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I'm about to drill holes thru my firewall, specifically for the adjustable
rudder pedal assembly, rudder cable eyebolt, rudder pedal return spring
bracket.  I'm sure there will be more holes later, for oil coolers,
gascolators, etc, etc, etc.

My firewall presently has the three layers of fiberfrax and the stainless
steel face sheet in place.  I cut holes in the fiberfrax under the engine
mount pads and have area washers between the stainless and the plywood, i.e.
there is no fiberfrax between the engine mount pad and the plywood.

My concern today is for the smaller bolts (i.e. the non-engine-mount-bolts)
loosening over time as the fiberfrax compresses (what the material scientists
would call 'creep' I suppose, although I know it ain't quite the same).  It
would be nice to have area washers in place of the fiberfrax wherever there
is a bolt, but as Orin says, "you can go nuts in there...", which was his way
of saying that my concern is unwarranted and if I want to fly before I die of
old age (I'm only 43 now) I'd better just throw the bolt in, put an area
washer between the bolt head and the stainless, crank it down to compress the
fiberfrax, and move on.

I'd like to take his advice and ignore this potential non-issue, but I'd like
to ask y'all how these attachments hold up in service.  I know the "Grand
Champion" route is to yank the engine when all my holes are drilled, trim
fiberfrax circles, and replace with area washers, and reinstall firewall
forward, but I'd prefer feedback from you Lancair flyers that says it's not
needed.

Please advise.

- Rob Wolf


[The trickest (?) solution that has been offered here is in the archives
somewhere, but in a nutshell one of our illustrious 360 builders chose
to mockup the entire firewall with a sheet of 1/4" plexi.  He located
everything that needed to go on either side of the firewall on that plexi
panel and once it was completed he transferred all the holes to the actual
firewall, cutting out the fibrefrax for phenolic hardpoints wherever
necessary.  If you look on the LML website in the Engine, Prop, Cowling
area you'll find some nice photos courtesy of Chris Zavatson showing his
approach.  He may even be the guy with the plexi trick.

There was one instance I know of where hard points weren't provided under
the engine mount points.  Even with the proper torque on the bolts the
fibrefrax hadn't compressed completely until the pilot pulled a few
G's during a maneuver and the whole engine sagged about 1/2" at the
spinner.

I'm putting phenolic hardpoints under every bolt so I don't dent up my
pretty engine-turned stainless sheet when I start tightening everything up.
Then again, there's the process that Orin recommends, and it'll work but
your stainless panel will have some serious dimpling before it's all
said and done.  The stuff is going to compress, and the heavier the
object the higher the potential for loosening bolts when it compresses
under some G-loads.  I guess that in the end it's your call as you'll be
the person who gets to live with the consequences.  I hope this helps you
formulate a reasonable approach.

  <Marv>            ]


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LML website:   http://www.olsusa.com/Users/Mkaye/maillist.html
Builders' Bookstore:   http://www.buildersbooks.com/lancair

Please send your photos and drawings to marvkaye@olsusa.com.
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