fastener help on new cowl

[iamcamper]

I wasn't happy with the fit of the two piece boot cowl that came with my frame kit, so I'm making a new one. I'm making the new one similar to a 3 piece but with the bottom half as one piece. I used the front edge of the old boot cowl to make the channel that attaches to the firewall for the new cowl. I know it looks kind of half ass-ed because of the over lap but I can live with it. I'd like to use screws so I can remove the cowling if needed. My question is what kind of fasteners to use to have a flush fit along the top cowl where the engine cowl over laps it? Or is a flush fitting screw not needed here? Any help would be great.
Thanks.

 

[kevin]

Use counter sunk machine screws and nut plates. Dimple the boot cowl and the flange on the firewall so it all fits flush. When you fit the top cowl over the boot cowl, you will not have any bumps from the screws. You will have a clean professional look. Nut plating is not cheap and it is time consuming but the end result is worth it.

 

[iamcamper]

Thanks Kevin,
I wasnt sure if I could use countersunk screws with nut plates. What size, 6's or 8's.
T

 

[mike mcs repair]

the top is going to have a thick piece of antichafe on it..... so would you really need to use flush??? I would think not, if the screws were like #6-32 ...........

but not much more work for a flush, but a flush is not as strong and vibration prof wise as a regular hole.......so........................

 

[Steve Pierce]

The Short Wing Pipers attach the bootcowl to the firewall like you are. They use #4 machine screws and #4 flat Tinnerman nuts. I use AN365 nuts. Like Mike says it goes over the anti-chafe material and doesn't even need to be dimpled. I would dimple rather than countersink if you see the need. It is nice to open the bottom up to access the belly. I put nut plates going for and aft and split down the middle.

 

[Darrel Starr]

Another similar way to do it is to use finish washers around the flat head #6 screws. I used an Atlee Dodge SS firewall that resisted dimpling so the finish washers worked well. I used them only where I had to. Just plain fillister head #6 screws were used down the sides except at the lower corners where the lower cowl comes up close to the boot cowl. The finish washers require a pretty good size hole, about 3/8 as I recall. I also put an array of small holes in the boot cowl to allow avionics heat to escape.
Darrel

 

[behindpropellers]

Darrel does nice work!!

I have been thinking about how to make my boot cowl for the J-5. My plan is to rivet it on the top and down the sides. Then put nutplates on the rest so I can open up the bottom of the boot cowl.

Tim

 

[Darrel Starr]

Thanks Tim for the comment. Now here is another idea that I am glad Bob Eckstein and I thought of as he was welding on the fuselage in the early days of restoring N18SY. We were looking at the brake pedal locations and wanted easy access to that area. To get that access, Bob welded on another 3/8 channel framework on the bottom behind the brake pedals. I made two .025 thick alum covers as shown. Already in the first year of flying N18SY we have had the covers off to chase a brake oil leak and to check cable routing - very handy and worth the extra effort.
Darrel

 

[WindOnHisNose]

Hey, Darrel, you might want to photoshop your photos a bit...some of the grease spots and scratches on your airplane sort of destroys the beauty of your worksmanship

Your friend,

Randy

 

[Darrel Starr]

Yuk Yuk Randy, If you come out to ANE tomorrow it may still be covered with bugs and exhaust residue. See you soon.
Darrel

 

[kevin]

I wanted to use #6 machine screws but the dimpled nutplates only came in #8's. Those nut plates were not cheap. I did mine on a -12 so it is a little different than an PA-18. The -12 has the aluminum acess panels that help get to the bungees. I also have the stainless tunnel. It is much easier to do 2 side pieces and then a top saddle as Darrel has done. The smaller parts are easier to work with. I kept mine stock looking. Mike is right, if you use the thick antichafe material, you probably wouldn't see the bumps from the screws but some say that I am a perfectionist and wanted it to be just right. A lot of people rivet these pieces together and you can't tell that rivets are under the antichaffe. I wanted to have access if I needed. I too had to use some of those finishing washers that Darrel mentioned but I was not happy about it. In fact, I was P.O.'d about it but I didn't have a choice. Again, some say that I am too critical. Darrel gave me a lot of useful information before I started.

 

[Steve Pierce]

K1000-6 dimple very easily and are 45 cents each. http://www.aircraftspruce.com/catalog/hapages/anchornuts.php

 

[kevin]

I had to use the K1100's. This is the dimpled screw hole. I dimpled the rivet holes. It turned out very nice.

 

[iamcamper]

Thanks for the info guys. I forgot about the chafe tape so will likely not go with a flush fit.
Now all I have to do if figure out how to get the lower corners to lay flat with the side cheeks.