The PastoMount
Revised
May, 2004 - Gene Maurice, Plano, TX, made his version of this type of
mount but makes the ply rear face a square ply plate. The rear disc is
glued to the center of that square. I adapted his idea to my newest mount,
made for an OS 1.60 and intended for an Aries. Here's the diagram
The mount has undergone many variations through the years, most notably recently substituting 3/8" balsa sheet instead of endgrain for the balsa pucks. Cut out of a single sheet, it provides the resistance needed to the rubber bands, and since the pucks are not in compression fore-aft, it works just fine. This one change simplifies the construction of the motor mount by quite a lot.
I also have stopped using the 1/64" ply for facing, and rather, just sand as smooth as possible, and iron on a round patch of Monokote, making sure it is WELL adhered around all edges. With two pieces of Monokote facing each other, it's pretty slippery!
If you have the firewall space, there is no doubt that the easier to build version is the square ply rear face plate, mounted on four corners. This adds a feature of significantly improving the load distribution toward the outside of the firewall/fuselage, and permitting much easier and accurate thrust adjustments in smaller increments.
Revised October,
2003 - based
on feedback, and observing others who have built these mounts, I changed the
motor mounts themselves to a different brand/style to make installation
easier. Also modified the firewall mounting to use 4 main mounting
bolts. See explanation 
This version of a rotational damping mount is made from basic materials meeting the need for it to be inexpensive. And fully capable. Long lasting, with excellent vibration damping. Early versions of this are in use with literally thousands of flights, using large-propped YS 1.4's, and Supertigre 2300's. As the article says, the key here is the NOSE RING.
balsa stretches the inner tube pieces
more or less, providing more or less resistance to rotation. I recommend 3" balsa discs to start for 1.4cu.in.
2 or 4 stroke motors. Cut the blocks slightly oversize to allow final sanding
down to a constant O.D. Click on the thumbnail for a drawing of the parts. Discs
are shown glued to the front and rear ply plates, two pieces of innertube shown.
2. Cut the two balsa 3/8" slices from the block. THESE MUST BE PARALLEL FACED. **ABSOLUTELY CRITICAL** (I cut the discs slightly thicker, then face on a belt sander).
3. Glue 1/64 ply to one side of the balsa discs, using thick CA. These must NOT extend beyond the diameter of the balsa discs.
4. Drill a center hole through both discs/ply assemblies that is snug for a 6-32 bolt. Bolt them together with a long 6-32 bolt, chuck them in a drill press, hold sanding block against it to get them perfectly round. Messy, but it sure works! Makes 'em smooth and ROUND!!
5. Add 1/4" to the finished dimension of the balsa discs. This is the OD of the front and rear hard ply plates. Make two hard ply discs this diameter. Do same center hole, drill press sanding like the balsa discs. The two ply plates MUST end up being 1/8" GREATER around the balsa discs. VERY IMPORTANT!!!
6. IMPORTANT step. Using PLENTY of ventilation, apply thin CA to all faces of the balsa discs and thin ply. Let cure COMPLETELY before finish sanding smooth. Don't omit this step, as the hardening of the balsa is very important.
7. Cut FS60 Mount in two pieces. Sand rear of mount SQUARE to the beams. Dave Brown mounts have a slight angled rear face taper for tension against the motor crankcase. Not needed in this application. (You may choose to use the Sig 1.2 glass mounts; they work fine, also.)
8. Layout holes on FRONT ply plate, front face first, centering beams on center of mount based on width of motor used. Mark the FRONT ply plate face for the mount screw holes USING THE MOUNTS. There IS slight variation in the mount hole locations from batches of mounts. Later, you'll drill these to accept the 4-40 blind nuts.
9. Layout hole spacing for mount to firewall. Mark these holes on FRONT Ply plate. They are drilled later for head clearance of the mount screws which mount the whole thing to the firewall.
10. BOLT ALL THE discs together using the center hole with a long 6-32 bolt. Using a drill press, or other accurate vertical drilling device, drill holes in the front ply plate face ONLY for the motor mount screws marked earlier. Drill the three firewall mounting holes all the way through the entire discs assembly. Right now, drill only 1/16" holes as pilot holes. There should be 7 holes when you are done.
11. Remove the REAR hard ply disc. Drill THIS plate with CLEARANCE HOLES for the 4-40 screws. "DO NOT" oversize these holes!!!
12. Bolt the balsa discs and the front ply plate together, aligning all the pilot holes you drilled before. NOW, Drill all the way through these THREE holes with a 5/16" drill. These holes permit clearance for the heads of the 4-40 screws which attach the mount to the firewall by holding the REAR ply plate ONLY. (That is why the REAR ply plate has clearance holes ONLY for the 4-40 screws.)
