How to change crankshaft bearings
Hi, gang! Last time (December issue) I described what happened after the well-worn, somewhat corroded crankshaft bearing let go in the YS61 engine that powered my Tiger 60. It’s a dandy airplane, and I love to shoot tankfuls of touch-and-gos with it. When a few bits of rust flaked off the bearing and scratched the piston and cylinder liner, the first thing that went away was—no, not the horsepower—the reliable idle. A loss of idle reliability is often the first sign that something is wrong with an engine.
The rebuild will require new crankshaft bearings to go along with the new piston/liner I obtained from YS Parts and Service. Let’s do a how-to about changing crankshaft bearings. If you are smart (as I wasn’t), you’ll heed the warning signs that the crankshaft bearings are in the beginning stages of failure and replace them before they damage other, more expensive engine parts.
Warning signs
- A rumbly or gravelly feel and sound as you turn the propeller back and forth.
- A turbine-like whine.
- A loss of idle reliability (which can happen for several reasons).
- Visible rust when you remove the crankcase backplate.
The rear bearing typically fails first, although it is good practice to replace both at the same time. The front bearing rarely rusts like the rear one does, as a result of being exposed to fuel. I prefer to replace OEM rear bearings with stainless steel, because corrosion is the biggest enemy.
Preparation
Disassemble the engine. Take care to remove all gaskets, O-rings, and plastic parts from the crankcase, because the bearing-removal process involves heat that can destroy them.
At this point, I prefer to clean the entire engine using denatured alcohol. Not only is this helpful when taking apart the power plant, but the heat used in the bearing-removal process will bake any crud onto the engine’s innards and that can complicate reassembly.
Removing the piston and connecting rod
Removing the piston and connecting rod from most engines requires that the cylinder liner be lifted up out of the crankcase after the cylinder head is removed. With the piston at top dead center (TDC) there is room to slide the connecting rod aft and slide it off the crankpin. There is usually wiggling involved, and gummed-up oil and rust under the piston can make this difficult.
Four-strokes require a bit of extra care because of the camshaft. Take note of any timing marks on the camshaft with the power plant at TDC. If in doubt, consult the engine instructions or the Internet.
Tools and the "lollipop" driver
You need a special tool: make one from a piece of 1/2-inch-diameter wooden dowel, tapered so it fits snugly into the central passage on the crankshaft — like a lollipop. Remove the crankshaft from the engine. If it requires persuasion, remove the drive washer and collet or keyway, and put the prop nut back on the shaft so it protects the last thread. Gently strike the engine head-on against the edge of a wooden tabletop (no hammers, please!) and be prepared to catch the crank.
Make sure the back end of the wooden dowel is the right size for pushing the front bearing out of the crankcase later.
Removing the bearings
With the engine thoroughly cleaned, grab it by the mounting lug using a pair of vise-grip pliers. Light your propane torch and gently warm the bottom of the case all around for roughly one minute. Hold the case nose-up and never park the flame on one spot — a lot of heat should not be necessary. Sometimes the rear bearing will fall out on its own, but a sharp rap against a tabletop or wooden cutting block will do the trick.
Quickly (before the aluminum cools) push the front bearing out with the dowel and let everything cool completely before touching it. Some prefer to use an oven for heating the crankcase, but care must be taken not to set it higher than 300°F. Temperatures near 350°F can distort the case and ruin it. Place the engine nose-up on an old cookie sheet and listen for the "tink" as the rear bearing drops.
Reclean the crankcase with alcohol and an old toothbrush, paying special attention to the bearing seats.
Installing the new bearings
Assemble the lollipop with the rear bearing firmly in place against the crank disk. Have the front bearing ready, as well as the deep socket wrench that you will use to press the front bearing into place.
Warm the case with the torch, taking care to warm the front end as much as the rest. Moving quickly, push the lollipop home and place the front bearing onto the shaft. Press the whole assembly together using the socket against the outer race of the front bearing.
Immediately grab the entire assembly and thump it gently but firmly onto the table so the bearings are pressed against their seats. You can tap the top of the socket with a block of wood to be sure. Hammers are unacceptable.
As the crankcase cools and shrinks a few thousandths of an inch, the shaft and bearings may bind. Replace the drive washer and any washers or keys associated with it, and tighten a propeller onto the engine so it is snug but not tight. Ring the case like an aluminum bell with a wooden hammer handle; a few taps will usually free binding bearings. Repeat until the engine is cool, the propeller is tight, and everything rotates freely.
Some engines use sealed greased bearings. For those without seals (not metal shields), I like to wash the grease out with mineral spirits so that the crankshaft-binding test (described above) is sensitive. Afterward, relube the bearings with Mobil 1 synthetic oil and replace rubber seals, if any. The oil in the fuel will replenish lubrication under running conditions.
Final assembly
Reassemble the engine. Take care to correctly orient the piston and connecting rod — the hole in the connecting rod normally has a bigger chamfer on the front edge. If the piston has a ring, orient it so the ring collapses properly into its groove and carefully slide the bottom edge of the cylinder liner over it.
When reassembling, ensure the carburetor is fully seated and that all gaskets and screws are properly tightened. Nothing ruins the way a two-stroke runs like an air leak to the crankcase. For four-strokes, check camshaft alignment twice before you put on the cover.
Maybe someday I will go into more detail about the innards of our engines, but until then, have fun and do take care of yourself.
MA
Sources:
- YS Parts and Service
- (775) 267-9252
- www.yspartsandservice.com
Transcribed from original scans by AI. Minor OCR errors may remain.
