It’s a Knockout

Home shop machinist.

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Using a screwdriver (left) to knock out a bearing or bushing is not only bad for the screwdriver, it can cock the bearing sideways and scratch up the precision bore the bearing was pressed into. A knockout bar in the form of a close-fitting tube (center) is less likely to damage things, but still requires working the tool back and forth to pop the bearing out straight. A flared tool (right) is the best choice—and easy to make in the home shop!

When it comes time to remove and replace bearings or bushings that are press fit into tubes or deep recesses, don’t be that mechanic who grabs a screwdriver and starts hammering away. Aside from being against “The Code” to never use a screwdriver as a pry bar or punch, it’s about the worst tool you can use to remove a bearing.

The correct way to remove a bearing, bearing race or bushing is to use a knockout tool or puller. Bearing pullers are available from most aircraft tool suppliers and are a phone call away. Knockout tools tend to be more ad hoc. Case in point is this month’s project: A friend is restoring the nose gear on a ’50s-era Bonanza, which has a large V-shaped magnesium casting fitted with plain bearings. Most people would call them bushings, but since they’re called bearings in the Beechcraft service manual, that’s what we’ll go by. In any case, there’s one at each end to allow the steering cylinder to rotate in the casting. With the plane on jack stands, there was a good ¼ inch of slop in the steering mechanism. After 60-odd years, the original bearings were worn out. Go figure.

To make the flare tangs, start by drilling four ¼-inch holes 90° apart about 1½ to 2 inches from the end. Line up and drill through the top and bottom sides (left). While accuracy is not that critical, it makes for a more professional-looking tool if the holes are accurately spaced. A spare bit was used to provide a quick visual to square up the tube for the second set of holes. Note that a spotting drill was used to peck where the holes were to be drilled (center). Drilling through both sides completed the four holes (right).
A small machinist’s square was used to pencil a line from the center of each hole to the end of the tube.
The slits between the ¼-inch relief holes and the end of the tube were made with a fine-tooth hacksaw.
A short piece of bar stock was used to bend the tangs out to form the flared end. The end should be flared out just enough to spring snug against the housing of the bearings you need to knock out.

With the nose gear off and disassembled, getting the bearings out of the casting proved to be a challenge. The sleeves were worn so thin that there was hardly any shoulder left to catch the edge of a traditional knockout bar. Using a close-fitting tube proved futile as well. There was simply not enough of an edge to keep the tool from slipping off the shoulder.

As an avid bicyclist, over the years I have collected an abundance of bicycle-specific tools. I instantly realized the Bonanza steering design was basically an oversized bicycle headset—and I knew just the tool for the job: a bicycle head cup remover. The only catch was, the Bonanza steering cylinder is a bit larger than my bicycle tool, so it was out to the shop to make one.

To give credit where credit is due, what I made was a home shop version of a Park Tool RT-1 headset cup remover. It’s basically a long tube with a flared end that gets pulled through the bearing. As the tangs go past the bearing, they spring out against the cylinder walls and seat against the lip of the bearing. A few taps with a hammer and the bearing pops out. It is ingeniously simple.

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The knockout was topped with an end cap, which was turned on the lathe out of aluminum. The shoulder was sized to be “just snug” (left). Then, using the shoulder to register the cap in the chuck, the face was trued up (center) and then chamfered (right).
With the end cap installed, the knockout was ready to knock out some bearings.

Since the knockout pressure is spread evenly, bearings tend to pop out much easier than tapping back and forth with a knockout bar.

The tubing I used for this project was 1-5/8-inch diameter, 0.065-inch wall, 4130 chrome-moly steel. 4130 is ideal for this type of project. It’s tougher than mild steel and retains good springback characteristics, even after bending the flares.

To remove a bearing or bushing, insert the knockout tool as shown until the flared tangs squeeze past and spring out behind the piece to be removed (left). Tap with a mallet as needed (center) until the bearing pops out (right).

This is one of those tools that you don’t need until you do. My advice is to tuck this column away for down the road. After all, hardly any Experimental airplanes, and even fewer kitbuilt planes, are as old and worn-out as my friend’s Bonanza. Who knows? If the Bonanza is still flying 60 years from now, someone will no doubt need to use our knockout tool again.

That’s it for now. It’s time to get back out in the shop and make some chips!

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