Hello,im going to explain this as best as i can

I have a plane that is made from plastic and wood ribs throughout the plane. when i went to the field to fly , the instructor noticed my engine was flexing. he told me my firewall was loose.

we removed 3 of the 4 screws . all are srewed into a wood piece behind the firewall with a BLIND NUT at the end. the 4th screw can come out because the nut it spinning with the screw.


2 questions

1. how do i get access to the nut behind the firewall
2. what do i need to beef up the firewall to make it rock solid

im going to get pics soon but see if you can understand it

my plane is an airmaster 40t with a magnum xl .40 engine

thankx

correction: the 4th nut cannot come out and the 4screws secure the engine to the firewall

First try pulling the 4th scew out (away from the airplane) real hard. Hopefully the blind nut will grab and then you can try unscrewing the screw.

If that doesn't work, just stick a screwdriver down and into the fuselage against the back of the nut and knock the nut into the wood. Try again to take the screw out.

You might have to end up getting some needle nose pliers in there to hold the nut while you unscrew it...

Is the wood firewall glued to wood on the fuselage?

If it is, just re-glue it with some triangular stock on the sides and a bunch of exoxy.

First try to retighten the 4th screw to try to reseat the blind nut. you may have to pull on the bolt a little to get it started. This should work as long as the teeth on the blind nut are not bent over. If it works tighten it down well and then you should be able to remove the screw again. (remember what one it is so you don't push it back out when you replace the screw. If this does not work you may have to make a hatch to access the rear of the firewall. I am not sure about the platic fuse but with balsa you would go thru the bottem just behind the firewall. If you wish to beef it up you will most likely have to double up your firewall and this would mean accessing from the rear anyway.
Good luck!

k turns out my problem was due to 3 of the blind nuts went even there. they had dropped out. thats why the 3 screws came out so easyily.

is there an ALTERNATIVE way of securing the engine to the firewall

The engine mount is always bolted to the firewall, with 4 bolt, and mostly blind nuts. This works fine normally, however some actions are best taken to make sure the bllind nuts work correct.

First, they need to be in the wood. They have these little arms that are pulled into the wood, preventing them from slipping. They must be pulled in all the way, until they're flat against the back of the firewall. Next, drop some thin CA between the nut and the wood, which will prevent them from falling off too easy while installing the bolts.

You could use regular nuts, however that would require you to have physical access to the back of the firewall, which isn't always to case in some planes.

However, since you are going to rework it, maybe a removable hatch is't a bad idea. Mostly the fuel tank is located behind the firewall, so having good access there is helpful in case of tank problems as well. I always build my planes so that I have physical access through a hatch at the room behind the firewall.

Now, as far as I can tell, your engine was loose because it was only fixed with 1 bolt, so I assume the firewall itself is still firmly attached ? Adding some triangular stock at the back never hurts anyway, since you're at it....

How thick is the firewall? If it is 1/8 inch, I would take it out and replace it. You need 1/4 inch. If it is 1/4, or even if you replace it, put 1/4 inch triangle stock in the back from the firewall to the sides of the fuse to beef it up. If the holes are too big for the blind nuts, fill them with a dowel the correct size, glue them in and re-drill them. Get new blind nuts and screws. Don't use the old ones. Without the mount, put the screws through the hole, start the blind nut on the screws, screw them in till the screw pulls the blind nut and sets it into the firewall. Don't twist them off. Make sure the holes are the right size.

Next take the screws out, reinstall the mount and put some blue locktite on the screws and put them back in tight. Not too tight. ?Then re-mount the engine.

I did some surgery to the plane. it now has a removable hatch. Thankx Dazzler_BE for the idea. I put new blind nuts in but boy were they hard to position. The fuel tank kept getting in the way. the nuts are flush against the wood, and i have put CA on the nut. someone at the club has offered to cut me a thick firewall. I hope this engine holds on Thankx everyone who replied

Something else to look for as you are learning some repair procedures, is the length of the firewall bolts. Check the amount of the excess length that protrudes into the fuel tank area.
Either cut off the excess, or put a little foam rubber in FRONT of the tank too, to keep the bolt ends from rubbing the tank. This will prevent possible tank leaks in the future. .... lownslo.

i put the foam on during the initial build of the plane. But thankx for the comment