I seem to see planes with too tight a gap between the brake disk/brake rotor and the main gear fork. What’s the story on spacers to set the clearance correctly?
Search strings: wheel clearance, brake clearance, fork interference, disk interference, rotor hits the fork, disk hits the fork.
Technical Editor:
Here are some 1.5” axle spacer part numbers, for the 600×6 wheels, that I know about:
169-810000-9 Main gear wheel inner spacer, 2.00” long; for original Goodyear brake disks
169-810000-27 Main gear wheel short inner spacer for Sierras with Goodyear brakes (no data yet)
169-810000-131 Main gear wheel inner spacer, 2.10” long; for Cleveland brake disks
169-810064-1 Dual-diameter inner spacer for use with SI-1168 compliance
All of the above have a 1.51” ID. The first three have a OD of 1.625”. The last one has smaller OD on one end than on the other (no definite data yet).
While not normally a part of the spacer set-up, the standard Cleveland stainless-steel 600×6 grease seal washer is P/N 153-00300. This ‘washer’ clears the 1.5″ axle, and is .030″ thick (about 1/32″). If you remove your existing spacers, and slide one of these up against the fork, then reinstall your spacers, it will add .030″ clearance for the disk. That’s probably enough to add to anyone’s comfort level, and it should still allow insertion of the cotter pin in the nut. These washers are available from Aircraft Spruce; I keep some in my shop for local use on flown-in planes. Spruce info follows:
153-00300 Ring-Grease Seal 1 1 2 1 1 $5.50
It is a generalization, but indications are that:
The -9 was used with FG nose wheels, and main gear having Goodyear brakes.
The -27 was a short inner spacer used with the -9, on early Sierras having Goodyear brakes.
The -131 was the longer spacer used with Cleveland brakes, both original and conversions.
The 64-1 is an odd-ball spacer having a different OD at each end; the wide end is intended to better stabilize the wheel bearing.
There is a spacer, referred to in the IPC as a ‘washer’, which is P/N 100951S125WL. It has a 1.51” ID, a 1.745” OD, and a width of .118”. If your current inner spacer is the -9, you can add one of these thin spacers between the long inner spacer and the fork, and can usually still get the cotter pin into the hole and nut. But if you already have the -131 spacer, adding another one may move the nut out too far over the cotter pin hole in the axle. There is also a ‘washer’ P/N 100951S063WL, which I believe to be a thinner version of the 125WL, but I have no definite data yet. The original application of these spacers/washers was against the outer bearing, immediately under the big axle nut.
One can only imagine the grief that this conglomeration has caused some owners, as shops unknowingly mixed and match axle spacers (‘parts is parts’). The key aspects are:
The brake disk must clear the fork, and there can be no ‘rock’. Disk to fork contact must not be tolerated.
The spacers, inner and outer, must form a solid stack-up against the wheel bearings (not the grease seals or felts); when the wheel bearing is adjusted; the wheel must not be able to rock.
The cotter pin hole must still line up with the nut castellations, to allow pin insertion.
This will be updated as I find more of my missing data. None of the above information applies to the 500×5 nose gear wheel on the Sierra retracts.