engine shock mount SDR

[SteveG]

I recently undertook what at first appeared to be the straight forward task of replacing the rear engine shock mounts on an A model. But, as usual, the job goes south pretty quickly. The problem first manifested itself as not being able to torque the thru-bolt to the specified value without the nut just bottoming out on the last thread. Pulling it back off and inspecting to understand the problem it becomes apparent that the shock mount, part no. J-9613-31 by Lord, does not fit the application. The mount's design is intended to have the flange surface of the two halves meet in the middle of the engine mount spacer and the internal spacer mate with the upper and lower shock mount components once the assembly has been compressed to the proper preload. But in my instance the engine mount spacer plus the mounting plate is too thick to allow either of these parameters to be met. The removed parts are the same as the new so this aircraft has not had, for at least the last twenty years that I'm familiar with, a properly fitting shock mount. I believe this leads to premature failure, which at the usurious price of $300 each is undesirable, along with engine sagging misalignment and increased vibration being transmitted into the airframe and the resulting noise and stress fractures.

So, my questions to you all are: has anyone else previously identified this problem and if so how was it dealt with? how widespread is this condition? do other models have a different configuration or are we all operating with misfit shock mounts? are other make & model aircraft involved?

I've been corresponding with the product engineers at Lord but so far they have just said "we're looking into it". I "fixed" the problem by inserting an additional spacer between the mount halves and adding washers to the internal spacer to make up the gap. I'd be most interested to have feedback from other operators concerning this matter. It's hard to comprehend how the correct part no. does not fit the application.

Regards, Steve G.

[cessnadriver]

SteveG-
Hello, I replaced the rear mounts. But I used the Barry mounts, at $75 per set, bought them from a Global salvage. It was the correct mount listed on the Barry Mount website. I did have a problem with them, I had to remove one of the washer under the nut but it was fine since the mount has a very thick material on the mount halves. My biggest problem was to get the damn bolts to line-up. I had to use all sorts of pry bars and adjust the hoist to get them in line.
Good luck, BILLS

[SteveG]

Thanks for the reply Bill. Yes, the work requires some patience but if you only remove one mounting bolt at a time you can maintain the overall alignment and it will go back just as it came out.

In examining the various shock mount part numbers it appears that the Lord J-9613-31 and the Barry 94017-01 are only used on Skymasters but are common to all model years. Therefore it would appear that the issue is confined to Skymasters but affects the entire fleet.

My conversation with the engineers at Lord has come to an abrupt halt. Seems that the lawyers or accountants or both have gotten to the engineering department and told them they cannot discuss this outside the company because in a schizophrenic about face the discussion turned on a dime from one of concern and intellectual curiosity to adamant denial.

I have to admit that I've been somewhat amazed that I can suggest that we are all operating with shock mounts that do not fit and are not performing as intended and that not one person has any comment or concern or any insight to contribute. But at least the issue is now documented in the forum for the use of any future search. Still, was it something I said? "I washed me face and hands before I come, I did." Eliza Doolittle.

[mshac]

This thread would be SO much more effective if it had PHOTOS!

Most of owners don't know the ins and outs of Lord mounts, halves, spacers, etc. because we've never dealt with them. If this issue is serious, please DOCUMENT it with photos, and copies of your emails with the Lord engineers. Thank you!

[wslade2]

Yes, photo helpful; have not ignored your post but carefully followed the wording to be sure I understand. I'm straining my memory banks. I did mounts a couple years ago. I'm pretty sure they were Lord mounts. And difficult to find a complete set. Getting them in was a wrestle. Seems like they bottomed out (as you're describing), everything looked ok and rubber was snug, so I called it a day there. Will look next time I am out at airplane and review manual too.

Are you saying once everything is tightened the rubber And the mount isn't snug? There's play?

[SteveG]

Hopefully these photos illustrates the issue graphically. The mount halves are designed to meet using a 0.620" spacer between them but my (our?) spacers are 0.770" thick leaving a 0.150" gap that cannot be made up. This issue also translates to the internal spacer which defines the amount of compression preload. Its length is designed to permit 0.410" of compression but because the mount halves never meet one would have to compress the mount 0.560" to tighten, a distance that cannot be realized with the length of threads available and is 37% greater than intended. As is, the mount never becomes a unified, stressed assembly as designed. The engine is just hanging off the upper mount half.

The thickness of the engine mount spacer, not including the flange which extends through the mounting plate, is compatible with the 0.310" flange of the two shock mount halves. I'm beginning to believe that the part was never designed correctly by Cessna and that this issue has existed from birth. Hard to fathom but I have no other explanation.