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Nipper firewall

Posted: Sun Jun 13, 2021 5:32 pm
by JimCrawford
Good Evening All,

Anybody got a copy of a drawing of the firewall? Preferably dimensioned and scanned or in electronic format?

I've just abandoned trying to repair the one on my Nipper and bought some 0.5mm steel sheet to make a new one. It will be easier than all the patching up required on an already patched (badly) unit.


Re: Nipper firewall

Posted: Wed Jun 16, 2021 9:06 am
by Paul1954
Hi Jim,

I don’t have a drawing but in the past I have one. Ex RAF tin-basher so it was no biggie for me.

Use gripper pins or bolts to secure the old firewall to your new material by drilling the 2BA size holes through the bracket points. Then mark around the firewall perimeter using a pencil (use of masking tape on the new material will help see you see the line). Next add the allowance for bend and lap area. Cut to size and trim edges. Mark/drill any other holes you require. Easiest to make the required edge bends only after drilling and cutting all other required holes.

I think I used galvanised mild steel as it was the spec as my original one.

Re: Nipper firewall

Posted: Fri Jun 25, 2021 11:31 am
by JimCrawford
Thanks Paul,

I've reverse engineered my firewall to create a drawing which I believe is pretty well correct. I am going to print it out full size (A1 at the local print shop) and photo-mount it onto the new material. That should get all the fixing holes in the right place. Then I'll sandwich it between ply sheets and form the upstands round the edge. As an ex-RAF tin basher perhaps you could shed some light on this.
I originally thought that I could accommodate the excess material at the upstand curved edges by fluting the edges but it occurs to me that if the flutes are made into the circumference of the firewall then they will interfere with the forward lip of the fuel tank. If they go outwards then they could interfere with the cowling. I guess to replicate the plane flange of the original you have to shrink the edge of the flange as it goes down.
Am I correct and stating the bleeding obvious or have I misunderstood and barking up the wrong tree?

One of the posts on this site shows a firewall with a made up flange added to a plain sheet. The flange being fluted on the firewall face not on the circumference edge, so the upstand is plain without flutes.

I've used plain mild steel as I couldn't find galvanised at this thickness and I kept away from stainless as I could see problems with forming the edge. I'll etch prime and paint the mild steel.



Re: Nipper firewall

Posted: Sun Jun 27, 2021 1:18 pm
by Paul1954

Bending metal is not too difficult when it’s a straight bend, but bends on a radius you really need have access to a press and facilities to heat the material. However, for a cold bend, you should use the correct bend allowance for the material - preferably more than the minimum. To lose the excess material you will require a sheet metal shrinker of some kind. There is an alternative method, but it won’t suit a firewall. ISTR that the bends are not all 90*, have a look at the area that encompasses the fuel tank.

While I applaud you faith in measuring and drawing, personally I would make use of the original to place the key mounting holes. You could also use the original to cut the ply you are going to use as a former. Paint - I was advised to use an Intumescent paint on the firewall.

I had the use of tools c/o BAe Prestwick which did make things easy.

Re: Nipper firewall

Posted: Mon Jun 28, 2021 9:11 pm
by JimCrawford
Good Evening Paul,

This could get a little tricky with the ply patterns and hand bashing but at least the base material is cheap enough. I've managed to elevate myself to a sufficient level of competence with CAD to be able to draw up parts to print out full size and compare against the original. I've had success with this technique and, compared to the rather dodgy state of my original firewall, I think it should work to allow me to build a copy. Time to give it a go. I like your suggestion of using intumescent paint, I shall certainly follow that up.

I do appreciate your comment about having access to professional tools at your place of work to ease the job. During my working life I've had access to university workshops and also facilities at one of Auntie Betty's research establishments. Whilst I've not been able to operate some of the more sophisticated machines I've found that the best possible tool has been a bottle of the finest or a slab of beer which seemed to get the job done really very quickly.
I think my finest hour was when a close friend and colleague drew me aside one day to complain that he couldn't get parts made for his experiment on an international research satellite because all three available workshops were building parts for my Taylor Monoplane. Result!!! :lol:



Re: Nipper firewall

Posted: Thu Jan 06, 2022 3:18 pm
by JimCrawford
I've come up against a head-scratching moment in my firewall replacement, best illustrated by the attached pictures:

1. The clip from a Mk3 drawing shows all the attachment points for the cowling to be protruding from the firewall, this would result in the aft lip of the top cowling sitting with a gap (~10mm?) above the top edge of the firewall and also the front of the fuel tank.
2. This photo shows that the two attachments on the top of my old firewall to be flush with the edge and so the cowl would sit directly on the front of the tank.
3. If I fully assemble top and bottom cowls except for the two top fasteners, the top cowl naturally sits ~10mm high above the firewall and requires quite a push with associated flexing to get the fasteners to engage.

The old firewall is definitely a Nipper unit as it has the details shown on the drawing, along with quite a few horrendous mods. Witness marks demonstrate that the top attachments have always been in the location shown, although they may not be original Nipper units.

I'd appreciate any information you can give me about how the top cowl should be positioned. In particular a photo would be useful. It's not a detail that's usually spotted in casual shots.

firewall.jpg (82.86 KiB) Viewed 75 times

FirewallTopEdge.jpg (188.27 KiB) Viewed 75 times

Cowling.jpg (72.47 KiB) Viewed 75 times

Best wishes for 2022 for all Nipper fans