How do I replace the main gear donuts (gear cushions) on my fixed-gear plane?
(Search strings: cushion replacement, donut replacement, landing gear repair)
Technical Editor:
I am going to start on this FAQ, and expand on it over time. There will be some assumptions made. For example, that you are using the BAC pneumatic tool to set Jo-Bolts; and that you are using pre-compression fixtures originally provided by Mike Rellihan. There are other good descriptions on BAC for this process, which have worked for other members over time. Photos are also available in the Photo Gallery.
PRE-COMPRESSION, FIXED MAIN GEAR AND ALL NOSE GEARS
Warm the donuts until they are hot to the touch, but not too hot to hold. Stacking them in a box in front of a small plug-in heater works fine. The box traps the heat, so they get thoroughly warm. Just watch the heat. If they start looking shiny on the surface, they are getting too hot.
When they are ready, wipe them lightly all over with a silicone grease, such as the Dow Corning DC4 you use on oil filter gaskets. Avoid using silicone spray, which may have solvents or propellants that will attack the rubber. Slide the donuts onto the compressor fixture spacers. There is an inner and outer spacer. The inner spacer helps align the donut stack where the donuts overhang the end of the main spacer. As you tighten the stack, the inner spacer telescopes into the outer one, as the last two donuts are guided onto the main spacer.
To tighten them, you clamp the double-nut end of the compressor fixture into a solid vise (both nuts in the jaws). Get the vise as tight as you can. Make sure there is a good coating of anti-seize compound on the rod threads, on the face of the washer, and the face of the nut (where they mate). Make sure that the anti-seize on the rod threads extends under the two top donuts. Then use a 15/16” wrench to steadily tighten down the stack until it hits the stop (the spacer tube). It will hit the stop at 6.25” to 6.00”. With the pre-heated donuts, there is no need to do it in stages; just tighten it down until it bottoms out on the spacer, before the donuts have time to cool off. While the nuts and washers are Grade 8, use hand tools only; no impact guns or impact ratchets, which can cause spalling on the softer rod threads.
Once they are compressed, let the stacks cool down to room temperature; then toss them in the freezer (yours or the maintenance shop’s). They should stay in the freezer until they are ready to be placed on the compressor plate stack tube, on the gear. At that point you clamp the fixture in the vise again, remove the outside nut, and pull off the stack of cushions (spacer tube and all, if desired). Line it up over the compressor plate stack tube, slide the cushions from the spacer to the stack tube, and get the tube bolted back into the housing.
There is usually a .050” thick spacer plate under the cushions, abutting the compressor plate. If the plane came from Beech with the larger 17.5 tires rather than the 15’s, that spacer plate may be about .312” thick (about 5/16″). Whichever plate yours has, make sure that it doesn’t get left stuck to the bottom of the old donuts, and wind up in the trash.
GEAR BUSHINGS AND PINS, FIXED MAIN GEAR
While the main gear are apart is the right time to check both the compressor plate bushings and the knee bushings. All four pins should have grease fittings in the ends of them, and all four should readily take grease. If any are stubborn, the tiny .049” side holes need to be cleaned out with .032” safety wire, the pin clamped in a padded vise, and clean grease pumped through until it travels freely. The side holes are in the grooves around the pin (in the center of the bushing load area), and they intersect with the drilling for the Alemite flush fitting. There is one hole in each groove; it is usually aligned with the center hole for the Clevis pin. Those tiny side holes are the reason why the most common Aeroshell greases are unsuitable for the landing gear on our planes (in my opinion). If the grease mentions ‘Microgel’, I recommend that you don’t let a technician use it (you can show him “why”). If you don’t want to switch to a more common lithium-carrier grease (like the Kendall Super-Blu, or the Lubriplate white grease originally used on the Musketeer), then find and use Aeroshell 33. It is their latest universal grease; with a Lithium base instead of Microgel, which is powdered clay. In tiny passages, the oil drains out of the clay, the clay remains packed up solid, and you can’t get it to move with grease gun pressures. Aircraft Spruce now carries #33 grease at a reasonable cost.
If you find loose bushings (play in the pins), they should not just be reassembled that way. The bushings can easily be pressed out (and back in) with a C-clamp, bushing driver, and socket. The pins can be cleaned and polished on a buffing wheel; eccentric pin wear is seldom more than .0005″ (half of one thousandth of an inch). New bushings come from Beech with an undersize bore, so that they can be reamed to size. The ideal is for them to be align-reamed in place, using a piloted reamer (both the knee and the compressor plate pin). They are supposed to be reassembled and shimmed to .001” side clearance, which is pretty tough to achieve. The thin stainless-steel shims are an odd bore size, come in only one thickness, and are expensive; make sure none get lost when the gear come apart. Note that not all gear will have the shims, though most will need at least one when they are reassembled with new bushings.