Smith Cub Firewall hole locations

Looking for some help


I'm using a O-375 from Aerosport with vetterman exhaust
Looking for preferred locations for passing through the firewall for


1 Gascolator
2 Carb heat cable
3 Mixture cable
4 Starter cable
5 Primer line
6 Engine temps / lightspeed cable

Thanks........ Phil

Hang the engine and exhaust system . It may come more apparent where to place these on the firewall .

That's probably the question I should have asked.

Can all the holes be drilled with the engine on?

Although any drilled firewall photos would be appreciated

90 degree angle drill and Bob's your uncle.

Hang the engine without the firewall. Find the best locations. Take measurements. The engine will be on and off several times during the build process.

I had to move the gascolator to a position outboard of the engine mount. The stock Piper location places it just above the exhaust stack. I consider that unsafe because of the proximity of the hot exhaust to gasoline.

Don't forget the cabin heat control cable.

It is always good to try and look at similar planes and see what they did if you can. It is much easier to visualize your own installation if you have an initial frame of reference. If you are still not sure you can install a temporary plexiglas firewall (1/8" thick) and try different locations until you are happy. The temp firewall then becomes the pattern for making holes in the real thing. The plexiglas has the added benefit of letting you see obstructions and supports on both sides as you work. Just remember not to try and set the cowl with the plexiglas firewall in place since it will push the engine forward from where it will ultimately end up.

Aside from that suggestion here are some thoughts on basic layout:
1 Gascolator - usually lower left as seen by pilot, since the carb inlet fitting is on that side
2 Carb heat cable - again usually on left but higher to avoid pedals.
3 Mixture cable - also on left but shaded more towards center, since that is where carburetor is. Again watch out for interference with rudder pedals and brakes
4 Starter cable - often on right just to get it away from all the stuff on the left side, even though starter is on left. You have a lot of flexibility on this. Also depends on where you decide to mount battery and starter relay.
5 Primer line - primer comes off gascolator (on left) but you will l probably want to run primer lines up and possibly more towards center before going thru firewall. It depends on where you decide to locate primer pump on instrument panel.
6 Engine temps / Lightspeed cable - lots of flexibility here. Probably easier to run Lightspeed wires through around top center, assuming you put box inside cabin. Sensor wires can go anywhere but you may find it convenient to run them in same place as Lightspeed wires or at least near them.

Be sure to protect all firewall penetrations with eyeball fittings for cables and sealed penetrations for wires such as Aircraft Spruce part #09-00978. and use a 2000 degree rated sealant not red RTV.