Session 5

Time to Mount the Engine

Here is a little hint that I didn't mention the last time. We have the gyro sitting on top of a set of plastic egg-crate filing boxes. This makes it much easier to work around the machine as you don't spend half your time on the floor!

Lower Engine Bearers

The steps which follow will describe how to mount the lower engine bearers to the engine support struts. You will need to separate the upper bearers (that attach to the engine) from the lower ones which we will mount to the struts. The upper bearers have in-line mounting holes and can be set-aside for the moment. There will be two lower bearers, with offset mounting holes, that we will be working with.

(1) First, we need to put some masking tape on the bottom of the rear (12-inch) and front (10-inch) bearers. Carefully measure and pencil-in the center line on both pieces. The center line will be inset 5 inches on the front bearer and 6 inches on the longer (rear) bearer. Now carefully pencil two parallel lines exactly 2 19/32 inches (2.59 inches or 67.8 mm) on either side of both center-lines. Here are the three-lines (center with two on each side) as marked on the bottom of our rear lower bearer:

The two side lines on each piece will let us center the bearers on the mounting struts by positioning the pieces laterally so the edge of the struts are aligned with the pair of side-lines.

(2) Take the rear/longer lower bearer and clamp it on the struts, flush with the end of the struts and centered side-by-side using the pencil reference marks:

(3) When you are certain that everything is aligned, match-drill the offset mounting holes using a 1/4 inch bit. Remove the clamps and masking tape, deburr the holes on the strut, and mount the rear bearer using the AN4 hardware provided by Star Bee. IMPORTANT NOTE: Star Bee provides two washers for the nut/lower-side of the attachment. You MUST use three washers or the nut may bottom out on the shoulder of the bold. In that were to happen, a very serious fatigue point would be created. Use three washers! Alternatively, if you have access to aircraft hardware, use a pair of AN4-10 bolts and you only need one washer under the head and one under the nut!

(4) Now take a piece of poster board or thin cardboard (from a cereal box is just fine, about 4-6 inches long. The width of the cardboard strip will depend on the engine you are mounting:

Rotax 503 = 102 mm

Rotax 447 = 96 mm

Make your measurements and cuts very carefully - the width should exactly 102 or 96 mm and the sides must be parallel. Using the piece of cardboard as a spacer, center the front lower engine bearer and clamp it in place:

(5) When everything is aligned, match-drill the offset mounting holes using a 1/4 inch bit. Remove the clamps and masking tape, deburr the holes on the strut, and mount the front bearer using the AN4 hardware provided by Star Bee. IMPORTANT NOTE: Star Bee provides two washers for the nut/lower-side of the attachment. You MUST use three washers or the nut may bottom out on the shoulder of the bold. In that were to happen, a very serious fatigue point would be created. Use three washers! Alternatively, if you have access to aircraft hardware, use a pair of AN4-10 bolts and you only need one washer under the head and one under the nut!

At this point you will have to reinsert the four loose Barry mount pieces back into the lower bearers from below. They will be very tight, so the use of a lubricant, such as liquid dishwashing detergent, will make the job much easier.

Upper Engine Bearers

The upper engine mounts now have to be secured to the engine case and here some people make a serious mistake. The studs that come with the engine should not be used to mount the engine in service. Rotax never referes to these studs and mounting hardware but rather labels them with some variation of retaining studs for shipping! These studs are a soft grade of steel and completely unsuited for use on aircraft.  Because of mount clearance problems and the unthreaded center section, using them (if we ignored the quality of the steel) would require spaces that would unnecessarily raise the engine thrust line. DON'T USE THESE STUDS to mount the engine!

Instead, remove the studs and gently use a small wire brush to clean and corrosion and old "loctite" out of the threaded bosses under the engine case.  You will need four 10-30 metric bolts (10mm in diameter with a 30 mm grip) and four metric 10mm lock-washers. The bolts should either be stainless steel or Grade 8 (typically marked with an 8.8 on the head). Do not use either Grade 5 or ungraded bolts. The proper bolts can be obtained at any well-equipped hardware store. Metric bolts are commonly available with either "standard" or "fine" threads - get the "standard" thread!

Put a bolt through the lockwasher and through the mount from the bottom, using a torque wrench to run it up into the threaded boss. Torque the bolts to 30 ft.lbs, 360 in.lbs, or 40 Nm. Repeat until both bearer pices have been mounted. A medium grade of loctite (red) can be used to help secure the bolts.

Finishing the Job

This part should be pretty easy. Simply use the supplied 3/8-inch hardware (bolt inserted from above with a standard 3/8-inch washer), the rubber Barry mounts, a fender washer, and a nut to tie the upper and lower bearers together:

However, the bolts supplied are not aircraft, but rather are coarse-thread grade 8. No problem in itself, but the bolts are also 3-inches long. This leaves an excessive length of threaded section (see above) and comes close to allowing the nut to bottom-out on the shoulder of the bolt shank. By my measurements, an AN6-22 aircraft bolt with matching hardware should be about perfect, but AN6 hardware is definitely harder to find. Grade 8 or stainless hardware will do the job, but the results will be more professional if you use 2.5-inch bolts in place of the 3-inch bolts supplied from Star Bee:

With everything bolted in place, you should have plenty of clrearance between the front of the engine and the mast:

Mounting is, in fact, complete, but this photo reveals   something we hadn't previously recognized:

Given the way this engine was set up in the trike, the gearbox ended up facing up instead of down! This is  easily corrected (and it must be if the thrust-line isn't to end up about 6-inches higher than it should be!). For the mopment, we will put off engine wiring and move on to the tail group.

Session time: ~3 hour      Total Time: ~16 hours