There have been several notes suggesting a hanger to support the rear of the
muffler on the Mk 1 would reduce the possibility of cracking the J pipes. In
making this hanger I found that it took longer to work out the details than
to actually fabricate it. The result is given here in an attempt to save
others some time.
The material for the hanger was ordered from Aircraft Spruce: 4130 strip, 5/8
by 36 inches, .040 thick, cost 89 cents. Stainless would be preferred, but
wasn't found in the catalog. (File the edges of this strip - they're very
sharp as received.) The other materials were leftovers and scraps found in
the junk box.
The distance between the lower engine mount cross tube and the muffler is
about 12cm on my Mk 1 - adjust the length of the hanger strip to accommodate
production variations.
To make the muffler clamp bend the strip 90 degrees at 3cm from the end. Form
the strip to fit around the rear chamber of the muffler, with the bent end at
top center. Use scraps of 1/8 in thick silicone baffle material to space the
clamp away from the muffler at each side, plus center top and bottom. Mark
the strip so that there is enough material for a matching 3cm 90 degree bend
on the other end with about 1/8 inch space between these 3cm sections. Cut
the strip and bend this end so that it is parallel to the previously bent
end. Drill 2 holes with a #10 bit in these 3cm ends, the first as close as
possible to the band around the muffler while allowing access to the bolt
head for tightening, the second close to the end of the strip, clear of the
first bolt head and with space so the second bolt head doesn't extend beyond
the end of the strip.
Bolt this clamp onto the muffler at about the center of the rear chamber
using the bolt holes closest to the muffler. Include the 1/8 silicone to
provide some cushioning and thermal isolation. Leave an opening of about 2mm
between the 3cm sections.
Construct a large cushioned "P" clamp by bending the remaining section of
strip 100 degrees or so, 2cm from the end. Use a piece of material left over
---From trimming the fuel filler hose; a piece about 3cm wide is fine, cut an
arc of it so that it wraps completely around the lower cross tube of the Mk 1
engine mount -- this forms the cushion. Wrap the strip around this cushion;
this is most easily done using a piece of pipe clamped in a vise since it's
awkward to do in place on the aircraft. Leave about a 1/4 inch space between
the parts of the "P" clamp through which the bolt will pass. Once the "P"
clamp is formed, cut the strip so that the ends match and drill it
approximately centered in the 2cm section, mark the other side to allow the
bolt through and drill it too. Round the corners of the straight section of
the "P" clip to make it easier to insert into the hose (below).
Use a 3 inch long section of 1/2 in id hose - a leftover from the oil hose -
as the vibration isolater. Drill a 1/4 inch hole 1.5cm from the end. Push
this over the straight end of the "P" clamp until the holes line up and push
a # 3 bolt (with washer) through temporarily. Drill the other end of the
hose at 90 degrees to this bolt. Drill the remaining piece of strip 1 cm
---From the end and round the corners; push this into the hose and bolt with a
# 3 -- this strip will line up with the ends of the muffler clamp to allow
through bolting.
Mount the "P" clip in the center of the lower cross tube. Install the muffler
temporarily with the clamp in place. Adjust the position of the clamp fore
or aft to ensure that it clears the rudder cross tubes. Position the
protruding section of strip so that it fits between the 3cm upward pointing
parts of the muffler clamp. The end of the strip must be trimmed to fit so
that it is long enough to allow drilling to match the upper hole in the 3cm
section and short enough so it doesn't hit the lower bolt - a bit awkward
because the end of the strip is in the way, may require a couple of trials to
trim slowly to the right length.
Once the final section of strip is trimmed, with the muffler pushed upward to
its desired position, hold the strip hanger between the 3cm ends and mark
through the upper hole onto the hanger strip and drill the hole. The bolt
through this hole connects the "P" clip/isolater/hanger strip to the muffler
clamp; use a stainless bolt here if possible to simplify future maintenance;
tighten until the strip is securely clamped to the 3cm sections.
Remove the hanger and clamp; paint all steel parts with high temperature
paint, then re-assemble. About 3 inches of the original 36 in strip will be
left over for inclusion in your scrap box.
As usual, there is a question: is the rear muffler temperature low enough
that the silicone will survive or will folded fiberglass cushions be needed
instead?
Comments ?
John A044 Newtown, CT
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