Johnson Service Bulletin SB-818 - 18 HP Cylinder Assemblies

December 8, 1960. Johnson Motors explained how to distinguish cylinder assemblies 376814 and 377643, including differences in tapped mounting holes and water-jacket passages.

Johnson Motors service bulletin letterhead
December 8, 1960
Bulletin No. SB-818 Cylinders & Crankcases
SUBJECT: 18 H.P. Cylinder Assemblies 376814 & 377643; 376814 used on Models FD & FDL-11, 11K & 12; 377643 used on Models FD & FDL-13, 13R, 14, 14B & 15.
When the thermostat controlled cooling system was incorporated in the 18 H.P. engines a few years ago, several changes had to be made in the casting die. These changes altered the physical appearance of the cylinder assembly and a new part number was assigned (377643).
The same die is now also being used to make cylinder assembly 376814. Consequently, this cylinder now has the same outward appearance as cylinder 377643. The similarity ends here, however, as there are many important differences in machining of the two assemblies that effects their usage.
In Fig. 1 the cylinder marked 376814 has only five tapped holes in the mounting surface. There are seven tapped holes in cylinder 377643 in the same surface. Obviously the cylinders cannot be mounted on the wrong lower units. In addition, you can see the drilled hole in the water outlet of 376814 while this spot is blank in 377643.
Figure 1 comparing five tapped holes on cylinder 376814 and seven tapped holes on cylinder 377643
FIGURE 1
SB-818 Page 2
The difference in the water jacket surrounding the exhaust ports is also readily apparent in Fig. 2. Notice the difference in the drilled water jacket and tapped holes between the two cylinder assemblies.
Figure 2 comparing the untapped blank water jacket of 376814 with the tapped and drilled water jacket of 377643
FIGURE 2
Another quick check of the two castings will also disclose the different drilling of the water transfer opening which leads to the thermostat (not shown). One (376814) is blank; the other (377643) is drilled out to allow water to flow to the thermostat when it is only incorporated in the water system.
This is only one example of parts that are similar in appearance but different in function. Always check your service parts thoroughly to be sure they are correct for the particular application. Do not assume they are wrong because they look different and resemble new parts but not the old part they are to replace. You can avoid lost time and eliminate confusion by closer observation and inspection.
JOHNSON MOTORS
W. W. DeWees
W. W. DeWees Service Department
WWD:eb
NOTE: Post on your FD series index - 18 H.P.

Related reference

Compare the two cylinder assemblies by part number: 376814 and 377643. The photographs and captions are reproduced from Figures 1 and 2 above.