9-Series Ring Mount (Suspension Frame) Rubber Mounts

"Look at he Cessna mount kit, look at one put together by Skytec (for the Zenair 750 with 912iS engine)  This kit they make works with their matching truss mount.  The spacing within the truss holders determines what bolts and sleeves are to be used by the mount OEM."

I've been using "motor mount" but we'll use "truss mount" here for clarity...

The old Cessna truss mount that interfaces with the J3608 is basically a plate welded to the truss tubing for the 3/8" bolt to go through. The J3608 rubber set lives in the hole in the O-200 engine case.

The spacing within the truss holders is determined by the dimensions of the rubber mount set. The size of the required ring mount sleeve is determined by the size of the bolt that will fit through the hole in the rubber mount set.

That rubber kit in the picture looks identical to the J3608 sitting in front of me, and the hole in that big washer, and rubber, and spacer and other rubber and other big washer is definitely 3/8". But the diagram shows 7/16" bolts because that's the size of the hole in the ring mount. I'd like them to show me a picture of that bolt going through that rubber mount set.

I've been looking high and low for a 40mm long (≈1-1/2") sleeve that is 3/8"ID and 7/16"OD to stuff in the ring mount hole so I can use the J3608 and appropriate 3/8" bolt.  Of course that is AFTER I create a rubber mount holder from scratch, because as I've already pointed out, no certified manufacturer ever made one because the J3608 was designed to live in engine case holes.

So, we're back to square one. Why did they pick this rubber set?

And two, has anybody found a different rubber set that has a 7/16" hole (or has found a reducing sleeve for the ring mount) AND provides the appropriate 912 vibration dampening, AND maybe has had a past life in a certified truss holder? For example, you can buy dynafocal cups from AS. But what rubber that fits it would be appropriate for the 912? I have no idea.

Expensive but would work, cut to length. If you search their site they make have other options. You could call Zenith, ask for Roger, they may not sell retail to you if you’re not a Zenith aircraft owner but may share the source.

https://www.mcmaster.com/products/tubing/material~stainless-steel-2/stainless-steel~/od~7-16/id~3-8-2/id~0-375/

"Expensive but would work, cut to length."

Good info!! That would work but you're right, expensive. Can't hurt giving Zenith a call either. Thanks!

Hi all

The question is why Rotax suggest this mount.  The answer is actually simple.  The early 9 series engines were first used  as replacements in certified motor gliders in Europe.  The existing engines were about the same weight and had truss mounts to connect to the legacy engines, these were adapted to the early ring mounts from Rotax to make the transition from the legacy engine to the Rotax.  The call out for the mount is still used today even by the test aircraft at Rotax, it works.  The installation manual gives you data and clearly this mount is a suggestion and not an absolute.  If you want to build or test an option it is your call for sure.  The mounting requirements are also shown in the IM.  

Cheers

"The call out for the mount is still used today even by the test aircraft at Rotax, it works."

Did they change the bolt hole diameter in the ring mount at some point because the current version of the ring mount has a 7/16" hole and the rubber mount hole is 3/8". That doesn't work.