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Old 06-20-2006, 01:17 AM   #1
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busted engine mount bolts.

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I have a reoccuring problem, and I have no idea why. I have always had an Eagle to bash around with, and I have continually busted the bolts holding the motor to the plywood plate. I'm using 4-40 allen bolts, the black, hardened ones, and I think they are considered grade 8.

This has happened with a weak OS 40, an OS 40 FX, ST 40, and ST 45 now. The most recent incarnation of my favourite trainer has maybe 6 flights on it, and today I started flying it like it should be. I was doing multiple snaps up and down the runway, and all kinds of stuff a normal trainer does. As I was trying to "encourage" a relatively new flyer to do something other than flat circuits, I noticed a weird sound coming from my airframe. Upon landing, I noticed that the "old" problem had returned. I had broken both of the rear mounting bolts on my engine.

Any ideas what's contributing to this happening? The engine mounts directly to a plywood plate that I've epoxied in place (it's supposed to be bolted , so it will crash in the event of a training accident). Engine runs really smooth, prop is balanced. Bolts were tight before taking off.

I've asked a couple guy's if 4-40 is too small, and they say it's fine for a 40 sized engine. Manual says 4-40, and my Giles, which is subjected to even more abuse, has 4-40. The only structural difference is the Giles has a Dubro rubber isolated aluminum mount.

I'm tempted to just swap them for 6-32, but then I may be hiding another problem.

Thanks in advance for any input!
Michael Gyger
MAAC 58295
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Old 06-20-2006, 02:41 AM   #2
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if you are using blind nuts in the plywood are ya over torqng the little 4-40 fellas are they hobby store stuff or walmart , most of the stuff from a hardware store is pretty soft. are they sheered or stretched ? 6.32 should be end of story,or try lock tight or glue on the threads with less torgue on the ball driver
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Old 06-20-2006, 07:58 AM   #3
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Mike for your flying style, 6/32 minimum. 4/40=too small!
Ron Mattiuz

Flying Tigers RC Club
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Old 06-20-2006, 05:01 PM   #4
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4-40 is pretty small for engine bolts. You probably torque them up a bit and the vibration finishes the job.

Go to 6-32, use locktite and just do the bolts firm. Should be end of story.
John Kovats

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Old 06-21-2006, 02:54 AM   #5
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Thanks for the input. I'm going to try 4-40 again, but use less torque. Maybe I'll get the wife to tighten them up, she's terrified of wrecking my "fragile" beauties.

To go to 6-32, I'll need to drill out the mounting holes in the engine, and the ply mount is very thin as it is. If the 4-40's break again, I may go so far as to chop off the nose, install an aluminum mount with the iolators on the firewall, and build a cowl. I really like these things, they make the airframe so much quieter.

Thanks for the info about the torque, I think that is the problem.
Michael Gyger
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Old 06-21-2006, 03:36 PM   #6
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My $.02: The comment about the "very thin" ply mount may say it all. Vibration can do wonders, and your one-cylinder engine working with the flexibility of the plywood might have found the weak point in the system--the mounting bolts. You might consider beefing up the plywood mount. Just a thought, Dzl
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