ENGINE FITTING AND COWL CUTTING

How many times, despite all your careful measuring and trial fitting, have you had the spinner not quite line up with the cowl or the holes cut for the cylinder head or exhaust not be where you expected.

Bl**dy frustrating, isn't it?

What follows is as fool proof a method as I've come across.

Materials required

• Ply, no thinner than1/4 (6mm)
• Ply, between 1-3mm
• Scrap balsa
• 8 or10mm nut and bolt
• 4 self tappers long enough to go through the engine mount plus 5mm.
• 2 penny washers to fit nut and bolt
• CD

1. Cut the ¼ ply to the same size as the firewall and cut a hole to match where the fuel pipes pass through the firewall.

2. Cut a circle of the 1-3mm ply and glue it to the CD (photo) this will be used to set the distance between the spinner back plate and the cowl. [don't throw this away, you can use it on future projects]

3. Using the self tappers, screw the engine mount to the 1/4 ply plate you made earlier, with the mount as close to central as possible and cut off any excess ply to save weight.

4. Using the measurements from the firewall to the front face of the prop driver provided in your instructions/plan, cut two lengths of scrap balsa to that size MINUS the thickness of the false firewall.

5. Glue the scrap balsa to the false firewall so it runs along the engine bearer (photo) and stick some masking tape to the face of the bearers that will be in contact with the engine mounting lugs.

 

6. Attach the CD to the engine with the ply side towards the engine if the plans show distance to cowl face, ply side away from the engine if they show distance to prop driver. (I've found that a broken prop with the blades cut off makes an ideal spacer.)

7. With the ply plate resting flat on the bench, slide the engine down until CD is resting on the scrap balsa spacers.

8. The engine is now the perfect distance from the firewall and you can push a pencil or scribe through the bolt holes on the engine lugs ensuring you mark the masking tape.

9. You have now identified the correct positions to drill the bearers.

Your engine is now the perfect distance from the firewall. The next stage is to cut the cowl.

1. Remove the engine from the engine bearers and using the (wing)nut, penny washers and bolt, fix the false firewall to the true firewall with the nut to the inside of the fus.

2. Temporarily reattach the engine.

3. cut strips of card long enough to stretch from the fus and cover anything that will protrude through the cowl(cylinder head, exhaust, needle valve, remote filler/glow, etc) and cut holes in the card to match the profile of the protrusion. (photo)

 

4. With the card positioned so that the appropriate engine part is protruding through the hole you cut, tape the other end of the card to the fus with sticky tape and repeat for all protrusions.

5. Bend the card out of the way and remove the engine.

6. Fit the cowl and bend the card back over the cowl and mark the outline of the hole onto the cowl.

7. You now have all the cut marks on the cowl. Carefully cut these out using a dremmel or chain drill & file. (chain drilling will be explained in a future episode). Remember, cut a little and measure a lot. You can always remove more but you can't put it back.

8. Replace the engine and trial fit the cowl. You should aim to have 1-2mm clearance around the cylinder head & exhaust and some fine fettling will be required to get a perfect fit.

9. Use this method for any and all access holes in the cowl.

The final stage is to align the centre line of the engine with the centre line of the cowl, easy in two dimensions, not so easy in three, however this method addresses that problem.

1. With the engine & cowl fitted, slacken off the nut holding the false firewall to the true firewall just enough to allow you to move the false firewall against the true firewall with a good friction fit.
2. Fit the prop and spinner, then slide them around until the back plate of the spinner lines up all round the front face of the cowl
3. Tighten the nut to lock the false firewall rigidly in place, and taking care not to move the firewall/engine mount, remove the engine.
4. Remove one of the self tappers securing the engine mount to the false firewall and using the mounting hole on the engine mount as a guide, carefully drill through the false and true firewalls to accept a securing bolt and T nut, repeating this process for each of the securing points.
5. If the engine mount interferes with the drill and you can't line it up square to the face of the firewall, use a pencil or scribe to mark the drilling points.

With everything securely fixed, you can remove the nut, bolt and washers, happy in the knowledge that everything is lined up perfectly.

©2007 BUFF