13. Enlarge the holes for the motor mounts on the FRONT ply plate, and INSTALL the blind nuts from the rear. Use the motor mount pieces to pull the nuts snug. CA rear of blind nuts in place. Relieve the FRONT balsa disc where the blind nuts contact it as necessary for the blind nuts so the FRONT ply plate will lie perfectly flat.
14. Align the holes of the front balsa disc with the front ply plate. The center hole, and the relieved areas for the blind nuts should permit this. Use a pencil and mark a line for reference, then carefully thick CA the rear of the front ply plate to the 1/4" front face of the balsa disc (not the side of the disc with the plywood glued on!!!)
[The PastoMount is held to the firewall by the three screws holding the REAR ply plate up against the firewall. The front balsa disc and ply plate/motor beams assembly are held only by the rubber donuts which you will make shortly.]
15. Carefully center and align the SMALL holes in the REAR plywood plate, and head clearance holes in the rear balsa discs. Keeping the discs bolted together here really helps!!! Using thick CA, glue the rear balsa disc (the balsa side), to the FRONT of the rear ply plate. Measure the dimension between the rear of the front ply face, and front of the rear ply face and make a note of this. Check that this dimension is the same at various places around the circumference.
16. CAREFULLY, and PRECISELY cut TWO 'donuts' from the motorcycle inner tube using the measurement from 15, PLUS a small "guess factor" to allow for the shrinking which occurs in the WIDTH, when stretching the DIAMETER to fit the balsa discs. Thoroughly CLEAN the rubber with alcohol. Edges need to be even, smooth, and square. This edge is the bonding edge of the whole assembly. It literally holds the motor in the airframe!!!!
17. You need patience and three hands from here on. Bolt the front and rear assemblies so that the 64th ply faces touch, and the firewall mounting holes align. While holding this, "roll" on ONE of the rubber donuts. BE CERTAIN THE EDGES OF THE RUBBER DONUTS ARE SNUG AND GAPLESS IN CONTACT WITH THE PLYWOOD FACES!!!! VITALLY IMPORTANT - CAN'T BE STRESSED ENOUGH >>> GOTTA HAVE IT THIS WAY >>>HAS TO BE "RIGHT">>>> (capishe?)
18. Leave the assemblies bolted together using that bolt through the center hole. Rotate the assemblies as needed to adjust and align the firewall mounting holes all the way from the front ply plate to the rear. Then, USING THIN CA, glue the edges of the FIRST rubber donuts to the ply plate faces on both assemblies. You'll have a rear ply/balsa disc assembly with three clearance holes for 4-40 screws held to a FRONT ply/balsa disc assembly by a rubber "donut", glued at the front and rear edges, all around the circumference of the plates. Give this chunk some CA cure time, maybe ten minutes to be safe. Then, stretch and install ("roll") the SECOND rubber donut on top of the first, and thin CA this one to the ply plates (edges ONLY) just like the first piece. Don't use excessive CA, as it can run in between the donuts, causing the mount to be stiff, and perhaps not rotate. DO NOT GLUE THIS DONUT TO THE FIRST !!!!!
19. That's it. Attach the motor mount beams to the front ply plate. Use 4-40 socket head cap screws only long enough to go through the blind nuts. No more. Put steel washers which will fit inside the larger head clearance holes for the firewall screws on 4-40 screws. Carefully apply CA to the washer, then drop the screw through the front of the mount so that the washer gets glued to the front face of the REAR ply plate. Do this for each firewall mounting hole, then you will mount the Pastomount to the firewall, capturing the rear plate against the firewall. The rubber donuts hold the front assy to the rear, and actually provides the damping and securing of the motor. Under thrust, the mount pulls forward slightly, separating the inner 1/64th ply faces, so there is actually NO contact between the front assy that has the motor and the rear assembly that is screwed to the firewall.
20. YOU MUST USE A NOSE RING. Buy one, or make one, BUT USE ONE!!!! The thrust changes are dramatic without it. If you insist on NOT using a nose ring, make one anyway, because after you observe the "wobble" of the spinner ring, you'll decide otherwise. To mount the Pastomount to the firewall, locate the center hole on the firewall as needed for the thrust line desired. Put a long machine screw through the Pastomount and hold it in place against the firewall using the center hole for axis and alignment. Mark the firewall through the Pastomount firewall holes, then drill and insert three 4-40 steel blind nuts from the rear of the firewall. Any thrust offsets can be accomplished by oversizing the firewall holes, installing the blind nuts on a ply "Y" plate, then moving the mount around as needed. THE "Y" plate does NOT need to be glued to the rear of the firewall. Trust me on this. Use hard ply, tighten screws well, apply Loctite to rear of screws. It WILL NOT move in normal use. Reinstall the the 4-40's for the firewall mounting, tighten.
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