VO wrote:

Paul,
Which rubber mounts I need to use in case 20B for Velocity?
Any suggestions for bolt size as well?
FYI - firewall is playwood with TRIAX on both side and
reinforcement TRIAX points for suggested five points of attachment.
Thanks.
Vlad
N1UT

What is the distance from the bottom mount hole on the firewall to
the top mount hole on the firewall? If it is at least 12 inches
a Barry 22003-14 should work. More is better. Separate these holes as far
apart vertically and horizontally as possible. 24 inches apart vertically
is way better. In that case you can go down to a 22003-13. You might need
to beef up the skin near the top holes with several additional layers
of glass cloth.

Copy Perry's design as close as you can. You will really need the
22003-14 at the 860 pound axial rating. The radial rating is
385 pounds each which is more than enough for a 20B. The fire wall
thickness must be built up to 0.875 inches thickness with glass and
plywood in the area of the mounts. Say three or four inches in diameter.
The center of the mount hole  should be as close to the fuselage skin as
possible. Say not more than about two inches in on the inside.

The bolts are 5/8 with the 22003. You can reduce this to 7/16 as per Bulents
trick of a bushing. Bulent used Delrin but I would use steel. Or you can drill
3/8 holes in the bolts stopping short of the threaded portion if the weight
seems too much. Use these types of fittings shown in the drawing where the
mount goes through the firewall to keep the length of the bolts reasonable.
These are for the 22002 size so you will have to scale them up slightly.

Don't forget the large diameter thick washers on the inside of the firewall.

Paul Lamar

VO wrote:

Paul,
Thank you for explanation.
Just one question about bolts. For IO-540 mounting on
firewall, Velocity supply AN6 type bolts (0.373) which is
smaller then 0.625 bolts diameter which you suggest.
What is right decision? Could you also recommend an
specific type of bolts with around 3.75 in length, which will
be Ok for this application.
Attached Barry picture and my firewall picture.
Thanks.
Vlad
N1UT

When you put the rubber bushings in the firewall the load on the
bolts is different so you need larger bolts. 

I take it there is a third bolt up at the top of the firewall as shown 
by the red circle. In that case you could go down in load rating to 
22003-12 or even a 22003-11 for all  mounts. Beef the inner skins near
the upper two outer circles with several layers of glass cloth.

Connect all the cup weld fittings with tubes as shown. 1/2 dia. 
0.049 inch wall would be fine for this purpose.

Paul Lamar
 
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