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Robert Williams told me to pick out the insulation in the firewall blanket behind the engine mount flanges because it will deteriorate over time. He said to make small cuts around the bolt hole and pick out the insulation, but leave the silver outer skin intact. Unfortunately, he told me AFTER I was done with the engine and cowl installation. I checked the bolts at every condition inspection and had to tighten them the first couple years as the insulation in the blanket broke down. I got to the point where I needed an extra washer because the nut was bottoming out on the grip area of the bolt. If the nut is tight and the bolt still turns easily, you've definitely bottomed out on the threads and need to add a washer or two. It's a little tough to count threads on a finished airplane! I had an engine mount crack a couple years ago and went ahead and removed the insulation from the blanket while the mount was off the airplane. I put an area washer between the mount and the firewall for spacing so the cowl would still fit correctly.
I remember Lancair giving me torque values for the engine mount bolts (engine to mount). I don't remember whether Lancair gave me a torque value for the mount to firewall bolts in the build manual or I used a standard torque value for that bolt size. It was a bit of a guessing game since the nuts are on the inside where you could never get a torque wrench after the plane is finished. I used the torque wrench on the bolt head and held the nut behind the instrument panel when I torqued the bolts. I estimated how many foot pounds were from the friction of the bolt passing through the mount and firewall, and added that to the torque wrench value.
I don't think there's an issue with crushing the firewall since the core has been removed and those mounting points are reinforced pretty heavily. As a general rule, use standard torque values unless Lancair specifically gives you a number.
Mike Easley
Colorado Springs
In a message dated 12/13/09 15:37:54 Mountain Standard Time, steve@oasissolutions.com writes:
I would contact Lancair. If I remember poorly, the torque required is lower than the bolt will take. You may crush the firewall.
Steve
Michael,
The bolt should be torqued to a value specified for the bolt size. Thus, the preload will be correct for the specified bolt. While your mechanic may be right about the blanket compressing, there are other considerations. Heated Fiber glass and carbon fiber can flow a bit over time - this can cause some loosening and these bolts should be checked at each annual condition inspection. Another problem could be if more three threads show outside the nut - it is possible to torque the nut onto the bolt shoulder at the end of the threads so that it cannot tighten the parts it is fastening any more.
Scott Krueger
In a message dated 12/11/2009 12:42:03 P.M. Central Standard Time, mnewman@dragonnorth.com writes:
The engine mount bolts on my IV-P (through the firewall) spin fairly freely when I turn them with a socket and my fingers.
Does anyone have the proper torque values for these bolts?
Any ideas why they might have loosened up? I have a mechanic saying that the fireproof blanket compresses over time and that this is why they loosen.
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