Firewall Grommet and Hose Shields

I wanted all the cables, wires, and hoses passing through the firewall to go through rubber grommets.  The grommets seal each item so air and oil from the engine compartment do not easily pass into the cockpit.  Because they are made of rubber they are not fire proof.  A grommet shield is like a metal top hat which covers the grommet to protect it in case of a fire.  Some like this one are simple covers made from one piece of galvanized steel.  These work great for a wire to pass through before adding a connector on the end.  For most things like the tachometer cable, the shield has to be made from 2 overlapping pieces since the nut on the end is so big the hole would never protect the grommet.

You can buy these but they are not cheap and they are easy to make from scrap metal.  Because of the spacing of some holes in the old firewall I needed some non-standard ones anyway.
We need a punch to drive the top of the hat into the die.  For the small (3/4" dia.) grommets I bought a short 3/4" bolt and sawed off the threads.  You need to support the threads to get a clean square cut.   a piece of 3/16" steel worked fine for this.
I also used a piece of 3/16" steel to support the bolt while grinding the end square on the belt sander.

I only need a small end of the bolt to make the pocket for the grommet so I used a stack of washers to give something to hold while hammering the bolt and to control how deep the packet would be.  The bolt can be ground to whatever length needed.  The pocket being a little extra deep won't hurt.  The washers also help flatten the metal around the pocket, at the bottom of the stroke, since the sheet steel is not being clamped and it will wrinkle a little.

To put the radius on the corner of the punch (bolt) I used a rubber cup to hold it and a nut for a spacer.  A socket wrench and magnet would have worked well also.  You gust need to hold it while gently grinding a radius.

With the die made I could see it would move around in the washers making it hard to control centering the punch on the die (another washer).  To fill the gap I put a piece of heat shrink tubing on the bolt. It did the job.

Using another washer for the die worked because the hole size was just big enough to provide clearance for the steel around the punch.

I ground a radius on the inside edge of the washer with the Dremel tool.

The strip of galvanized steel was placed over the die (washer) and the other washers stacked on top of the steel.

With the punch (bolt) in the stack of washers all the washers were aligned before hitting the bolt with a 6 lb. sledge hammer.

It made a nice pocket in the steel.

There are radii in the corners and the pocket is deep enough to allow room for the firewall to fit in the groove and for the grommet to be nicely protected.

I then made enough pockets for all the 3/4" grommets.

To center punch the location for the grommet hole I used a duplicating punch which fit snugly in the grommet.

I needed 2 mounting holes for each shield or shield half, if they were a 2 piece shield.  For the first one I drew the locations and center punched the holes.  The holes were then punched with the Whitney punch.

The piece was cut out and filed smooth.  It became my master for punching the holes in the remaining shields.

The center hole varied in size to fit the hole of the grommet with a little clearance so whatever was passing through the grommet could not rub on the shield.

It would defeat much of the purpose of the grommet if it rubbed on the steel.

The mounting holes were than duplicated for the remaining pockets.  The first hole was punched just holding the master snug in the pocket.  A Cleco was installed and the second hole punched.

Blue felt marker was used to layout the area for the cut line.  The master was Clecoed in place and the cut line scribed.  The parts were cut with snips, the edges ground on the belt sander and then filed smooth.

Center holes were added.

For larger grommets a socket wrench was used as the die.

For the fuel hose and oil pressure hose a slightly different shield was made.  The base was made by the same process from 20 gauge (0.035" mild steel).
A tube gets welded on to attach a fireproof sleeve over the hose.  The tube was cut from a scrap of 0.035" 4130 steel tubing just large enough for the hose to easily pass through.
The tube was inserted in the base leaving about 1 metal thickness sticking out the back to make welding easier.

The tube was welded to the base on the grommet side.  The assembly was grit blasted primed and painted.

Here are all the finished parts ready to install with their grommets.

You can see that the 2 piece shields are cut to leave plenty of overlap.

The shields are held to the firewall with sheet metal screws.  I always start with the smallest crew which will hold so when the holes get worn you can go to a bigger screw size.

You can see how the hoses fit through the steel tubing.

The firewall is ready for the engine to bolt back on and the fire sleeves to be installed on the hoses.

