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Motor Mount sag

pylotttt

Active Member
My rv-6a with an o320 has about 830hrs since built in 2005. The spinner sits about 1/2" low compared to the cowling. The gap at top and bottom are equal.

My question is, when the motor mounts get old and the engine "sags" does it normally drop straight down resulting in a mismatch of the spinner/cowl but still have the same distance at top and bottom, or do they normally just drop the front of the engine causing a smaller gap at the bottom of the spinner vs the top?

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Engine sag

Normally the engine mounts (4) are replaced at overhaul.
When sitting on the ground there is a pushing force on the lower mounts
and a pulling force on the upper mounts. This creates a little wider gap on
the top of the spinner vs the cowling and it increases with age.
In the air the prop is pulling the air frame and the gap is more or less
equal top and bottom.
 
Depends a bit if your engine is Dynafocal or Conical mounted. Also depends on the original engine installed angle. Also depends on the original cowl installation.
 
On mine (180 dynafocal), it sagged 1/4" or so since I last replaced the mounts a few hundred hours ago; if there was an angle it wasn't noticeable.
I put one shim (washer) under each of my bottom mounts this weekend, all lined up now!
 

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Yes, my 8 looked like that at 750 hrs and 16 yrs later. I changed them because the alternator pulley started to rub the cowling. After new mounts, I couldn’t get cowling on due to top of spinner touching. After a five minute run, the engine settled enough for cowling to go on but the gap at top was very close. It now has 100 hrs since new mounts and the top gap has opened up a little more, but I was afraid to fly it inverted for the first few hours because of such a small gap. They appear to sag evenly around spinner gap, but when new, they are angled such that upper gap is smaller than lower gap.
 
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Lycoming Isolators

I've added a washer to the bottom mounts on sagging Lycomings a time or two over a 40-odd year career. It works, I guess, but it's not really the recommended fix for a saggy engine. Hey, you have the cowl off, the engine hoist in place and you've already unbolted the bottom mounts. Why not just pop for a new set of isolators while you're at it?
 
Just some anecdote to add to the convo

It was pretty common in Mooney world to swap top and bottom isolators or rotate in place when the sag got noticeable. They were symmetrical/omnidirectional so orientation didn't matter. I have no idea if the isolator OEMs would have approved but I know which way to bet.

One thing that was pretty indisputable and another that became accepted or at least tribal knowledge for that part of the flying community:

The older polymers lasted way longer than the newer stuff e.g. resisting stiffness and sag. That seemed true for any of the manufacturers.

The Lords gave better performance and life than their competitors . Yes, they cost more. No, I'm not wishing to start a debate.
 
“The older polymers lasted way longer than the newer stuff e.g. resisting stiffness and sag. That seemed true for any of the manufacturers”

When you say “old”, what time frame are we talking here? My originals were installed in 2003 and replaced in 2021.
 
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There's no reason to address the anecdotal data; you mention a single data point in a population that experiences a lot of variables. Plus, I framed it in the "accepted or tribal knowledge" category as to try and avoid debate. There was no specific crossover date from better to worse vulcanization. It seems to be more of a trend towards such. The polymer engineers and chemist claims I've read surmise the newer polymers may crosslink more over time to make the parent materials stiffen. I'll let those real experts weigh in. Chemistry/formulation or process changes to lessen environmental impacts or to save money, who knows? With a very limited supply base, what would be the manufacturer's motivation to maintain or extend product life? Just sayin'
 

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After a five minute run, the engine settled enough for cowling to go on but the gap at top was very close.

I think this happened on my new installation a couple years ago. I thought I had a pretty good cowl fit, but after the move to the airport (on a trailer) and the first engine run (and wet-dog shake down), the cowling fit looked like the engine drooped 3/16" to 1/4" below the original setup. If I build again, I will make sure I yank the engine up, down and sideways on the mounts before I start fitting the cowl.
 
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