I got a new ARF for Christmas (Seagull Spacewalker II) which has the engine mount already attached. But I want to use a different (larger) engine than the provided mount will handle. Since the original mount is NOT centered on the hole drilled in the firewall for fuel lines, I can't simply extend lines out from the center to locate the new mount. What is a good way to locate the new mount so that I have not changed the thrust line? I thought maybe I could put the original mount back on, clamp on engine that fits it, and try to go from there, but that seems a little dicey to me. Looking for better ideas!

I apologize if someone has answered this question elsewhere, but I searched and did not find anything.

Thanks for taking the time to read this post. Dzl

Remove the engine mount.

Put your engine on the mount you intend to use.

Hold the plane nose UP (You might want to get a helper for the next few steps)

Place the engine and mount on the firewall (No screws)

place the cowl on.

Align the engine in the cowl

Carefully remove the cowl so as not to disturb the placement of the engine

Mark where the mount is and drill.

Now THAT'S a solution I would never have thought of! And elegant in its simplicity, too. Thanks, Dzl

I think MinnFlyer's idea would work in most cases, but it did not in this one. I couldn't establish the reference point without first doing some surgery on the cowling to accommodate the engine cylinder (plus intake tube). Also, my cowling fit allowed too much to chance; upon wiggling it around, I could make a poorly-aligned engine "fit" in the center.

After considering the options, here's what I did, and it seems to have worked, though I won't know until the whole thing is in the air, will I?

Pictures would be nice, but I don't have the equipment for closeup work. But it is pretty simple.

First, take the original engine mount off, and place a piece of paper over the firewall.

Second, locate the original engine mount's mounting holes, and carefully poke a hole through each of them so that you have all four.

Third, put the paper on the back of the original mount and install it on the firewall.

Fourth, locate the thrust line and the center line of the original mount by marking on the paper on the outside of the mount.

Fifth, remove the original mount and draw lines on the paper making an "X" showing the center of the engine mount; extend these lines out.

Sixth, place the new mount over the paper, and locate the "X" in the center of the circle in the new engine mount, making sure you have the thrust line "spot on"; then mark the new engine mount hole locations and an outline of the new mount on the paper. Poke holes in the paper for the new mount's location.

Seventh, put the paper on the firewall, lining it up with the old engine mount holes; now mark the new engine mount holes.

Eighth, if necessary, plug the old holes before proceeding to drill SMALL pilot holes for any two of the new engine mount's mounting holes.

Ninth, mount the paper again along with new engine mount using small wood screws in the small holes you drilled in step 8.

Tenth, mount your engine and start opening up the cowling as much as necessary to get the cowling on straight. Worry about cooling, etc. later.

Eleventh, mount your cowling as you think it should be, and check to see if your engine shaft is in the center of the exit hole in the front of it. Mine was.

Finally, drill the firewall for T-nuts, and install the new mount.