How do I replace the main gear donuts (gear cushions) on my retractable-gear plane (Super Rs and Sierras)?
(Search strings: cushion replacement, donut replacement, landing gear repair)
Technical Editor:
You will need three compression fixtures. These can be borrowed from BAC (by BAC members); but I have also included instructions on how to easily make them (below).
The nose gear fixture can utilize quarter-inch thick end-plates (4″ by 4″ square, or 4″ round); but the Sierra main gear fixtures must be made of at least 3/8″ thick steel. I now use 3/4″-10 threaded rod, in 12” lengths. You’ll need six 3/4″-10 coupling nuts, six Grade 8 nuts, and twelve Grade 8 washers (high strength, hardened steel, usually gold colored).
You will need 6″ long center spacer-guides made of 1” OD aluminum (outer tube), and a 6″ inner tube that will telescope within the outer tube. Proper 6061-T6 tubing is available from Aircraft Spruce, with wall thicknesses and ID-OD combinations that enable a nested set to fit over the 3/4″ rod. The exact 1” smooth OD is important for the outer tube; DO NOT try to get by with 1″ threaded rod. If the cushion ID is not kept to at least 1” during pre-compression, over a smooth surface, you will have a very difficult time transferring the frozen cushions onto the gear stack tubes.
When you do the donut installation, make sure that the steel or aluminum spacer is still on the stack tube, against the compressor plate.
To do the pre-compression, heat the cushions evenly until they are uncomfortable to hold. Not hot enough to burn your hand, but too hot to keep holding. A small portable electric heater blowing into a box standing on end, with the cushions stacked up in it, works well. If the cushions start looking shiny, they are getting too hot in front, and need to be rotated. They need to warm up for a couple of hours, to make sure they are hot all the way through.
Once they are hot, you can compress them non-stop, without having to spread it out over days. Just coat them lightly with silicone grease (like the Dow Corning DC4 used on oil filter gaskets; don’t use silicone spray unless it is a food-grade spray which has no harmful petroleum content), slide them on the center spacers, and crank them down. The inner telescoping spacer guides the two end cushions onto the outer 6″ spacer, as you tighten down the nut. The stack is automatically at the correct length when the nut stops, due to the center spacer length. Then toss them in the freezer. Leave them in the freezer until you are ready to install them. This will give you ten or fifteen minutes (nose gear), and about five minutes (Sierra main gear) to get them off the fixtures, onto the stack tubes, and get the nuts tightened to the correct length, while placing no stress on the aluminum stack tube (and its retaining pin in the landing gear compressor plate).
In addition to the telescoping spacers, the BAC fixtures have a coupling-nut set-up on the ‘handle end’. The coupling nut provides a good surface for holdng the fixture in a vise.
IMPORTANT NOTE #1- Be sure to put some anti-seize compound on the rod threads where the moving nut will be; and on the washer and nut where the nut turns against the washer. This reduces the turning force required, and keeps the rod threads from galling. DO NOT USE an air wrench or impact gun, or you may seize the nut on the rod.
IMPORTANT NOTE #2- The Shop Manual is in error on the correct stack height. You must leave the nose gear stack tube nut so that the top tube thread barely clears the plastic locking collar in the nut (by the minimum single thread). Otherwise the nose gear tire is almost certain to rub on the hydraulic lines, in the left side of the nose wheel-well. For this reason, I highly advise you to get a new nut for the nose stack tube. They are a standard AN364-1612 or MS20374-1612 low-height elastic lock-nut, in the one-inch size. They take a 1 and 7/16” wrench. If your shop does not have any hardware this size, I can send you one. If your main gear nuts are all rusty, I can send all three; you definitely don’t want a corroding nut to seize on the stack tube. They are a bit hard for me to find, so I only replace the one on the nose, and any rusty ones on the main gear. The Shop Manual says to set the initial main gear stack height to 6.5”. If you do, you will not have sufficient tire clearance on the forward edge of the retracted main gear tire. That’s probably what happened to Mark Weiss in Australia, when his main gear jammed in the wells following gear cushion replacement. The main gear stack height needs to be set to 6.25” to 6.30″ instead. This does not hurt anything or affect anything, and it will give ½” to ¾” clearance at the front edge (and at least one inch on the aft side).
IMPORTANT NOTE #3- The nose gear stack tube is supposed to have an abrasion washer, a flat washer, and the big nut. In some cases the abrasion washer will be missing. It needs to be replaced if it isn’t there.
IMPORTANT NOTE #4- New AEC donut composition and or size will effect compression, Do not over compress your main gear donuts, splitting may occur. 6.30″ or 6.25″ is the max.
GEAR BUSHINGS AND PINS
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” or .041″ 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. 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.
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) to one-thousandth of an inch, which is fine. 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. That is pretty tough to achieve, unless repair bushings are mad with the flanges machined to the exact width needed. The thin stainless-steel shims offered by Beech are an odd bore size, come in only one thickness (.012”), 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 from Beech.
NOSE GEAR PIVOT BEARING/BUSHING
The nose gear needs to have the main pivot bearing examined and re-shimmed. Do a BAC search on ‘nose gear play’ for advice on that subject.