Repairing Oversize Camlock holes in an aircraft cowling. by John Rogers & Joe Bachofen

So, what do you do, if you have camlock fasteners that have worked their way through your cowling, leaving an enlarged hole AND no longer hold the cowling in place?

1What do you do if you know nothing about fibreglass repair? Why, you call on a friend who can help you do the repair.  In my case, that friend was Joe Bachofen – an individual who is building a Glasair in his garage. Not only is this friend talented – but he is also educated. To hone his skills, Joe actually went away and took a College course on fibreglass techniques. What more could you want than to have such an acquaintance on hand to assist and teach you in your repair requirements?





2To begin with, the cowling needs to be removed, and thoroughly cleaned. Fibreglass and hydrocarbon residue does not mix. To assist with proper cleaning – and to facilitate repairs, I removed all remaining camlocks. A pair of camlock pliers , purchased from an aircraft parts company, or borrowed from your local AME, can make short work of an otherwise frustrating procedure.





3Once the cowling is cleaned, you can begin to prepare the area around the camlock hole where the repair is required. A piece of carpet on the work area can help prevent scratching of the cowling. On the inside of the cowling, sand an area larger than the size of the largest patch to be used. A piece of cardboard can be used to provide a contrast to assist in grinding, and to protect the work bench.





4Sand and bevel an area on the outside of the cowling, so that the bevelled edge of the hole forms a “ > ” , with an equal bevel on the outside and inside of the hole. Once the patch is in place, this will give a small mechanical structure to assist holding it in place.














6Use wax paper and masking tape to provide a “back stop” on which to lay the fibreglass cloth.


8Cut the  circle of wax paper as small as possible, to allow the masking tape as much physical contact, as close to the hole as possible. Cut several “circles” (or the shape of the repair required) of increasing size out of fibreglass cloth.






9I won’t go into the mixing of the various parts of fibreglass resin we used. That could be a future article written by an individual, knowledgeable in this area. Needless to say, portions need to be prepared and weighed properly, to get the mix required for the repair in question. Once mixed, a timer should be started; this will give you a timed warning, prompting you cease using the resin before it sets up too much.





10Begin with the smallest size piece of fibreglass cloth patch, that will just fit in the prepared camlock hole. Using your finger as a backing on the repair area to prevent pressure from the brush, pushing the wax paper and tape back stop away from the repair area, apply a layer of resin onto the wax paper, and around the tapered hole.






12Use a short bristled brush, and a “stipple” stroke to the top of the cloth, “drawing” the base coat of resin through the cloth layer.

If necessary, apply additional resin to the fibreglass cloth, and stipple stroke it into the cloth, ensuring that the cloth is thoroughly impregnated with the resin. As the resin sets, apply additional of fibreglass cloth circles in subsequent layers. Each circle of cloth should be a little bigger, as you build up layers from small to big, to match the bevel that was created earlier.




14When you reach the depth of layers needed to complete the outside portion of the repair, (The layers should protrude slightly above the elevation of the outside cowling, to facilitate sanding it down to a smooth finish.) and the resin has set sufficiently, remove the tape and the wax paper back stop.





16Begin the inside cowling repair by cutting a number of patches from the fibreglass cloth. Placing the fibreglass cloth on a hardboard backing, a sharp, rotary cutter easily cuts the cloth into strips. (Make sure that you cut the cloth on an “angle” to the fabric’s weave. That way, you do not risk losing a thread along an entire cut – leaving a frayed edge. Each patch should be about ˝ inch larger than the previous piece. Cut a small piece off each corner, leaving an angle instead of a square. (This will reduce the possibility of a corner lifting, once resin is applied.)

The method for applying the patch to the inside of the cowling is similar to that used on the outside of the cowling. EXCEPT, begin with the largest patch first. (By going from large to small, there is less risk of having an air bubble forming in the gap created when a large patch is placed over a small patch. Apply a layer of resin to the entire area to be covered by the first, largest layer of fibreglass cloth. Make sure there are no air bubbles anywhere – especially in the gap between the patch just completed on the outside, and the one that is now underway on the inside of the cowling. As done previously, apply a layer of cloth, and stipple it to draw the resin through the cloth. If required, add some more resin to the top of the cloth layer, and stipple it in, ensuring the entire layer is impregnated with resin. When slightly set, add subsequent, smaller patches until you reach the desired depth of the patch.


17Note how the corners of the patches have been cut off – allowing it to adhere to the lower layer without lifting.







19When the patch has cured sufficiently, (Because we had cool temperatures, I waited four days.) use a fine hack saw to cut off the excess material.  Sand the exterior side of the patch, using a manual or rotary sander. (Pictures #21, #22) Picture #23 shows the patch, with basic sanding completed – ready for drilling the hole for the camlock fitting.






21There are no doubt a number of methods for determining just where to drill the camlock hole, to ensure it ends up in the correct location. I’ll outline a method I found to be simple and workable.






25Install a camlock into the fuselage receptacle. Draw a vertical line through the centre of the camlock, onto the fuselage. Measure and make a number of marks along this line, showing the distance from the centre of the camlock receptacle. To ensure accuracy, identify the measured distance of a few of the cross hatched lines. I identified 1.5 and 2 inches. ) Mount the cowling on the aircraft. Now, draw an extension of the line from the fuselage, onto the cowling, and across the repair. Measure an appropriate distance back from one of the previously identified cross hatched lines. You will now know exactly where the centre of the underlying camlock receptacle is located.



27Use a suitable size hole saw, to drill a hole for the new camlock installation. (Picture #26) If necessary, a rotary rasp can be used to expand the hole slightly to ensure a tight fit.






28Test fit a new camlock in the hole just drilled. Push and turn the camlock into the receptacle, to ensure that the fit is correct.






29If no adjustment is necessary, the camlock can be removed once again, allowing for a final sanding and paint.

When the paint is dry, re-install all remaining camlocks, lock them in place and your repair job is complete.

Enjoy a newly repaired, and tight fitting cowling once again.