Cowling Countersinks – Lots and Lots Of Countersinks

Yesterday there was a window of opportunity between rain storms so the fuselage was rolled outside to countersink the 150 plus recently drilled #40 holes in the cowling. All but the six holes that were drilled into the front of the cowling require countersinking for AN426AD3 rivets. There is nothing difficult about this task once the countersink cage depth is set. Van’s suggests leaving the rivets a little high (up to .005 thousands) so the fiberglass is not weakened as much … so I set the countersink cage so the rivet was just not quite flush by  a click or two on the cage. It was a little tricky countersinking the sides of the cowling because when pushing the countersink cage onto the cowling, the cowling tended to flex inward … so it was necessary to hold onto an adjacent Cleco to prevent the inward flexing. I was making it a point to make sure Clecos were on either side of the hole being countersunk.

Just beginning the countersinking of cowling for flush rivets … a few holes done and 150 yet to do.

Did not take more photos because it was all the same, but the following are some tips for fellow builders that worked well for me. Reduce the air pressure to the drill so the countersink bit turns slowly … it only takes a couple of bursts of the trigger on the drill to make the countersink in the fiberglass. The countersinks were made with the cowling on the fuselage … which is a good idea because the metal hinge strips will prevent the countersink bit from elongating the holes in the fiberglass (especially with the bit turning slowly). The depth of each countersink was tested with a long AN426AD-3 rivet. Sometimes it was hard to remove the rivet from the hole, so a piece of tape was used to lift it enough so a fingernail could get under the rivet to remove it. There were three or four locations where the outer edge of the countersink cage was sitting on the fuselage skins and the countersinks were not as deep as they needed to be …. rather than reset the cage depth, the hole locations were marked with a red sharpie and the countersinks were made deeper when the cowling was off the fuselage.


One of the things I forgot to do prior to drilling the cowlings was to place paper extending from the firewall to over the engine to catch the debris from drilling. So when the top cowling was removed, it was a mess and necessary to borrow Jan’s portable vacuum cleaner (the one that will pick up a bowling ball) and suck up all the debris. So fellow builders fitting the cowling with the engine installed as I have, do yourself a favor, place paper or plastic sheet over the entire engine area … you will save yourself at least 15 minutes or so of vacuuming fiberglass dust and metal shavings.


After everything was cleaned up around the engine, the F-12115L&R side hinges were deburred and readied for riveting. Ran out of time so will begin riveting the hinges to the cowling sections this afternoon.

Deburring one of the 44 holes in the F-12115L hinge.

While the countersinking and deburring was going on, I was also performing a fuel leak test of sorts. I placed two gallons of fuel into the fuel tank and every couple of hours I turned the tank onto a new side so see if any leaks appear. As mentioned in a previous post, I don’t think the tank will leak  ... but when the tank was pressurized to 24 inches of water for the pressure test, it was around 78 degrees and during the evening the temps went into the 30’s and the next afternoon when I got to the hangar the temps were around 43 degrees. So the gauge was showing pressure, but nowhere near 24 inches of water ... at the time, I placed the tank in front of the kerosene heater and after only a few seconds the pressure began to rise on the gauge leading me to believe the drop was due to temperature fluctuations. However, felt prior to installing the tank in the RV-12, it would be prudent to pour a couple of gallons of fuel into it, hope for the best ... and wait and see what happens.

So far the fuel tank bottom and now front have passed the “drip test”.

The Eagle Grows A Beak!

Yesterday, after borrowing a bunch of Clecos from fellow builder Tom, commenced drilling the upper cowling into the F-12115 side hinges. Prior to drilling, duct tape was used to keep the cowling halves tightly together. Mike K. was available to lend a hand and assumed the role as the “Pusher Man” by gently pushing a block of wood up against the backside of the F-12115 side hinge to keep it pressed snugly against the upper cowling while the #40 holes were being drilled. When near the battery box, we switched to a dowel rod fed through the round hole in the front of the cowling. Our efforts resulted in what I feel is a very nice fit.

While Mike the “Pusher Man” was pushing a piece of dowel rod against the hinge from inside the cowling to keep it snug against the upper cowling as the #40 holes were being  drilled.


After the right F-12115 hinge was drilled, we moved over the left side and repeated the process. Both side hinge seams are nice and snug as can be seen in the photo below of the right side after drilling … (and it is the worst fit of the two). Also pulled out the hinge pins one at a time and they both were  very easy to reinstall

The left side turned out even better than the right side shown in this photo … and this side looks just fine.

After finishing up drilling both sides of the upper cowling, decided to take advantage of the calm non-rainy day and roll the fuselage outside to check out the GPS2020 receiver’s satellite strength. After powering up the SkyView, went and checked the GPS signal strength and it looked great. While the fuselage was outside, decided to sweep the hangar floor which was a long overdue task.

The Eagle basking in the sunlight with its newly fitted beak.

Except for a few screw holes on the bottom of the cowling that have yet to be drilled, all the holes are now drilled. However, there is still a lot of work necessary before the cowlings will be completed … all the hinges need to be riveted in place and the lower cowling requires a little fiberglass work to install an air duct for the oil & coolant radiators. Except for the six holes that were drilled into the front of the cowling, all the remaining #40 holes that were drilled now need to be countersunk for solid flush rivets as called for by Van’s. Hoping the rains forecasted for today are of short duration so the cowling can be countersunk outside to avoid getting fiberglass dust inside the hangar.

Upper and Lower Cowlings Drilled

Yesterday, with the assistance of Mike K., the upper and lower sections of the RV-12’s cowling were drilled. The duct tape that was put on the cowling pieces to hold positioning was still doing its job … in that, there was still a nice snug fit to the fuselage for drilling. Mike helped hold a little pressure while two rivet holes were drilled into the F-1201-J&K hinges (on either side of the access cutout for the upper cowling’s hinge pins) and secured with Clecos per the plans.


At this point a deviation was made from the plans. The plans would have the builder drill the remaining holes into the F-1201-J&K hinges upper cowling first, then drill the holes for the F-12115 side hinges into the upper cowling, and then drill the holes in the lower cowling. However, the decision was made to change that because, in my case, I could tell, that would likely create an issue … in that, the upper cowling slightly overlapped the lower cowling when held down tightly so I decided to change things up and drilled the lower cowling first while it was still positioned nice and snug.


After drilling and inserting Clecos in all the mounting holes for the lower cowling, the rivet holes for the upper cowling were drilled into the F-1201-J&K hinges. This will allow for removing some of the Clecos so the side edge of the cowling can be lifted for a few cycles of sanding until there is a snug fit to the lower cowling without any bulging as there would have been if the plans would have been followed in order.

Mike K. keeping a little aft pressure on the upper cowling while it is being drilled.

Now that both the upper and lower cowling sections have been drilled, it will be easy to remove a few Clecos and lift the edge of the upper cowling up so the side edge can be sanded to eliminate the small overlap. In my case, there was about 3/64" overlap on both sides, mostly at the firewall, that tapered to about perfect near the front of the F-12115 side hinge.

Both cowlings drilled and secured with Clecos to the fuselage. Only the holes for the F-12115 hinges remain to be drilled after a little material is removed from the upper cowling along the line my finger is pointing towards.


Did not complete the task of drilling the F-12115 side hinges for a couple of reasons … Mike had a commitment he needed to get to so he could not wait around while I was sanding down the side edges of the upper cowling. The other reason is … I have run out of #40 Clecos and really do not have enough to drill the side hinges in the cowling. I had extra #40 Clecos but still not enough for my liking (this task takes far more than I thought it would). I reached out to Tom, a fellow builder in the area who is putting together a RV-10. Tom lives a couple of miles from the airport and said it would be no problem to loan me a few handfuls of Clecos … so the plan is to finish drilling the F-12115 side hinges this afternoon.

More Progress On The RV-12’s Cowling

One of the reasons there has been a lull in the activity on the cowling is a replacement piece of hinge material was ordered to remake the F-1201J hinge. As mentioned in a prior post, I had been attempting to install the F-1201J hinge upside down last week and had fluted the hinge material. Once the mistake was caught, the flutes were pounded out and the hinge material was reshaped to conform to the curve of the fuselage skin. I’m sure it would have been OK, but decided to just remake the hinge piece because it just bothered me. Unfortunately, could not move forward until the upper cowling was fitted and that was held up by the hinge.


Once the F-1201J hinge was remade, the upper cowling was set in place so the approximate overall fit could be checked. Once it was determined it would be OK to move forward, two holes were drilled into each side of the lower cowling to allow for Clecos to temporarily hold the lower cowling in place. The red marks placed onto the hinge material to denote the rivet hole locations are hard to see through the fiberglass unless there is plenty of light … so a flashlight was used to make the marks easy to see.

Bernie holding the flashlight so the red sharpie marks on the hinge material denoting the rivet hole locations could be easily seen through the fiberglass. Drilling one of the four #40 holes that will be used to temporarily secure the cowling to the fuselage. After starting a little dimple in the fiberglass, I reached inside to hold the hinge tightly to the cowling while drilling.


Now with the lower cowling secured with two Clecos on either side, the plans instruct the builder to drill the lower cowling for the F-12115 side hinges. The F-12115 hinges were drilled after clamping them to the side of the lower cowling with Cleco clamps. The upper edge of the cutouts on the hinge material is to be aligned with the top edge of the fiberglass as can be seen in the photo below. The previously marked rivet hole locations that were marked onto the hinge material were drilled using a #40 drill bit.

Aligning the top edge of the cutouts on the F-12115-L hinge level with the edge of the lower cowling and securing it with Cleco clamps for drilling.

Drilling through the fiberglass with a #40 drill bit and into the F-12115-L side hinge.

After drilling the lower cowling for the F-12115-L&R side hinges, the lower cowling’s side hinge pins were fit. I made the decision to move the mounting location for the tab on the hinge pin further aft to about 9/16" forward of the F-12115 hinges. Also decided to make the cut out for the hinge now … so the upper half of the cowling was loosely set in place so measurements could be taken for the cut out or “knuckle” as Van’s calls it. I may need to elongate the knuckle a little more, but for now it is fine. This is a little out of order with Van’s plans … but I wanted to verify the clearances between the spinner backing plate and the cowling once again prior to doing any more drilling. Cutting the knuckle now allowed for a round of and final sanding to remove more material from the aft edge of the upper cowling to achieve a snug fit with the fuselage skin.

