Sonex Engine Cowl Modification

Top and Bottom Split Cowling

I have received many requests for the “how to” on modifying the Sonex cowling, so that the top and bottom halves separate, rather than R and L halves. This modification makes for a very easy removal of the top cowling for preflight, to check or add oil, etc. There are no piano hinges to struggle with - merely turn the fasteners 1/4 turn and off the upper portion of the cowling. BTW, it is mportant to use quality fasteners to hold the two halves of the cowling together. The fasteners that are called out on the plans will not do a good enough job for this type of installation. I really recommend this modification.

Here are step-by-step procedure for modifying the cowling, along with some crude diagrams. It is rather straight forward even though there are a lot of steps. If you have any questions, or don’t understand my explanation, let me know and I’ll answer as best I can. If you develop a better or easier method, let me (and/or) the ASA know. Rod Panzer rbpanzer@ips.net (510) 758-6532

1. Fit the R and L halves per plans, be sure to leave about 1/4 inch between the cowling and the prop.

2. Place piano hinges on sides and across bottom back of cowling. Do not place the midline hinges.

3. If you already have your midline hinges in place, leave on temporarily.

4. If you haven't placed the midline hinges, fabricate four straps of .060 sheet (about 2”x 6”), to cleco across the midline to temporarily hold the halves together. Bend these straps to fit the curves of the cowl. (See diag. #1)

5.When you are satisfied with the cowling fit, remove the straps, and remove the cowling from the plane.

6. Re-assemble the R and L halves, using the clecoed straps. Keep the cowl as near the original shape as possible. i.e the contours across the midline are the normal curve.

7. If you DON’T have the midline hinges in place, keep the overall shape of the cowling as near “normal” as possible and fiberglass the two halves together ON THE INSIDE ONLY. Use three layers of 4 inch fiberglass tape. If you are building the tri-gear, you do NOT need to fiberglass the midline aft the hole for the landing gear. If you already have the midline hinges in place, you can leave them in, and fiberglass over, or fiberglass the outside first, and then remove the hinges before fiber-glassing the inside.

8. When the fiberglass hardens, remove the straps and clecos

9. Cover the outside of the seam with a 2inch fiberglass tape, followed by a layer of 4 inch tape.

10. Draw a line on the outside of the cowling, from just above the side hinges to the middle of the intake opening, and from the middle of the other side of the intake to the middle of the prop shaft hole. (see diag. #1)

11. Shine a light from the outside. The line should show clearly on the inside. Re-draw the line on the inside.

12. Place TWO layers of 2” masking tape ON and ABOVE the line on the inside, from the hinge area to the intake opening. From the intake opening to the prop hole, place the tape ON and BELOW the line. (see diag. #2)

13. For the sides, layer 4 or 5 layers of 4 inch fiberglass tape below and onto the masking tape (DO NOT go past the top of the tape). In front, place the tape ABOVE and onto the masking tape.

14. When all is set, use a large diameter, thin, Dremmel disc, CAREFULLY cut on the outside line just down to the masking tape. (see diag. #3)

15. The top and bottom halves should now separate (yeah, it’s scary!)

16. Cut out the exhaust opening in the bottom rear of the cowling. IF YOU HAVE A TRIGEAR, make a cut per diag. #4. Fabricate a plate with the exhaust deflector included, per the diagram. Attach with Skybolt fasteners. This is so you can get the bottom half off past the forward nose gear.

17. Finish all the fiberglass and paint the cowling. Add Skybolt fasteners every 5 - 6 inches. Do NOT use Cam-Lok or cheap fasteners, they are a pain. Skybolt fasteners are quality and their receptacles are adjustable for different thickness of fiberglass.

Skybolt part numbers

Receptacles	SK213-28 2000 series	buy about 26
Studs	SK28S3-3S Wide Flange	8
Studs	SK28S3-4S Wide Flange	10
Stud	SK28S3-6S Wide Flange	8
Retainer rings	SK2600-LWS series retaining rings	30
Fastener tool	SK-T26 1/4 turn Fastener Tool	1

See their web site at Skybolt.com