before inserting the bolt. The effect is that you sort of glue the bolt to the engine so it cannot ring. It makes a giant mess but it is cheap. I only had two bolts break in 30 years. Later bolts are not necked down. I never lost one of those. I never used Mocal coolers so I cannot comment.
Lynn E. Hanover
In a message dated 7/4/2018 7:49:44 AM Eastern Standard Time, flyrotary@lancaironline.net writes: