Time: 2.5 hrs
I finally finished up the heater box. That took a lot longer than expected, but I think it came out pretty good. The mounting holes lined up well except for the top right one which I had to open up a little so I could get the M3 screw to go in. I also used aluminum rivets on the bottom instead of the stainless steel ones I bought. The thought is that I may need to take the heater box apart to replace the foam or silicone, etc. and the stainless steel rivets are REALLY hard to drill out. Since there are 6 stainless steel M3 screws holding everything together in addition to the rivets the screws would probably out last the rivets in a fire and still hold the box together.
I also did some work looking at the different push/pull cables I have and some firewall pass-thrus that I bought for the cables. The heater cable is a fairly large (0.25″) cable and the standard pass-thru fits well on the cable. The choke cable that I purchased is a bit thinner so I also bought a few reducers that fit around the cable and into the pass-thru. The throttle cable works with the standard pass-thru, but the catch is that it already has cable terminates on it so I’m not able to slide the pass-thru over the end. I found they make a split pass-thru as well and this allows you to put the pass-thru on without having to slide it over the cable end. The other parts (nuts and lock washer) fit fine over the cable end so that’s not a problem.
Some photos of the completed heater box. The base plate, flap, and cable attachment (on the flap) are stainless steel.
I made a gasket out of 0.40″ silicone sheet. I used a single hole punch to make the holes for the rivnut heads and the two rivet heads that I added on the firewall.
The heater box fits well and the gasket seats against the firewall fine. I only had an issue with the top right hole not lining up so I had to slightly elongate the hole so the M3 screw would screw in OK.
I found these pass-thru bushings on Aircraft Spruce. They’re called “Cable Safe” (the standard one) and “Cable Safe II” (the split bushing). The regular one was around $10 and the split one is around $18. I thought these would be nicer to use then the rubber grommets that TAF uses. I had to figure out the combination of fittings though since the push/pull cables I have are all a little different. Above is a split bushing. a standard bushing, and a reducer (in the bag).
The standard fitting just slips over the end of the cable and fits fine on the larger (0.25″ outer diameter) heater box cable I purchased.
The choke cable I bought is a dual cable (because the Rotax 914 has 2 carburetors) and each cable is fairly thin so you can use the standard fitting, but it needs a reducer ring so it fits tighter.
The throttle cables (there are 2… remember 2 carburetors) are large like the heater box cable, but since there are ends already on them I had to use the split bushing. These are the TAF supplied throttle cables. The split bushing is a bit of a pain to get the nut onto because the halves move around and the treads don’t always line up correctly, but with some patience it threads on fine.
Here’s a photo from inside the firewall. This is the right side so there are 3 cables here: the heater box (bottom), Choke (middle), and throttle (top). I got lucky because there was just enough room for the the lock washers to fit. I had to upsize the holes a little bit so that the bushing would go through. BTW The fittings on the far right are -6AN fittings for the fuel send and return lines.
Here’s the firewall so far. All the pass-thrus are done and almost all the rivnuts are installed, except the ones for the fuel pumps. Once I get the overflow bottle I can make the bracket for that and then figure out the final location for the 2 GPS antenna brackets and mount those. I’ll mount those with M3 screws and rivnuts. When that’s done I’ll put on the heat blanket and finish up cutting the holes in that. Then the firewall will be ready for the engine install.