Quite often, a biker will, when
he starts building, have to spend a lot of time and effort looking for a
rigid frame for his scooter. It may be because he has a swing arm that he
wants to pull the engine and tranny out of, or that he wants to build a late
shovelhead-to-rigid chopper, or simply because he is starting off with just
a pile of parts including an engine
Locating a rigid frame isn’t hard, so he
buys one, takes it home, goes
through the long and involved process of molding, painting, ect., and then,
too late,
finds out that he has a knuckhead
frame and that the rear jug of his panhead
engine just wont fit in the frame.The language at this point usually isn’t
something you would want your mother to hear.
It is possible to fit a panhead engine into a knucklehead frame,
but
it takes some cutting, grinding, welding, filing and
the use of a big hammer. On another
day, we’ll have to take a look at just how it can be done. On the other
hand you can slip a knuckle engine into a pan frame with only minor changes,
if that happens to be what you are planning on building. If it’s a
panhead you’re building, though, the hot setup is to make sure the
frame you are going to buy is a panhead to start with, so that you won’t
need to do all that surgery.
There are several differences between the pan and knuckle frames, but most
of them are rather subtle and hard to spot. Some are "sometimes" differences,
like the straight front downtubes. The knuckles have straight tubes’ while
most of the rigid pans have "bowleg" tubes. However, the early 1948,
late’56, and ’57 pan frames were also straightbars. The one sure
way to tell the difference is the top motor mount. A look at the pictures will
show that the top motormount on a knuckle is a straight-sided rectangle tab,
while the top mount on a panhead is a sort of a trianglar thingie. If the top
mount is missing, or looks like it might have been changed for one reason or
another, a final check involves the front tubes. If it is a straightbar and
there is a lock built into the neck, it’s a pan. If it’s a straightbar
and there’s no lock it’s a knuckle. If the neck has been molded
and you cant tell you got troubles.