Parts Needed

All parts are Century brand.

The parts needed to implement this upgrade are:

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Part Name: Main Gear with Autorotation Hub
Mfg Part Number: CNE250
Vendor part number: CNE250
Purchased from: Heli-World
Purchase link: link
Cost: $17.95 USD

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Part Name: Main Shaft Pin & Collar
Mfg Part Number: CNE219
Vendor part number: CNE219
Purchased from: Heli-World
Purchase link: link
Cost: $3.50 USD

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Figure 1 shows the parts.

Introduction

I purchased both parts from HeliWorld. They were delivered quickly and in good shape. I did not like paying over $11.00 for shipping from California to Wisconsin however. I suggest googling on the part numbers shown above and see if you can't do better.

A lot of folks have switched out the stock, one-piece main gear and shaft for the Century brand main gear with auto-rotation capabilities, and separate main shaft. Let's look at the positive side of doing this conversion:

1. The stock main shaft is notoriously soft and bends easily during minor crashes. The Century main shaft is reportedly constructed from a harder metal. I have no evidence to support or refute this.
2. It is much easier to replace the Century main shaft if it does get damaged.
3. The auto-rotation system allows you to perform auto-rotations. This is useful in the event of an in-flight main motor failure.
4. Replacing the Century main shaft is a couple bucks cheaper than replacing the stock main gear and shaft.
5. The Century main gear is known to spin truer. The stock main gears are usually out-of-round directly from the manufacturer.
6. During power down the main rotor will free-wheel which allows the main motor to stop spinning sooner resulting in less brush wear (OK that's really reaching).
7. Modifying the Blade CP is fun!

OK - now let's look at the negative aspects:

1. The Century main gear and shaft combo is slightly heavier than the stock one-piece gear/shaft assembly.
2. Some tinkering is needed to get the gears to align properly.

That's all the negativity I can come up with. That's a pretty good ratio of positive to negative.

All right - let's modify this thing.

Remove the old parts

Remove the stock landing gear. If you have the Super landing gear, just move the battery tray all the way forward to allow the main gear and shaft assembly to be lowered out of the airframe.

Pull the Retaining Pin (035).

Loosen the Main Shaft Retaining Collar (047).

Pull the Main Shaft & Drive Gear (050) out through the bottom of the airframe. If the retaining pin hole on the main shaft is deformed, and they usually are, you will have to clean up any burrs with a suitable tool like a jeweler's file (Figure 2). A burr here will cause the shaft to bind on the Center Hub (008), Swashplate Ball (043), Main Shaft Retaining Collar (047), and the Bearings (014) as you try to remove it. Resist the temptation to cut off the main shaft for quick removal - we're going to need it intact in a subsequent step.

Prepare the new parts for installation

Notice that there are two holes in the new main shaft, one on either end (Figure 3). One hole is drilled nearer to the end of the shaft than the other. The hole that is nearer the shaft end is the bottom of the shaft. Take one of the two included short pins and insert it into the bottom shaft hole as shown in Figure 4. Make sure that equal amounts of pin protrude through each side of the shaft. Place a small drop of silicone oil or your favorite light oil into the auto-rotation mechanism and the outside of the bushing as shown in (Figure 5). Insert the bushing into the auto-rotation mechanism ensuring that the end with the groove is showing after installation (Figure 6). Insert the shaft into the bushing hole and slide it all the way up. Engage the pin into the bushing groove (Figure 7). The shaft should now rotate freely in one direction and not the other while holding the gear stationary.

There is a critical dimension that must be checked and maintained. Let's do that now. (Figure 8) shows you how to measure the distance between the top of the gear and the retaining pin hole. Measure both the old main shaft and the new. This dimension should be identical. Mine was, and it measured in at 2 11/32 inches. If this dimension is not identical between the old and new assemblies, you will have to verify your neutral pitch settings.

Another thing that should be done now that will help you in the future is to measure the distance from the gear to the top of the main shaft on both assemblies, old and new. What the heck for you ask? Well, as you probably know, the swashplate is supposed to be set so that there is one-half inch of main shaft protrusion through the top of the swash when the controls are neutralized. The new shaft is shorter than the old one by 1/8 inch so you will need to subtract that from one-half inch and retain that number (3/8 inch) for future use when setting the swash up.

