Engine Mount Alignment Ajustment

[Mark Goldberg]

This is not the first instance of our engine mounts not mating up properly with scratch built fuselages with their attach bushings already welded in place. We just added a note to our web store urging scratch builders to have our mount in hand before they weld the attach bushings in place on their fuselage. This instance was somewhat different as Ryan bought a flying plane. Mark

[Chris In Milwaukee]

I can see this being a common problem. Mark's planes and mounts are all built on the same jig to make sure everything lines up. There are bound to be mismatches when folk scratch build their planes. I'd definitely want to have Mark's mount on hand before I mocked up the front framework.

[James]

Hi Guys - well, I'm now up to hanging the engine, and I need some guidance on modifying the engine mount / fuselage.

I have the Avipro O-360 mount, but as described above, it is the variant with 5 arms.

The outside 4 tubes on the fuselage have a 3/8" inside diameter, but the tube at the top centre cluster is 5/16" ID.
The corresponding tube on the engine mount has a 3/8" ID.

Should I:
a) ream out the fuselage top centre tube to 3/8" to match the mount, or
b) bush down the engine mount tube to 5/16" (with ACS tube PN 03-01200)?

Either approach seems reasonable - the top centre fuselage tubing has a thicker wall than the outside corners, and so if I ream it out to 3/8", it'll match the tubes in the outside locations.

Bonus question - should I place the boltheads for the engine mount forward or aft? Looking at photos on the forum, people install them variously with heads forward or aft, but having them aft with the nuts accessible on the firewall side might be smarter for access down the track.

Thanks again everyone,

James

[jaredyates]

I used option A and mounted the nuts out where I could more easily install the cotter keys and keep an eye on them. And JimParker256 might be as dissapointed with that as how unshined my boots are.

[James]

Yeah ZK, but as long as you meet min torque, I feel a nut + cotter pin is preferable to a metal locknut without a locking device, but then I think the Cessna 150 I sometimes fly has AN363 nuts on this application. I certainly wouldn't want a nylock nut here, not in a hot engine bay.

When you were talking about torquing a nut or lining it up for the cotter pin, maybe I misunderstood - it's not either/or, it's both. Castellated nuts are often installed with a torque range (min to max torque), to allow for this alignment. If there's no range, well then that's min torque, and some guys might just keep changing nuts until they find the right one that lines up at the set torque. My rule of thumb is (on my own plane, and depending on what I'm torquing) - if it's within half a hole at max torque, just keep torquing until it lines up. If it's past the hole on max torque, change the nut / washer.

Given that I've just reamed the hole to 3/8" and you have the weight of the engine hanging off it, I'd be surprised if this bolt turned. But a nut can still rattle off a bolt that doesn't turn. With the AN363, I'd place some torque stripe or something on the nut so I could be sure it hasn't moved since I torqued it. But then, a cotter pin still installed will tell you the same thing at a glance.

James

[JimParker256]

Too funny, Jared... But there's always an "exception" for practical purposes like that -- more so because those bolts would not be exposed to the airstream.... (And by the way, even the Army doesn't polish the boots any more... Only the dress shoes get spit-shined! LOL)

[James]

Well guys, after incidentally reading through the Tony Bingellis book FWF, he says that metal locknuts are ok in this application, and that the bolts should be placed head aft. That's good enough for me, but I'll probably still go with the cotter pin option.

[Mark Goldberg]

That top center attach is not really even needed for the four cylinder installation. Anything you do is OK. But I would use it for sure. Mark

[James]

Thanks Jared and Mark - once again it's great to have your experience here on the forum.

I didn't realise that we had to get the blessing of JimParker256before we could do dodgy stuff on our aircraft...

Luckily, down here in Australia, all my workmanship will be hidden by the curvature of the earth!

James

[JimParker256]

Sheesh! I go out of town for a couple of days, and suddenly it's "pick on Jim week"?

I'll have you know that LOTS of people do dodgy stuff to their airplanes without my permission! LOL.

[jaredyates]

You know it's only a reflection of our admiration Jim!

[JimParker256]

Yeah, Jared, I'm sure that must be it... My wife one time asked me "Why is it that when two guys are friends, and like each other, they tend to insult each other all the time?" I said "Well, it's complicated... But it's one way you know you're friends – you can tease each other pretty mercilessly, and nobody gets upset." She said "That would never work with women..." At that point, I wisely chose to keep my mouth firmly closed...