Cutting A Firewall

[James40]

Hey folks,

 

I've got to the stage of fitting a Saito FG-20 to my H9 P-51.  I fitted the engine and made a card replica of the cowl to mark up the real thing ready for cutting.

 

What worries me is the gap between the cowl and the spinner. This kit was designed to run the FG-20, yet whan it's all offered up, there is an ugly 9mm gap between cowl and spinner.

 

The carb on the engine is flush against the firewall but I still have to move this forward 2mm to allow a clearance gap for a breather hole on the carb, that will give me an 11mm gap that will look a little daft on a scale model.

 

Is it safe to cut a square hole in the fire wall to allow the carb to protrude inside by around 5 to 6mm to reduce this unsightly gap ? The hole will be aprox 20mm x 20mm.

 

I have the option of fuel proofing a 3mm piece of ply and sticking it on the inside of the firewall but this will still have to have a hole drilled through it to allow the carb breather to function.

 

What's the best options do you think ?

[Martin Harris - Moderator]

First thought if the model was desgned around the engine being used is whether the spinner was also supplied or if it is intended to use a special one?

 

A round hole would be a better option as it reduces stress raisers - alternatively round the corners off with a reasonable radius - although ply would be relatively unaffected.  You should create a recessed box to prevent oil getting into the structure. Without knowing the model, I wouldn't be able to comment on the effect on the fuel tank mounting, control runs etc. but I would not imagine such a hole would have any serious implications on the firewall strength unless the hole is less than 1/2 an inch or so from the engine mount fixing locations - most ARTFs have similar sized holes for the fuel tank neck...

[A.A. Barry]

James, like Martin, not knowing the model, why not just add 4//8 mm on the nose cone (false spinner ) to take up the gap, a gap there may also help with cooling also

A.A.Barry

[James40]

The spinner came with the kit, it's an alloy disc with a plastic cone so unless I buy a new round section of alloy and turn it to the correct size and shape on the lathe then it won't cure the gap problem.

 

I think cutting the firewall is the simplest option though so I'll look at cutting a rounded off square hole, a round hole would be much too large and there is already a round hole where the fuel tank passes through the firewall.

[Andy Gates]

How about making up a wooden washer to fit on the front of the cowl to hide the gap?

 

Suitable wood? Nice piece of MDF.

 

Draw a circle slightly oversize with compasses.

Rough cut it out.

Drill through the center and fit a bolt though with 2 nuts to lock it all together.

Now spin the assembly in a firmly held drill (wearing suitable PPE of course).

Now with a file or sanding pad, gently sand smooth and down to size.

 

Once at the right size, a fret or coping saw can be used to cut the center away so the additional weight is negligable.

 

Fasten onto  the front of the cowl - job done.

Edited By Andy Gates on 28/02/2010 20:06:45