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Mounting an engine in a model may seem daunting, but it is easy, and model pilots eventually need to know how to do it. Although many of today's RTF trainers' engines are already mounted, hard landings may damage the original mounts. An ARF trainer requires the assembler to mount the engine.

Depending on the airframe, you may need to adjust the engine's "thrust angle," which is the angle between the airframe's horizontal centerline through the fuselage and the direction—right, left, up, or down—in which the engine is pointing in relation to that centerline. Remounting in a slightly larger mount is usually the best way to make thrust adjustments, especially if the engine is cowled.

There are four types of engine mounts most commonly in use today: aluminum "clamp-on" mounts; adjustable fiberglass or solid fiberglass mounts; and independent, twin I-Beam, fiberglass mounts.

Of those, the aluminum clamp-on mount is the easiest and the hardest to use correctly. It's easy because two clamps hold the engine in place; there is no need to drill mounting holes into the mount. It's difficult to ensure that the engine is centered and aligned inside the mount.

Clamp-on mounts are larger than the engine's crankcase, allowing the engine to be mounted too far to one side or twisted between the mounting beams. Both situations affect the engine's thrustline and consequently the airplane's handling characteristics—never for the better. Compounding the alignment problem is that most trainers and sport ARFs have right and/or downthrust built into the firewall (the wood faceplate to which the mount is bolted).

The firewall's offset means that it is impossible to align the engine inside the mount by measuring from any point on the airframe, unless you are a surveyor or mathematician. If you are not, all measurements must be done in relation to the mount itself.

The initial step is to determine how far forward in the mount the engine needs to be. If your model has a cowling and spinner, make sure there is at least 1/16 inch clearance between the front of the cowling and the rear of the spinner. A photo shows what happens without this clearance. If the engine is not cowled, make sure the propeller will clear the fuselage side plates.

Once you have established the engine's fore and aft placement, make a mark at the rear and front of the engine's mounting plate. Measure the mount's outside width at the front and the rear of the marks. Measure the width of the engine's mounting plates. Subtract this number from the mount's width, and the result is the total extra side space at the front and rear of the engine's position. Divide this extra space—front and rear each—by two, measure in from the outside of the mount by this amount at the proper locations, and mark. Draw a line between the two marks on each side. Aligning the outside of the engine's mounting plates to these two lines centers the engine in all directions inside the mount.

Lightly clamp only one side of the engine. Ensure that the engine hasn't moved by checking the reference line on the unclamped side, and—just to make sure that everything is straight—mount the propeller.

Make a mark in the top middle of the mount's faceplate (the rear mount part that holds the aluminum mounting beams), and measure from this center mark to each propeller tip as a check. The distances should be the same. If not, they will not be too different and can easily be adjusted without moving the engine sideways.

Do not use this check measurement without centering the engine in the mount first. If you do, it is possible to have the engine too far to one side. Equal propeller-tip distances will then ensure that the engine is twisted inside the mount.

Once everything checks out, install and tighten the second clamp, and then secure the first clamp. It takes longer to read this than to do it.