The plans call out for 6061-T6 tubing 1 3/4" x 2 1/4" x 1/8" wall, which is hard to get. The following is a discussion about how to substitute this part.
Steve Mahoney to the mailing list, Wed, 18 Nov 1998:
Johnny,
I have about 750 hours on my BD and at about the 550 hour mark my top
left mount cracked.
I'm referring to the rectangular extrusion where it is cut out for the
bushing. I have a Lyc-0360 with a fixed pitch prop.
As I couldn't find a suitable replacement extrusion, I machined a new C
channel from 2024-t3 I made them quite allot thicker than on the
extrusion, can't remember the exact numbers but they seem to be performing
well. I
replaced both sides of the top mounts.
Steve M.
Bruce Frank to the mailing list, Thu, 09 May 2002:
I don't have a set of firewall drawings for the BD4 so I don't remember
specifically how these pieces fasten to the fuselage, but why is this size
so critical. Seems the only dimension that would be significant is the 1/8"
wall from a strength standpoint. Wouldn't anything that comes close such as
2" X 2.5" X 1/8" wall. Then build the ends on the motor mount
to fit?
As for substituting steel, the dimensions need to be no where close to
equal the strength of the aluminum it would replace. A couple of angle iron
brackets would do the job.
I can imagine no real reason to go to the expense of even a small
production run of aluminum extrusion just to get these numbers. Did Jim
really calculate the exact requirements or did he select something
available at the time that would meet the strength requirements?
Bruce A. Frank
Robert Bollinger to the mailing list, Thu, 9 May 2002:
Bruce is right. I made my engine mounts from 1/8 in. flat 4130 stock and
than used bronze flange bushings for the bolt holes. It is very strong and
only slightly heavier and very cheap.
Robert Bollinger
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