O-200 Motor Mount Bolts

[atypicalguy]

I had a similar thought about the mount. Not sure how to inspect that. I will ask the IA helping me. Unclear if they just reused bolts from old mount or if bolt wear was from this mount. The mount has a big metal flange where this bolt wear occurred; i dont see that flange wearing down or being a problem but I can check the holes for roundness I guess. The mount bushings I just removed were in adequate shape, just cracked on the outermost part of each one where it gets compressed by the washers. So whatever damage hit these bolts is not from the most recent bushings wearing thin.

When I think about it, aren't these bolts all going to wear where they exit the mount on the engine side? The motor is cantilevered off the mount bolt and the bolt is not constrained in bending, due to the entire engine-side mount assembly being rubber. So there is both a shear load and a bending load on all the bolts, and no bushing or liner between the bolt and the mount tube exit. This explains why the grooves are only 180 degree.

My motor has aluminum washers inside the steel washers, with a tang hanging down to contact the motor case, providing a ground from the case to the motor mount bolts. These were present on all four bolts, but the tang contacting the case was broken off of the bottom washers. So all the ground was through the top two bolts.

[6643]

It just occurred to me that the bolt info you posted was for the early mounts using the conical motor mount cushions, not the later mount with the lord mounts.

The bolts called for are AN6-44A and AN6-54A with AN365-624 elastic stop nuts.

Both installations specify a bonding strap. I don't see how aluminum washers with tangs could be reliable.

[atypicalguy]

Thanks.

Yes I have the older mount with the conical rubber bushings sitting inside aluminum cups in which insert into the block from the front and back. Sorry if I am posting in the wrong thread but I figured motor mount bolts only need one thread probably

Anyway the new bolts showed up yesterday so I will head down tomorrow and see if they fit. Mine has castle nuts so I got drilled bolts like the old ones, rather than elastomer, and some cotter pins.

I will try to check the roundness of the exit holes but as mentioned there is a huge flange on those which backs up the rubber cone, so the only way the bolt holes are coming out of round is if the material has been worn away.

Bonding strap comment noted. They seem to have been working just fine to provide a current path from the block to the bolt, but they don't really look like they belong. Anyone have a photo of where their bonding strap runs from and to?

[6643]

Yes I have the older mount with the conical rubber bushings sitting inside aluminum cups

Ah!

What do your logs say about that? Strictly speaking, you should have a field approval as it is a major modification. I have no idea what the right bolts are for that.

Mine has castle nuts so I got drilled bolts like the old ones, rather than elastomer

The elastomer nuts are only appropriate if there is a steel tube running the entire length of the rubber mount, so the nuts have something incompressible to torque down against.

Anyone have a photo of where their bonding strap runs from and to?

OK, you got me thinking...
I recall seeing the washers with tabs about 40 years ago. they're probably in a bucket of old hardware I have at the hangar... They were worn and the IA I was working with fashioned a jumper with about 6 ga. wire and ring terminals. It goes from the stud on the engine case opposite the one the oil filler neck support bolts to, to under the head of the engine mount bolt.

If you have the washers, there should be 4 of them. If you only have two I'd definitely fashion the jumper and fly worry free.

[atypicalguy]

What do your logs say about that? Strictly speaking, you should have a field approval as it is a major modification. I have no idea what the right bolts are for that.

Well, sticking an o-200 case and rotating assembly into the plane and calling it a c85 is pretty major. The Swords STC has a huge amount of documentation. Maybe that mod is in there somewhere.

Anyone have a photo of where their bonding strap runs from and to?

OK, you got me thinking...
I recall seeing the washers with tabs about 40 years ago. they're probably in a bucket of old hardware I have at the hangar... They were worn and the IA I was working with fashioned a jumper with about 6 ga. wire and ring terminals. It goes from the stud on the engine case opposite the one the oil filler neck support bolts to, to under the head of the engine mount bolt.

If you have the washers, there should be 4 of them. If you only have two I'd definitely fashion the jumper and fly worry free.

OK thanks. There are four of them, but the tabs contacting the case have broken off of two of them, so I wanted to replace them all. I can see that Cessna still sells something similar, but it has a larger center hole than mine do. The wire with ring terminals sounds like a good solution, though less redundant. I will see if my IA will accept the jumper idea. I suspect these washers were original parts that the installation shop just reused and the tabs failed over time on the lower ones.

[6597]

Anyone have a photo of where their bonding strap runs from and to?

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Have another one on the other side, plane came with 2 so I did the same. 0-200.