Engine Mount Vibration Isolators

One of my 'vibration isolators' (rubber shocks) on the engine mount of my Glasair has a small split in it, and I'm wondering when should I plan to relace all four shocks? The split is 1/8 deep max x 1/4 inch long. It'll be a big job to replace them, so not interested in doing that any sooner than necessary. But dont want to risk a major event in flight either.

I installed these isolators as I was building the Glasair about 20 years ago but have only about 120 flying hours on the Glasair so far. The engine (IO-360 C1E6) has 6th-order counterweights and I had the engine/prop dynamically balanced (during run-up on the ground) so the engine runs pretty smoothly and vibes are not getting worse. The gap between spinner & cowl is good, so engine is not 'sagging' much yet. I'm not an A&P and haven't seen many old shocks so not sure how much to be concerned. Should I just watch for increased vibes, or is there a risk this one (or more) of the four shocks will fail in flight and cause big problems quickly?
Thanks

I've actually never heard of an engine shock mount failing catastrophically. Generally the problem is that they sag progressively over time, and if they sag enough the engine or propeller spinner can start contacting things that they shouldn't be contacting.

I am not familiar with the Glasair engine installation, but usually replacing shock mounts is NOT a big job. Typically the engine simply needs to be supported (but not removed) by an engine hoist to relieve the stress on the mounts, at which point they can be replaced fairly easily. Is there something unique about your Glasair installation that would make shock mount replacement a big job?

Mike
Thanks for the quick response.
Good to hear you've never heard of a catastrophic shock failure in flight. Niether have I.

My Engine will only need to move forward about 1/2" to replace shocks, but the challenging would be the accessories on the back of the engine...they are all very close to the steel tubes of the mount, so several will probably have to be removed (both mags, vacuum pump, air intake, exhaust, fuel line, other smaller stuff).

Take a good look. I bet you can move the engine enough to replace the shock mounts without removing accessories. There has to be enough clearance between the accessories and the mount tubes for the engine to move a fair amount on the shock mounts in normal operation (particularly engine start and shutdown), so usually there's enough clearance to replace the mounts without significant disassembly. No guarantees, mind you, as I've not worked on a Glasair..

Thanks Mike
Sounds encouraging. And I see from your profile that you know what you're talking about! So I really appreciate your feedback.
I will take a closer look at the job and hopefully get a local A&P to look at replacing the shocks maybe at one of the the next few Annuals. Meanwhile, I think I will keep flying and watch for signs of change in vibes and/or sagging.

Do you think isolators should be replaced periodically, even if they have no signs of damage or sagging? ie is there any 'age limit' on them since they are rubber (ie calendar time, irrespective of tach hours?). The engine has been hanging on mine for 20 years, but I've heard some original isolators have been left in for the life of the aircraft (which I assume is longer than 20 years).

Thanks again for your helpful feedback...this foum is great!
Brian

Quote:

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Originally Posted by bwbeaird

Do you think isolators should be replaced periodically, even if they have no signs of damage or sagging? ie is there any 'age limit' on them since they are rubber (ie calendar time, irrespective of tach hours?).

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No, they should be replaced strictly on-condition. Replacing them on calendar time does not make sense since they are not safety-of-flight items. It is normal to replace them at engine change/overhaul (for convenience), and also if there is excessive engine sag that threatens to allow the engine or things attached to the engine to contact the airframe or things attached to the airframe. (Recently had a Cessna 210 whose engine mounts had sagged to the point that engine's oil pan was chafing against the top of the nosewheel well...that's not good.) Since the engine can move significantly on its shock mounts, we always want to make sure there is plenty of clearance between anything attached to the engine and anything attached to the airframe, so that there's no way they can touch one another even when the engine moves maximally on its shock mounts.

I've seen some pretty ugly rubber engine mounts, but like Mike I've never seen one fail. the typical Glassair mount is the conical type with off the shelf Lycoming engine mounts. being the Gassair is very tight spacing it will be a difficult chore to change them with the engine on, I'd wait until the engine sags or is off to change them.

I deal with two Glassairs on a regular bases, they are tighter spacing than a Moonie, this job won't be fun with the engine on.

Thanks, Tom. Nothing like hearing from a mechanic who actually has worked on Glasairs. I haven't, but if they're worse than Mooneys, then that's pretty bad. LOL.

Thanks Mike & Tom for the great info.
I will take a really good look around the inside of my Glasair's cowling to ensure there's still nothing on the engine close to anything on the airframe/cowling...that's good advice. An good excuse to use my new VideoStik fiber optic camera I bought at Oshkosh last summer!..
Brian

I work in a shop that deals with Glasairs primarily and the bushings can be changed without removing the accessories. It is close and a bit fiddly, but it is regularly done.

Thanks Neil that is very good news. Let me describe my firewall-forward layout. Mine is a Glasair II RG, with a Lyc IO-360C1E6 engine and standard 6-point mount from S-H. This is 200 hp engine so a little wider and heavier than the typical 180hp, but fits onto the mount nicely. Accessories mounted on back include the usual two mags and vacuum pump, oil filter, fuel pump. Air intake is also in back. Air intake is on the pilot's side...ie the injection servo sits near the firewall, with a 90 deg elbow leading into the engine. Look like the mags are the tightest clearance from the engine mount tubes. I can post pictures if that would help. From your experience, does that set-up lend itself to replacement of the isolator bushings without removing accessories?
Brian

Shouldn't be a problem. You just have to use a hoist to hold the weight, work methodically and be patient. I think getting to the cotter pins is the worse part. On rare occasions it becomes necessary to move something out of the way but not often. Your installation is very typical. It may be a bit more difficult if you have Bendix mags as they take up a little more space than the Slicks.

Thanks again Neil. Bad news is I have the bigger (Bendix) mags, but good news is I do have an engine hoist and patience (mostly acquired by >15 years of building the Glasair).