Installation

Make sure that the old shaft bearings are still installed and fully seated in the airframe.

Insert the new shaft and gear assembly up into the airframe from the bottom of the helicopter. Pass it through the bearings (014) and then install the new retaining collar. There is a step on both ends of the new retaining collar so it doesn't matter which end goes against the bearing. If you choose to reuse the original Main Shaft Retaining Collar (047), make sure the tapered end goes against the Bearing (014). Continue to insert the shaft up through the Swashplate Ball (043) and Center Hub (008).

It's interesting enough for me to mention that the new gear has an outside diameter of 2.79 inches while the old measures in at 2.75 inches. I thought that this must mean that the new gear has more teeth, but no, they both have 140 teeth. The spacing between teeth must be slightly different however. After much consideration I believe that there will be no effect on head speed but you will have to move the main motor over a bit in its adjustable range.

Make sure you have loosened the Main Motor Screws (027) and slid the motor over to allow for the slightly larger new gear. Now slide the main shaft with the new gear attached into position. Make sure it is fully seated against the bottom bearing and that the gears are properly meshed. Right about now you should be realizing that the 2 gears don't align properly. The new main gear is significantly lower than the stock gear. I found this to be unacceptable and spent hours going over possible solutions. I tried removing the aluminum hub with autorotation mechanism from the plastic gear and installing it on the other side (henceforth referred to as "flipping the hub") (Figure 9), but that just made the misalignment move to the other end of the pinion. A person could remove material from the top of the hub (many people have done this), but I eliminate this as a solution because I don't want to make the retaining pin hole move up or down as it throws off the head settings.

Here is my list of solutions (along with my opinion of each):

• Leave the hub installed as it comes from the manufacturer and live with the misalignment. (No good)
• Flip the hub and live with the misalignment. (No good)
• Leave the hub installed as it comes from the manufacturer and pull the pinion further out on the motor shaft. (No good)
• Flip the hub and push the pinion further onto the motor shaft. (Acceptable)
• Flip the hub, manufacture and install a shim of proper thickness between the motor and frame. (Acceptable)

I finally decided to go with flipping the hub and pushing the pinion further onto the motor shaft (Figure 10). This puts the main gear very close to the bottom of the airframe. I'm sure I will have to trim the canopy to keep the main gear from rubbing it.

The shim solution is just as good, but you may have to come up with longer Main Motor Screws (027). Also, the shim should cover the entire contact surface between frame and motor.

Using hand pressure only pinch the main gear up and the retaining collar down as you tighten the setscrew on the Main Shaft Retaining Collar (Figure 11). Some people recommend allowing a little vertical play here, but I say let the bearings do their job. You don't want the main shaft spinning inside the bearing inner-race.

To properly set the gear mesh between the Main Motor pinion and the new main gear cut a small strip of common writing or printer paper. All you need is a strip about one-half inch by one inch. Place the strip of paper between the two gears where they mesh as shown in Figure 12. Pinch the two gears using firm hand pressure. The strip of paper should "crinkle" and conform to the shape of the gears as it is squeezed between the teeth. Tighten the two Main Motor Screws (027) securely. This sets the proper amount of play between the gears to prevent binding or slipping. To remove the paper simply rotate the main gear until it is free. Rotate the main gear slowly and observe the mesh between the gears. There should be the slightest amount of free play between the gears throughout an entire revolution of the main gear. Make sure that the new main gear does not contact any wiring or cable ties.

Align the hole near the top of the shaft with the Center Hub retaining pin hole, and insert the Retaining Pin (035). You're allowed to swear and curse here as much as needed to get that pin in.

Put the landing gear back on or reposition the battery tray depending on what style gear you have.

If the previously mentioned "critical dimension" was the same on both the old and new shaft assembly, you're good to go. Fire it up and go fly. If not you will need to setup the Swashplate.

Summary

This is a very worthwhile modification for the money in my humble opinion. Again, look at the ratio of positives to negatives, more commonly referred to as a "risk to reward ratio" and you'll see that this should be at the top of one's mod list, especially for newer pilots. This is a 100% functional mod - no bling here at all. Thanks to all of the site members that have provided valuable input to these instructions by participating in our ever-growing user forum.