I recently purchased a 1/4 scale Extra 260 ARF, built by some individuals that were going to start kitting these, but I guess they never took off. The 260 is built, and I have no plans or instructions, and I really need to know where to mount my engine. Is there a formula to find the thrust line? There is no markings on the engine box.

here are 3 pics of the front end if it helps at all....

Well, the thrust line should be in the center of the fuse as viewed from the top. Position it top to bottom so the prop shaft is centered in the cowl opening. Keep in mind there is probably right thrust built into the firewall so you will have to shift the engine a little to the left (pilot's left) to get the spinner backplate directly on the thrust line.

The Chevy looks sharp.....

The center of the engine should be 1/4 inch to the left of the center of the firewall. That's how my last two quarter scale aerobats were setup. Up/down center of the engine was on center of the firewall. You will be able to see if your firwall has right and down thrust. It's easy to see by eye. It probably does have it built in.

You know I just went to look at your first pic and it almost seems like the engine box is shifted to the left already. Then you would use the center of the engine box to center your engine (IF the box has right thrust built into it). Again you should be able to eye it up and tell what thrust was built into it.

Rich:

Lay a large piece of paper on a flat surface and draw horizontal and vertical centerlines on the paper. Set the back of the cowl on the paper and use a plumb-bob or a square-ended ruler through the prop-shaft hole to center the hole over the junction of the centerlines on the paper. Without moving the cowl, trace the outline of the back of the cowl on the paper.

This will tell you where the prop-shaft will exit the cowl in relation to the firewall. If you have no right or down-thrust, just measure the distance from the top of the cowl to the centerlines on the paper and use that measurement to find the distance from the top of the front former to the center of your engine mount.

If you have right thrust, offset the engine mount 3/16" to 1/4" to the right (looking at the front of the firewall) and if you have down thrust (unlikely in an Extra), offset the engine mount upward 3/16" to 1/4".

After you have made all of the measurement, glue the engine mount to the firewall and check that the prop-shaft will come out where it should before you drill the mounting holes.

Jim

that sounds easy enough ! thanks Jim

We had a little club get together today at a members house, worked on a few radio programs, and finished up someone's C-47, afterwards, we worked on my 260. It seems that the builder mounted the engine box dead center fo where the thrust line is... so its pretty much a no brainer from here as to where to mount the engine.