20601 Old State Road Haubstadt, Indiana 47639
glenn.karch@gte.net
Have you ever been puzzled about the main bearings in the small
block (3 inch bore) Hercules-related engines? The following story
and pictures will help explain it all. The main engine casting has
two cavities for the location of the main bearings as shown in the
first picture.
When the appearance is like this, it causes some to immediately
think that it originally had poured main bearings. Not soor only
half so. Only a babbitt liner is poured into that cavity. It
provides the location for a replaceable bearing insert.
The second picture shows the babbitt bearing liner as poured at
the factory in place. Occasionally someone will mistakenly chop
that liner out, or it may have been melted out of an engine that
has been in a fire. This causes a real problem, but read on, there
is a simple solution.
The third picture shows two views of the replaceable bearing
insert. These are currently available from several suppliers. It
may be necessary to trim or scrape a little to get a good fit.
The fourth picture shows the bearing insert in place.
What do you do if the babbitt liner has been destroyed? There
are several options. A complete new bearing can be poured in place
and then fitted. Although it can be done, many gas engine hobbyists
don’t have the capability or equipment to do babbitt work. An
easier way is to first get a new set of bearing inserts and a can
of Bondo or other suitable epoxy filler product. Also needed will
be a 1 inch piece of round shafting long enough to go through the
main bearings (any material as long as it is straight), a straight
edge, ruler and a square.
Do a few practice alignment exercises so that when the Bondo is
mixed up, you will be ready. With pieces of cardboard cut to fit,
tape them to the sides of the bearing cavity to keep the Bondo in
place. Coat the outsides of the inserts with grease so they
don’t stick. Put the Bondo into the cavity and squish the
inserts into place. Be sure that there is enough room for the
crankshaft throw between the bearings and that the bearings are
even with the top of the main casting. Check to see that the shaft
is perpendicular to the engine bore. Let it set up and you are in
business. The procedure for making new liners in the bearing caps
is much the same.
The fifth picture is of the three stages of the bearing caps
(hollow, liner in place, and insert in place). The question comes
up sometimes about how tight to make the bearings when the engine
is assembled. For these slow running engines, tolerances are not
critical. I make the thicker shims out of head gasket material and
the thinner ones out of any handy cardboard. Tighten the bearings
until there is a slight drag and then back off a quarter or half
turn. There will then be slight play when the crankshaft is wiggled
up and down. That is about right to allow for a coating of grease
for the shaft to ride on. What kind of grease should be used?
Again, this is not critical. Any grease currently used for most
machinery and automotive applications will do quite well. It will
likely be much superior to what your grandfather dug out of a can
with a stick for that purpose many years ago. Hey, the foregoing
information is all free but not guaranteed. Good luck!