A very neat professional job which will give a few minutes longer to get safely on the ground in case of a fire.  With a wooden airframe and the fuel tank in your lap a few seconds could mean the difference between life or a horrible death.  It was probably the thing WWI pilots feared the most, burning to death in  their airplane.

Cabin Heat Box

I may have told this story, one day I decided to go flying when it was 15 degrees on the ground.  I had on my snowmobile suit and other warm cloths, and I've hang glided when it was 2 degrees out and snowing.  It never occurred to me how quickly I would get cold.  After 15 minutes I was shaking so badly I wasn't sure I could get back to the airport or land if I did.  Right then I decided to add some cabin heat.  It wouldn't have to make the cockpit warm but if it raised the temp. even 15 to 20 degrees it would help.  I've since read a book where they were teaching people to fly on Lake Fenton, where my wife grew up in Michigan, in winter flying Curtiss Pusher knock-offs.  I have no idea how they survived it.  The interesting thing is that I have a WACO and one of the founders of WACO, Sam Junkin, was working at this company at that time.  I've read letters between him and Clayton Bruckner, another WACO founder, written while working there.  Small world.

One of the things needed to safely have cabin heat is a fireproof valve to turn the heat on or off and which will prevent flames getting into the cockpit if there is an engine fire.  The valve box gets mounted on the engine side of the firewall.  I'm using 20 gauge (0.035") 1018 steel to make the box because I have lots of scrap from making Curtiss OX-5 Exhaust Manifolds and elevator horns for the WACO NINE.  I made the box from one piece of steel with the sides welded together.  The holes are where the inlet and outlet tubes connect to the top and bottom of the box.  When the heat is off the hot air from the exhaust heat muff passes through the box and out the bottom of the engine cowl.  The tabs sticking up here are the corner flanges which will get welded to the sides when the box is folded.

The rings were made from strips of steel formed around a socket wrench and spot welded together.  I had to buy a cheap spot welder to make the exhaust manifolds.  It has been a useful tool.

If you look at the first picture I left about 1 metal thickness sticking up to use for the corner weld.  The corner is just melted together with the torch.

Before folding the weld scale was grit blasted off.  The tabs sticking up are the mounting flanges for the top and bottom of the box.  We'll form the other 2 mounting flanges after the box is folded.  I want all 4 flanges in a plane.  I'm good but not good enough to make all these bends and have the flanges actually line up.

The first bend was made over an oak form block with a small bend radius and a few degree spring back angle so the bend could be made square.  The piece of plywood is a backing block.  It's needed so the box wall will stay flat and the bend will be tight to the block.

The same process was used for the second bend.  We now have 2 sides of the box folded.

A form block was made to set in the box while forming up each end.  The top and bottom, sides with tubes had the mounting flange  and corner tabs formed before welding on the tubes.

The first 2 sides prevent over bending the top and bottom enough to get them to spring back to a square bend.  As a result they need to be held tight with clamps while spot welding the corner tabs.

A couple welds were made near the open end of the box.

The clamps were then removed and more welds added to each corner tab.

Now we need to form the last 2 mounting tabs so they all set tight to the firewall.

The form block was clamped to the side using the thickness of my thin steel scale (ruler) as a gauge to offset the block for the thickness of the flange.

The tab could then be hammed to  90 degrees and the mounting surface aligns with the top and bottom mounting flanges.

I always hammer metal using a block of hardwood, not shown, so the metal being hammered over won't get dented from the concentrated blows of the hammer.

We need a stop for the flapper valve so when it's open it can't get stuck.  This hat shaped piece gets welded into the front of the box.

With the basic box formed I clamped it to a piece of 3/4" plywood to hold it flat while I welded the corners shut.  It might not have warped but this seemed an easy precaution.

A steel hinge was welded into the box and the the valve (door) welded to the hinge.  A spot welder made all this so easy.

The closed (heat off) position.


When it's in the on position the valve sets tight to the stop and very little hot air will leak around the valve.

The box is test mounted to the firewall to work out the hole size and see that it works properly.

The eye bolt is to attach the control to operate the valve.

The box needs to be painted and the outlet tube riveted on.

We'll come back to this when I hook up the control to operate it.