What’s the story on the tubing and rubber grommets that were used in the battery vent and drain tube holes, in the aft fuselage belly, on the 1977 and earlier planes? What about the smaller 3/8″ tubing that extends from the lower fuselage beneath the windshield area; and below the aft fuselage area on the 1977 and later airframes?
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Technical Editor:
Here is the data I have in my personally-compiled parts listing, for the tubing and grommets. These are the ‘official’ MilSpec AN931/MS35489 grommet sizes for the required Tygon tubing OD, as listed in Aircraft Spruce and the MilSpec chart.
Tygon battery vent tubing: Large is 1” ID by 1.25” OD; small is ¾” ID by 1” OD
Tygon water drain tubing: 3/8″ ID by 1/2″ OD
McMaster-Carr Tygon ordering information:
3/8″ ID by 1/2″ OD: 5114K26; $1.51/ft in a fifty-foot roll
7/8″ ID by 1″ OD: 5554K22; $5.95/foot for five feet
1″ ID by 1.25″ OD: 5554K51; $8.74/foot for five feet
AN931-16-22: Hole grommet for 1” OD battery vent tubing; 1.375” skin hole/mounting hole; alternate number MS35489-23
AN931-20-38: Hole grommet for 1.25” OD battery vent tubing; 2.375” skin hole/mounting hole; alternate number MS35489-25
AN931-8-13: Hole grommet for .5” OD water drain tubing; .812” skin hole/mounting hole; alternate number MS35489-14
These do not match the IPC parts list for the battery vent hole grommets. The IPC calls out an MS35489-27 grommet size for BOTH battery drain holes (larger and smaller), as a supersedure for the original AN931-24S28 grommet (also shown for BOTH holes). Since the -27 has a 1.5″ center-hole ID, that means that Beech had to then cut the grommet to length for BOTH holes. I do have some New Old Stock of the Beech split grommet; but I prefer to use an un-cut grommet having the proper size.
The 24S28 suffix used to mean a Split grommet; but grommets are no longer made with a factory split; they must be cut during assembly, if required. They also clearly cannot have been all the same size, as there is a quarter-inch difference in the hole diameter in the aircraft skin. All of these grommet sizes have a groove width of .062″, which is the thinnest groove available.
Note that the dash numbers are not always consecutive when it comes to the grommet tube ID. For example, an MS35489-140 has a center hole of .125″, and an installation hole diameter of .188″. There are MilSpec grommets that have an overall OD of 1.25″, to install in a 1″ diameter skin hole, but having only a .125″ center hole. If you wish to select a grommet size for a particular purpose, and the size doesn’t seem to be shown in the Spruce listing, you’ll need the MilSpec chart. The Genuine Aircraft Hardware toolbox handbook is an excellent reference for things like this. I have also discovered that Spruce actually carries a lot of items in sizes that do not appear in either of their printed and online catalogs. In fact, the Spruce part number indexing and searching capability is some of the worst I have used. McMaster-Carr’s is some of the best.
What I typically find in the battery vent holes is a grommet that has a ‘baloney cut’ in it, so that it has the correct fit when installed. This is clearly the result of Beech having originally used a Split grommet that had to be cut to length for each hole. Using a baloney-cut rather than a straight-across cut, to shorten the grommet, ensures that the cut rubber ends will overlap to some degree when installed. It also makes getting the exact length a bit less critical. The tubing is then inserted into the grommet, effectively locking the grommet in place. With new tubing and a new grommet (so that both are flexible), the grommet can be installed with the tubing already installed. It’s a bit more difficult to do it that way when the tubing is old and stiff.