More on Alumimium engine mount bushings

[pdb]

I have just begun the overhaul on my O-300A. After fixing and powder coating the engine mount, my mechanic is suggesting that I replace the original rubber 170 engine mount bushings with the alumimium bushings from a 180. I am inclined not to.

The original rubber bushings can be replaced for well under $100, the aluminium bushings will cost more than $800. My question is, does it make economic sense to replace the existing worn bushings with metal?

I understand that the rubber bushings will start to sag at some point but the existing ones have worked out ok for the last 18 years or so that I have owned the plane and I would be happy to get another 15 or 20 years from another set. I don't beat the plane up too much on seriously rough fields and haven't operated it on skis yet but I do plan to put it on skis this winter. Nor do I plan to put it on floats ever.

Am I being too cheap here?

[DaveF]

I don't understand why you'd use the hard mounts. Rubber will reduce vibration and shock transmitted to the airframe, which I think is a good thing, especially because the engine mount attach area of the fuselage is a known trouble spot. The rubber mounts will eventually sag, but they're easy to replace.

[pdb]

As I understand it, the hard mounts are commonly used in applications like seaplanes where the mount and airframe are subject to greater stress. It sure seems to me that the rubber mounts would be better tor vibration however, they seem to do just fine in the C180 and its heavier engine.

[flyboy122]

The stock mounts are relatively thin, and relatively hard rubber. If they sagged, I can't imagine they would sag much. I replaced the 35+ year old ones in my plane last year and didn't really notice much difference. The old ones were a bit smushed, but it didn't seem to affect anything.

DEM

[blueldr]

Why in the world would anyone want to replace the TYPE of a part that has previously given satisfactory service for twenty years or more?

[hilltop170]

One reason is if the rubber bushings smash out and the engine mount sags to the point the bolts start to contact the firewall, the system has failed and next thing to happen will be for the relative motion between the engine mount and firewall to wear the holes in the firewall out of round. Not saying that will always happen but it has.

The aluminum bushings clamp the firewall making a solid connection without relative movement between the mount and firewall insuring the bolts will never contact the firewall and the engine mount will never sag. The rubber lord mounts between the engine mount and engine absorb engine vibration adequately. I have never heard of the aluminum bushings causing any problem. Having the aluminum bushings has not changed the vibration level at all in my plane.

[blueldr]

Since i acquired my airplane when it was well aged, I have no knowledge of the age of the rubber mounting bushings. However, I never noticed any significant sag in the years that I owned it. Is it possible that some, perhaps after market, bushings were produced with an off standard rubber of the wrong durability?

[hilltop170]

That is entirely possible although I have never heard of that happening.

[pdb]

Thanks for all of the responses. I spoke to my engine rebuilder yesterday and he echoed the recommendation of my mechanic. I value both of their opinions a lot.

It seems that the concern is that the rubber mounts allow much more engine movement during normal operation than the aluminium mounts. The issue of sagging prematurely is a bit of a red herring.

The consequence of the engine movement is supposed to be greater wear on the firewall as Richard suggests. I am not quick to dismiss the concerns because all of these guys see a lot of planes with high time in rough conditions and Cessna had some reason for using these in the 180 and floatplanes.

Are they better? Probably. Are they worth the additional cost? Now I am thinking twice?

[DaveF]

Yep, there's probably a reason that Cessna changed the design. FWIW, the Lycoming-powered 172s use a hard mount.

Screenshot 2015-09-05 at 2.47.05 PM.png

[hilltop170]

Since you mentioned it, I would definitely use the aluminum bushings with a Lycoming.

On a different note, I can't see $800 worth of material and labor, especially if a local machinist was to make owner-producted copies.

[pdb]

Does anyone have an idea how I can get the approved data including dimensions and specifications for material so I could have these machined?

I know there is a thread here which puportedly shows the dimensions but there is nothing with a Cessna stamp. Will Cessna provide this data?

[johneeb]

Motor Mount firewall spacer.jpg

Pete, I do not know where this drawing came from and unfortunately there is no information about material. You might try the R172E parts catalog.

[blueldr]

In consideration of the price of the aluminum bushings and the historical service of the stock rubber bushings, I consider even thinking about switching a serious reason to look for a personal psychiatric evaluation.

[MoonlightVFR]

What is the history of the Lord Rubber mount design?

Why did Cessna utilize Lord mounts on the 170?

Could it be that Cessna was influenced by the WW2 military aircraft designs?