Engine Mount

[cdmuse04]

Reviving this thread because my A&P is currently replacing the bushings on my ‘46 140, C-85-12. He noted the difference in spacer callouts in the Cessna and Continental manuals.

I have been chasing a vibration issue and in addition to changing out the conical bushings my prop is at the prop shop for service.

I would be interested to hear if anyone who removed the spacers permanently noticed any improvement or change in engine vibration with or without the spacers.

Thanks!
Chad Muse
N2287U
‘46 C-140
Manassas VA

[edidin]

I put my metal spacers back in, and will do so again soon given the short life of the rubber these days. However, having the prop overhauled did improve the smoothness substantially. All cylinders were replaced at the same time, so that may have played a role. So far all the 120/140 I've seen seem to have the metal insert.

Is your vibration continual or RPM specific?

[cdmuse04]

The vibration is most notable between 2000-2300 RPM. It seems to get a bit smoother at 2400 and above, but is still strong enough to shake the yokes and actuate the carb heat cable at full power in climb.

I tried to have a shop dynamically balance the prop but the initial reading was 1.4 IPS and was 1.9 after initial adjustment. The IA said he thought I should change the conical bushings before trying to balance the prop again, and the Dynavibe rep said that over 1.2 IPS reading the prop should go into the shop for service.

[6643]

The spacers squeeze the thin section of the rubber mount so thin that it can't help but transmit more vibration. In addition it doesn't take long for the rubber to wear through allowing the spacer to contact the engine case. Now you have metal to metal contact between the engine and the airframe.

I'm dredging up 40+ year old memory here, but I can't help but think some Cessna design engineer back in 1945 or 46 was confusing a Lycoming mount with a Continental one... To the best of my recollection, no other A- or C-series Continental installation has a spacer in there (Lord mounts excepted, of course).

[8425]

The C-85 does use a bushing, its part # is 0450195.

[6643]

The C-85 does use a bushing, its part # is 0450195.

Yes, but, the question is "why?"

The hole in the rubber bushing is designed for a 3/8" bolt and the hole in the engine case is designed for the bushing with a 3/8" bolt. There is no room allowed for the steel sleeve. Using it compresses the "nose" of the conical mount to 50% of its original thickness. That can't be right. I'm inclined to believe it was a mistake, maybe an artifact from a Lycoming design (little Lycomings used a bushing with a 7/16" hole), but, the truth is likely lost in the mists of time.