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<< Lancair Builders' Mail List >>
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<<Hey, what gives here? I torqued the firewall bolts holding on my engine
mount to the 43-13 guidelines, but I noticed slight dishing of the area
washers. I thought the bolt torque was too tight, but didn't want to go any
less. Any of you experts out there have any suggestions?>>
Rob,
The dishing you are experiencing is due the fire wall material not being able to
support the compressive load imposed by the bolt. At the same time, the area
washer is not stiff enough to adequately distribute the load across enough of
the softer wood/fiber glass below. The first thing I would recommend doing is
to remove the nuts and see if the fiberglass was damaged or is merely following
the new dished contour of the plywood. Any cracking of the epoxy up against the
bolt area is the concern. If the deformation is small, I would recommend
filling the depression with flox and bolting in new area washer with just enough
torque to snug it against the fire wall and squeeze out excess flox. Don't
worry about any release for this washer. There is no reason to remove it later.
Once cured add another area and one regular washer under the nut. This will
help distribute the load over a larger area. Even then, I would not go to full
torque on this installation. This joint will never be able to load the bolts to
the point of needing that much preload. Something like 35 ftlb will do just
fine in terms of holding your engine in, but more importantly will keep from
crushing your fire wall any further.
These bolts must be checked every annual. The plywood can relax over time
resulting in loss of torque and preload. The standard torques listed in 43-13
and elsewhere assume that the item being bolted can withstand the load. This is
not the case for this particular installation and there are plenty of other
examples in our planes where these torques will damage hardware.
Chris Zavatson
N91CZ 360std
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LML website: http://www.olsusa.com/mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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