I have been working on A Pacific Aeromodel Gee Bee Y 1.20 ARF. I have been having a heck of a time centering the engine in the center of the firewall. Hence, I have removed the engine as I had it mounted, somewhere off center, plugged up all of the holes in the firewall and am beginning to start over. The instructions in the manual say to mark the center of the firewall and then go 5/16 above and 5/16 to the right and use this as a centerline. This sounds good, but as a backup I went to the website and looked at the instructions posted there. There is no mention of this second marking of the firewall. I'm hoping some of you can walk me through this procedure step by step. I'm having trouble finding the center of the firewall seeing that it is round. Pictures will be a big help. I am mounting a Zenoah G-26. Thanks, Aframe2

A good way to find the center of a circle is to draw diameters in a bunch of directions/angles (if you measure across, the widest point point is the diameter). They will intersect at the center of the circle.

A pictorial idea would be like cutting a pizza. If all cuts are across the widest part of the pie, the slices will all meet in the center.

I have a clumsey but sure fire way to find the center if your circular firewall. I would use my mechanical drawing dividers or compass - or any drawing compass for that matter. Place the point of the compass where you think the center is and move the drawing point around (don't make a mark, substitute a piece of sharpened dowel if you want) move the point until you can "draw" a circle within the confines of your cowl. There is the center. The diagonal line piece is OK if you know the widest points of the circle. Like I said, my method is clumsey but effective - and sure fire.

Bill

For working on round firewalls, you need to find yourself a "center finder". I got mine from Lowes or Home Depot, I think, or maybe a woodworking store. It has ridges set at 90 deg on one side and 60 deg on the other for smaller items. Just lay it across the face with the ridges tight against the item. mark a line, and then rotate about 90 deg or so and mark another line. Where they intersect is the center. I used the lid from a butter dish to illustrate, but it's the same on a firewall.
Also you can use this chart to figure out how much to offset and engine from center: http://www.lcrcc.net/offset_calc.htm

TLH101, thanks for the information. I will go and try to find a center finder, it looks the easiest to use. Are you kidding me with that LRCC calculator. My worst class in school was math and that thing looks like it uses calculus or geometry, way beyond my comprehension. Thanks again, Aframe 2

Hi!
It's very easy to find the center so you can mount your engine properly...The answer is: You don't have to find it!!!

When you have an airplane such as The GEE BEE Z with a round radial engine and cowl you don't mount the glow engine first and then try to mount the engine cowl so that the engine protrudes exactly trough the center of the cowl...

What you do first is to mount the engine cowl so that it sits in a scale position!!!

Then...you remove the cowl. You take the engine with its engine mount mounted to the engine...and put some medium Ca glue on the back of the engine mount. Yes ! You put Ca glue on the mount (be it alu or plastic doesn't matter).

Then you put the GEE BEE Z engine cowl over the engine so that the engine housing protrudes through the hole in the scale cowl. Hold both the cowl and the glow engine and now carefully screw on the GEE BEE Z cowl!

When the cowl is securely tightened to the fuselage... you carefully push the glow engine with its engine mount against the firewall...and hold it there until the Ca glue hardens (use accelerator). Don't forget to check so that the glow engine axle centers in the hole.

Then you remove the cowl and drill the holes through the engine mount into the fire wall. Mount blind nuts and there you are!
That's it!