Cutting a Cowl


1. A mini drill or similar rotary drilling tool complete with cutting bit attachment.

2. Small needle files, fine sandpaper, masking tape and a marker pen.

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3. A face mask. The dust created during the cutting process is unhealthy and can cause serious irritation.  

1. In much the same way as a hacked or bodged cowl, a tidy, cleanly cut example says a lot about the builder. Often, bad workmanship draws the eye quicker than good. Why not take time to produce an example to be proud of? After all, what’s the hurry?

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2. A mini drill is the essential tool here, cordless versions more so. You’ll also need to have a cutting bit and a shaping attachment at the ready, along with some fine grit sandpaper.

3. Think about the holes you’ll need to cut, noting that the sizes and positions of such will differ depending on the type (two- or four-stroke), size and make of engine you’re fitting. Fit the engine to the model noting the correct distance from firewall to prop driver.

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4. Fit the cowl (you may need to remove the engine to do this) and check (by transferring measurements) that the prop driver will clear the cowl nose ring when fitted.

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5. Remove the cowl and refit the engine. Okay, let’s start with the cylinder head. Produce a paper template incorporating a hole that fits neatly (with suitable clearance) over the cylinder head of your model. Secure the template against the side of the fuselage with masking tape.

6. Remove the engine (if necessary), fold back the template and refit the cowl. You can employ several templates at once if you prefer.

7. Using your marker pen or soft pencil, transfer the cylinder head cut-out to the surface of the cowl.

8. Remove the cowl and begin cutting the hole using a multipurpose (Dremel, or similar) cutting bit. Looking much like a drill bit the latter are specifically designed for cutting wood, plastic, laminate, fibreglass, aluminium etc. It’ll go through your cowl like butter so be careful not to slip! 

9. Use the shaping attachment to open out the hole. Again, go carefully – it’s not easy to put material back so support your hand or rest it on a pad if that’s easier. 

10. Smooth the edges of the hole with fine sandpaper. Better accuracy can be achieved here by rolling the sandpaper into a tube or wrapping it around flat balsa to mirror the contours being worked. Refit the cowl from time-to-time trimming and cutting as work proceeds until an accurate fit is achieved.

11. Smaller holes can be drilled and then opened out with needle files. It’s easier to cut the main apertures first so the cowl will fit over the engine, then mark and cut smaller holes for the needle valve etc.

12. The cowl will crack where it touches the engine so the trick is to ensure that no part is in contact. There you go, stand back and admire a cowl you can be proud of. 


Over time the screws that secure the cowl to the mounting blocks on the firewall can work loose in their holes. To guard against this, a drop of thin cyano or PVA white glue can toughen up the hole and help lock the screw thread in situ. Fitting a washer behind the screw head is also a sensible idea to protect the underlying surface of the cowl and prevent the onset of stress cracks.

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