I'm mounting a .91fx on a GP Extra .60. The holes for the engine mount are located at the borderline of the top of the firewall. I'd like to drill them about 1/8" away from where they are currently marked to be. Is it a really bad idea to do this as this would shift the engine's center line 1/8" away from the supposed centerline on the firewall?

I wouldn't think that 1/8" low would make that much difference but the spinner may not be centered on the cowl. Have you looked at other motor mounts, IE the Dave Brown ones will let you squeze a mount where you don't really have room.

If the firewall is capped with a balsa top, you my have to do what I had to do with a Great Planes Super Aeromaster. I made a new firewall that was a full cross section of the front of the fuselage out of 1/4 aircraft ply. This repaced a rectangle firewall with a balsa former on top. That gave me just enough real meat to get the engine mounted. The plans called for a 91 or 120 four stroke, and I had to shoe horn the 91 in. I picked up my 120 and it just screamed NO, No way.

Don

To move the engine thrust line up or down a significant amount would need to be compensated for by a down thrust angle if moving the engine down.

But like was said 1/8" is not going to materially effect the flying characteristics

If you move it and it ever does affect the thrust line then just use some washers to correct it.

tIANci is talking about if you have problems, You can use washers to angle the engine up or down to counteract thrust reactions

Just be sure to check the lateral balance of the plane after everything is mounted and installed. [8D]

Instead of altering the centerline, why don't you get another motor mount and re-set the mounting holes. I do this all the time when I change motors and the old mount doesn't align with the new motor. While you can make all of the above mentioned adjustments to accomplish your modification, why mess with the design charateristics of the plane.

If you have a old plane without a manual how do you determine the center line?

If I had one like this I would mount the engine where it L.A.W. and fly the plane, making thrust angle adjustments by trial and error.

I cannot compensate with washers because its a side mounted engine. I already drilled the holes and I'm off the supposed centerline by only 3/32" which I'll assume its ok. If it seems awkward when I fly it, I'll just expand the screw slot on the mount a bit so I can slide the mount just a tad to compensate those 3/32"

Why can't you compensate with washers because it is side mounted??? The washers go between the engine mount and the firewall, not between the engine lugs and the engine. What you do is tilt the engine by putting more washers on the top than the bottom, or right or left by putting more on one side or the other. Just watch you don't screw up your thrust to badly.

By the way, you can't enlarge the holes enough to compensate for 3/32" That would put your screw holes about 1.5 times or more that stock size and you will probably run out of meat on the mount before you get it where you want it.

Don

Like is said above, you don't want to run out of "meat" on your mount.

If you have to go to the right (when setting in the pilots seat) you cn use washers. But to the left, you will need to move the mount.