Engine Cowl Fitment

Time: 20 Hours

Now that the engine is hung I could start on the engine cowl. The cowl comes it two parts, a top half and a bottom half, both come with extra length so you can get a proper fit to your firewall and spinner. First up in this process is to get the spinner bulkhead set in its proper position. Since I’m not in a position to but my propellor I’m going with the spacer method which simulates the propellor. I made a few calls to Hartzell and Jon Thocker from RedLine Airshows to ask questions and get measurements for the composite blended propellor that I will be using. The all metal prop uses a 2 1/4″ spacer which Jon used for his original prop. After discussions we decided that 2 1/2″ would be better to accommodate the different prop. The composite prop has a wider blade that requires the “G” hub which pushes the blades forward by one inch by a built in extension. That in turn moves the spinner bulkhead forward one inch as well. The difference is the aft lip of the Vans bulked is around 7/16″ deep and the Hartzell bulkhead is 1 1/8″ deep. So with the extra 1/4″ on my spacers and the added depth of the spinner bulkhead will get me to a 1/4″ spacer between the forward edge of the cowl and the aft edge of the spinner. I will have a little wiggle room that can be adjusted with washers when the bulkhead is mounted. To simulate the 1/4″ space I cut a disk out of 1/4″ plywood and bolted it to the bulkhead. I also cut out holes to clear the spacers. This way when I mount the bulkhead and spacers to the engine I can slide the cowl tight to the wood spacer. This will give me a consistent 1/4″ space when I remove the wood.

To support the forward end of the upper cowl I created a couple of threaded rod pushers that I could adjust to get the measurements just right. I used some aluminum angle, threaded rod and nuts. I then screwed them to the engine baffle mounting holes.

Once I had those mounted I bolted the spinner bulkhead/spacers into place.

Before I put the top cowl on I made a 2″ measurement aft of the firewall lip around the perimeter. This will be a reference line to measure the aft edge of the cowl. I will be able to measure forward from this line and mark the cowl to get an accurate cut line. With that marked off I put the cowl in place and got my laser out to line it up with the center of the crankshaft. The horizontal lines will tell me where the sides of the cowl need to be cut.

With the laser lines I was able to run tape along those lines to mark the cut lines. I also marked the aft end with tape as I measured forward 2″.

The only cut I wanted to do now was the aft end. I cut it a little long and sanded with a long sanding block to get a straight edge. To lock the back edge in I started the Skybolt installation. To get the holes drilled I needed to see where they are. I used a flood light to shine up through the flanges and cowling. I then drilled a pilot hole to start up sizing the hole to 15/32″ and keep it centered.

I snapped in one of the grommets to see how it lined up.

With the side lines established I was able to space the next two Skybolt tabs so that the side ones will line up perfectly.

Skybolt gives you temporary rubber grommets to hold the surface mounted grommets in place. I put all of them in place through the holes I just drilled so I could put the cowl back in place to drill the four tabs I just put on. Once the cowl was on I inserted the studs and tightened them to hold it in place.

I’m real happy with how these look and open. I have some filling that’s needed on the under side of the cowl to get it to fit flush with the aluminum. Now that I have the top cowl secured I could remove it and sand the horizontal sidelines down to the blue tape that indicates a level line.

Before I can work on fitting the bottom cowl to the fuselage I needed to meet very forward end of the two Cowls together. This is so I can get the circular portion exactly 13 inches in diameter. You need to slide the two portions together so that the nest well giving you that 13 inch diameter, in order to do this you must cut away the corners slowly as you work to get the 13 inch as well as keep the air inlets equal on both sides. I marked out a small section on both sides to cut away being very conservative.

I continue to work and sand down the corners double checking that I was keeping everything somewhat level. Once I had the diameter at 13 inches and everything nested well I drilled two holes on the inside of the inlets to lock the forward section in place.

Once I got the forward edges lined up and drilled I moved on to riveting the vertical row of SkyBolt tabs onto the firewall.

Now came the time to fit the two cowl half’s together on the fuselage. I used a couple of cinch straps to hold everything in place.

To start with I overlapped the top cowl with the bottom cowl along the sides as it fit better this way. I just needed to get everything lined up around the entire cowl so that I could measure the aft edge for that cut. I used the same process as the top cowl using a 2″ line measured aft from the firewall. Then measuring forward from that line 2″ onto the cowl surface. After the aft edge was cut and sanded I began the process of drilling the SkyBolt holes in the lower cowl. I started with one on each side using their clecko insert that they provided as well as drilling two holes thru the spinner bulkhead/spacer. Once I had the lower cowl locked into position I removed the top cowl to give me better access to drill the remaining holes.

After drilling all the lower cowl holes and inserting the SkyBolts I returned the upper cowl but this time I overlapped the upper cowl sides with the lower cowl sides. The upper cowl had the level line marked with blue tape and was sanded smooth right up to that edge. With the sides overlapping I could use that edge to run a pencil line on the lower cowl by tracing that edge. That would give me my cut line for the lower cowl sides.

So off came the lower cowl again to cut the sides. I used the Dremel tool to get it close to the line and then used my long sanding block to finish it up. After cleaning up all the dust I returned the cowls to check the fit…almost perfect!

Next up was the horizontal side SkyBolt tabs. I New I wanted to do 10 tabs per side so I laid the spacing out so they fit evenly.

I marked the cowl for where the center of each tab should line up so I could return them easily. I then marked the start and ending of the entire row to determine the rivet hole layout. I decided on a 1″ spacing pattern and removed all the tabs after marking where the first aft rivet hole should be. Once all the tabs were removed I drilled all the holes along a line 3/8″ below the cowl edge using my hole layout tool. With all the holes drilled through the cowl I could return each tab one at a time and match drill after clamping them in place. I needed to trim a little off each tab to accommodate the spacing I worked out. I also lowered the tabs from a 1/4″ reveal to a 1/8″ reveal so that I had edge distance for the rivet holes. That little change will throw off my alignment with the vertical tabs by 1/8″ but I don’t think you will notice. If I do I can remove one tab and adjust it later.

The next steps were tho countersink all the rivet holes in the cowl, mark each tab for their location and drill 1/4″ holes in the tables between the rice holes for epoxy/flox to help secure them. Then I riveted them in place so I could start drilling the upper cowl to them.

I then returned the upper cowl and started drilling the holes. The first couple were easy to get to but the next few posed problem as I couldn’t get a light behind them since the engine was in the way. I went back to the technique that I used with the canopy. Drawing two intersecting lines over the center of the hole and copying those lines on the upper cowl to pinpoint the center of the hole.

After drilling all the holes I put in all the parts on the cowl and riveted the barrels on the tabs. Here is what the final product looks like.

The inboard sections on the inlets get a couple of nutplates to hold them together.

So that’s the big step for this cowl…getting it to fit and all the attachment hardware installed. Next up will be the fine tuning of the fit and getting all the gaps to be around 1/16″ to account for the paint. I will also start working on the exterior surface as this thing is rough. The forward edges all have a little mismatch that will need work to get a smooth flow. I also have some work to do on the inside of the cowl including the cooling ramps.

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2 Replies to “Engine Cowl Fitment”

  1. Hi,
    I am thinking about adjusting cowlings to the fuselage of my RV8 .
    Before ordering the prop spacer I need to be sure to order the right one.
    Could you tell me if spacer lugs are threaded or not? It ‘s asked in the order form .
    Your photos and job skill are great ; thanks for the precious informations you gave to me.
    Best regards

    • Hi Dominique,

      I don’t think I will be able to help as I didn’t use a prop spacer in my setup! A call to Vans would probably be the best bet along with the engine manufacturer.


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