Engine mount to Firewall Torque?

[fenderjw]

I'm in the process of reinstalling my C145 on my 170A after an engine rebuild and after perusing the manuals ad-nauseum I have a question about torque values. The bolts that pas through the engine mount into the firewall (AN6-22A and AN6-32A) are both 3/8ths inch by 24. According to the Cessna 100 series service manual (pre-1962), these size bolts should be torqued to between 160 and 190 inch pounds. This equates to only about 15 foot pounds

Am I missing something? I cant find torque values anywhere else that specifically relate to the engine mount to firewall torque requirements so I am left with the generic values listed in the table on page 1-8 for the bolt sizes used. Its just a bit scary to think that the bolts that hole the engine mount (and the engine !!!) in place are only torqued to 15 foot pounds.

Thanks
Jim

[Bruce Fenstermacher]

Why is it scary?

You have to understand what the bolt is doing. It is holding two pieces together in one spot. Making it tighter won't hold then together any better or make the bolt stronger.

[fenderjw]

Thanks for the reply Mr. Fenstermacher
I understand your explanation intellectually and since no one has pointed out something that I may have overlooked yet, I guess I feel better. I'm new at this. I've only owned the plane since Feb and have been hip deep in an engine removal, rebuild and reinstall since Memorial Day. Its nice to know that you guys on this site are here and can perform a sanity check for me when I doubt myself.

Thanks again.
Jim

[ptporebski]

Jim,

Does the engine mount to firewall include an elastomer element of some sort? Perhaps to serve as an vibration isolater? If so, then I would expect that explains the relatively low torque value.

[fenderjw]

No Sir.
The mount is held on by a bolt with a washer that passes through the mount itself. Then there's a flat washer, the aluminum bushing that passes through the firewall, Another bushing on the other side of the firewall, then a flat (square) plate, another flat washer then the locking nut. No elastomer anywhere.

Thanks
Jim

[Bruce Fenstermacher]

As Jim has pointed out early 170 and 170A models like his and mine don't have any elastomeric bushing material in that location. That was started with serial 19200 and on.

[n2582d]

As Jim has pointed out early 170 and 170A models like his and mine don't have any elastomeric bushing material in that location. That was started with serial 19200 and on.

Bruce, I believe Jim's 170A is post s/n 19200 for he says,

...Then there's a flat washer, the aluminum bushing that passes through the firewall, Another bushing on the other side of the firewall, then a flat (square) plate, another flat washer then the locking nut. No elastomer anywhere.

I think his elastomer bushings have been replaced by the "seaplane bushings". This discussion refers to them. Here's a picture of the IPC for the C-170A:

C-170A Engine Mount Detail.jpg

If Jim wanted to he could go replace the aluminum bushings with rubber ones. See this discussion. Interestingly, in later model C-172's Cessna went back to omitting the bushings altogether. The '69-'76 C-172 service manual is also where one can find a torque value for those nuts.

Engine Mount Torque Values.jpg

The C-170A IPC calls for AN365-624 nuts on the mount to firewall bolts. I would recommend going with all metal nuts rather than those with the elastic inserts. The later C-172 IPC calls for MS20365-624C nuts. AN363 nuts would also work. If you're going to go to the trouble of torquing the nuts remember to add friction drag torque to the torque value of your calibrated torque wrench. (See AC 43.13-1B 7-40d).

[Bruce Fenstermacher]

Good catch Gary. I should have read more carefully.

[fenderjw]

Gary is correct, it looks like the bushings are the solid aluminum (seaplane) variety and yes my SN is post 19200. Thanks again for the pages from the IPC (which I have) and the Service Manual (which I dont have). As I mentioned earlier, my biggest concern was the proper torque values for the through the firewall bolts/nuts. It seems though that you guys have come to the same conclusion as I did which is 160-190 inch lbs (about 15 foot pounds). Thanks again for confirming this. Being a NOOB causes me to doubt myself especially when I see a torque value that's so low.

Thanks again
Jim

[n2582d]

Jim,
Thanks for asking. It was a good question. If you had the original C-170A rubber bushings in there the answer would have been different. For those C-170's with rubber bushings at the firewall here is what Cessna says in the 100 Series Service Manual, (paragraph 12-66 on pg. 12-32):

Engine Mount Torque.jpg

As far as getting a service manual goes, you can download one -- see "Free Cessna Manuals Online" under the Mx Library section or, for $20 you can get it on CD from McCurtain Technology Group.

[Brad Brady]

Gary,
I liked your post! there is really no torque on the rubber mounts to the firewall. Just push out till you get a round fit after the washer. the first flight everything (in my mind ) will change....That's why I wanted to put sea plane mounts on the 55 I was working on.....It's just that the price (650.00....to 65.00... round figures) was more than I wanted, and my owner was willing to work with. You can replace a lot of rubber mounts for 10 times...