7/16
1.94
3/16
M3414M
1/4" MALE
3/8" MALE
3/8" MALE
1/4" MALE
3/8" MALE
3/8" MALE
GMM-4M-675
CM-4S
MD3614M
MD3616M
MD3616M
1/2
1/4 STUD
1/4
1.94
2 3/4
1/2
7/16
1/2
3/16
3/16
2 1/4
1 3/4
3/16
2 1/4
STUD
3/16"
FEMALE
5/16"
FEMALE
1/8" MALE
1/4" MALE
1/4"
FEMALE
MM-3
CM-4M
F3514M
F3414M
PART NO.
MW-3M
7/16
5/16
3/8
7/16
1/4
3/16
1.94
1 1/4
3/16
3/16
1 3/4
1 3/4
THICKNESS
0.31
FIGURE 1: ROD-END BEARING REFERENCE CHART
BEARING HOLE
3/16
LENGTH
1 3/8
5.27 FLUID FITTINGS
PIPE THREAD FITTINGS
Many fittings used in RV plumbing systems have tapered pipe threads. Tapered pipe threads have an
outside diameter which decreases toward the opening. Therefore, as the fitting is threaded into a
normally threaded hole, the clearance between the two diminishes until the fitting becomes tight. To a
large degree this interference fit is what provides the high pressure seal. Some fittings have 45 or 90
degree angles which require positioning (clocking) in a specific direction and may prevent them from
being turned to the fully seated point (though this should be done on all straight fittings that allow doing
so).
NOTE: There are a few locations where the fittings used have straight threads (usually fittings or
sensors installed on engines). In these cases, some type of crush gasket or O-ring is used to
provide the seal (no paste or liquid sealant is used). One obvious indicator of a straight thread
fitting/sensor is that it can easily be fully threaded in until it bottoms out in the hole.
All of the threaded fittings used in RVs are NPT (National Pipe Thread), which is a U.S. standard for
tapered threads used on threaded pipe and fittings. There is often confusion regarding the size
designations because it pertains to the nominal inside diameter of the fluid passage, not the outside
diameter of the fitting.
Two methods are provided for tapered fitting identification. See Page 05-29, Figure 1 for full scale
drawings or refer to Table 1 for use in identifying a fitting's designated size. To use the table measure
the maximum outside diameter (OD) of the tapered thread, locate this value under the "Actual Size
(OD)" column, and read the corresponding NPT fractional value from the "Designated Size" column.
5.26 (continued)
PAGE
REVISION:
DATE:
VAN'S AIRCRAFT, INC.
PAGE
REVISION:
DATE:
09/24/13
05-32 RV-ALL
0
Actual Size (OD)
Designated Size
0.405 in. [10.29 mm]
1/8
0.540 in. [13.72 mm]
1/4
0.675 in. [17.15 mm]
3/8
TABLE 1: NPT SIZES
Because we cannot always fully tighten tapered thread fittings, and because even after fully tightening
the fitting a small spiral leak path remains along the full length of threads, a thread sealant must be
used during assembly. Two sealants popular for use on aircraft are Tite-seal and Permatex #2. Teflon
based pipe dopes and sealants, and some of the anaerobic thread sealants are also used by some
builders with success. Do not use Fuel Lube. It is not a sealant. It is meant for lubricating moving parts
in fuel valves, etc. Teflon tape is also not recommended. Small pieces of this tape may be cut by the
threads, become loose, and cause all kinds of problems in aircraft systems. Teflon tape has even been
the cause of engine stoppages.
When installing the fitting, be sure the threads on both parts are clean and dry since most sealants will
not tolerate any oil contamination. First determine what the clocking position needs to be by installing it
finger tight and marking the desired clocking. Remove the fitting and apply a small amount of sealant to
2 - 3 threads but leave the first 2 bare to prevent contamination inside the fluid path. Remember, this is
an interference fit so not much sealant is required.
Thread the fitting in with your fingers until you just begin to feel resistance and then an additional 1.5 - 2
turns. This is a general guideline… you must still use judgment to not over tighten and damage the
threads, but a properly installed fitting is quite tight. If the installation requires a specific clocking, when
approaching the correct position you must determine whether you will be able to make another full
rotation and still be within the 1.5 - 2 turns stopping range. You must avoid turning the fitting backwards
in the loosening direction because it will have a high probability of leaking. If you must do this, it is best
to completely remove the fitting, clean up the threads on both parts, and try again.