The cowling's side hinge pin for the left side in the position that will soon become “home”.

After spending quite a bit of time sanding, fitting, sanding and refitting (repeat many times) the fit is pretty good there is a small gap on the left side of the lower cowling that will close up if pushed on gently so will try to have a helper apply a little pressure to snug it up when final drilling the hinges. Below are photos of the fit prior to the final drilling of the cowling pieces which are planned for this afternoon.

Upper cowling left side prior to final drilling … (the right side looks about the same).

Lower cowling left side prior to drilling … this has a little gap which will close up significantly if a little pressure is applied.

Nice gap between the spinner back plate and cowling of approximately 1/8" ... hope it stays that way after final drilling.

Right side of lower cowling prior to drilling.

Bottom Cowling Tweaked For A Better Fit

After getting the fit of the bottom cowling in the ballpark the previous work session wanted to make sure the proper clearances would be achieved when the upper cowling is installed. After cutting the aft edges of the lower cowling to the scribe lines, decided to use duct tape to hold the positioning of the lower cowling so the upper cowling could be mated. This reveled the left side of the lower cowling needed to be made a little shorter to achieve the desired clearance around the spinner back plate. So the aft edges on the left side of the lower cowling received a little more sanding to pull the lower cowling away from the spinner backing plate.


Not sure all this work was necessary, because I have read where builders have not had the optimum clearances between the backing plate and cowling and Van’s provided a spacer plate to pull the backing plate forward to obtain the clearances. However, decided to try making an attempt to just shorten the left side a little so a spacer would not be needed … plus it will remove the slightly askew from center issue as well.

A piece of 1/8" aluminum was placed between the cowling and spinner back plate to use as a visual reference for achieving the desired clearances. I was working by myself so the yellow strap was used to catch the cowling if the duct tape gave way.


After removing some more material from the aft edges of the left side of the lower cowling, it was clamped in place then taped once again with duct tape so the clamps could be removed to test fit the upper cowling.

After removing material from the aft edges of the left cowling, it was taped back in place and the upper cowling was set in place and Clecoed to the bottom cowl for a test fitting. Now there is a good clearance between the spinner backing plate and the cowling. The metal spacer is not being pushed on as it was prior to tweaking the lower cowling.


Looking at the above photo, one can see the upper cowling which was trimmed to the cut line is just a little too long now and will also need to be trimmed back a little. To gain an idea of where to mark the cut, Bernie came up with the idea of placing blue tape on top of the hinge so it would show through the fiberglass allowing for the cut to be marked from above.

Looking closely at the photo, one can see the aft edge of the blue tape through the fiberglass. This is the cut line and was marked with a red sharpie.


This actually worked out quite well and after a little conservative sanding, the cowling was fitting nicely on the right side. The sanding began on the right side because that side started out not needing any sanding for the first inch because the upper cowling is slightly askew due to the left side of the lower cowling being pulled back a little. Ran out of time so will finish the sanding during the next work session.

Lower Cowling Half Sanded & Test Fitted

With the aid of friend Bernie, the lower cowling was held up against the firewall and it was determined that if the aft edges of the cowling were sanded down to the scribe marks, the fit of the cowling should be in the ball park. Prior to test fitting the lower cowling, the spinner back plate was mounted with temporary hardware so a visual could be obtained to check for both centering of the cowling and clearances between the  spinner back plate and the cowling.

The spinner backing plate is temporarily installed with temporary hardware for the test fitting.

Between rain showers, the afternoon was spent hand sanding the last 1/8" of fiberglass down to the edge of the scribe marks on the fiberglass. Of course, there are places where the scribe line is very difficult to see and there was one area on the right side that there appeared to be multiple markings for “the line” so only sanded to the first marks.


After spending the afternoon sanding and sanding, Bernie helped me position the lower cowling half on the RV-12. For the test fitting, the hinge pieces are installed and secured with the hinge pins and the lower cowling section is set in place so the “tab” on the cowling sits flat on the fuselage side skin and the aft portion of the cowling butts up against the firewall and follows the curve of the fuselage. Once in position the lower cowling is secured to the upper hinges with clamps.

The lower cowling is held in place with the C clamp my finger is pointing towards … there is one clamp on either side holding the cowling. The overall fit looks quite good, although there are a couple of places on each side that need a swipe or two of sandpaper to improve the fit. There needs to be at least 1/8" of clearance between the back plate and the cowling. This is a bad camera angle but it appeared that is approximately where it is at.


Just for the heck of it and to check how close the scribe marks were to being correct, the upper cowling was set in place. Even though it was touching the clamps and not fully in position it was possible to discern that if trimmed to about the trim line the overall fit would be seemingly OK.

Bernie giving me a hand in positioning the upper cowling so the scribe marks were approximately at the edge of the fuselage skin. Overall the fit looks great.