I had it tested at an FBO that does prop balance and they found about
0.2 ips at the prop (FAA acceptable) and about 0.4 (+?) ips at the
other end and told me it was the engine.

R1.... .2 ips is 'faa acceptable' but any tech should be able to get it
below 0.1 ips.
Sometimes I see a cross-effect in which the rear is running big because
of a prop being installed in an angle that is not optimun. Another factor
is the static weights on the prop itself. The adjustment may resolve an aft
imbalance. What was the phase angle difference?
However in most cases it is a crank or rod imblance that is probably
causing it.



But as Ben said I
could probably do the pistons/rods myself with a good scale. But
balancing complex rotating masses like a crankshaft requires a proper
machine.

R2. You need more than a scale for the rods. A rod is suspended by each
end and in turn the big ends and the small ends are weighed, the C.G. is
deduced. In other words the rotating and the reciprocating masses are
measured. Auto guys will then make up a bobweight of nuts and bolts
equal to the rotating part which they add to the crank throws and that is
then spun on a dynamic balance machine.



Sorry, I disagree that automotive people can do a better job.
How many auto shops check the nitriding and heat treatment?


R3: The issue isn't RPM unless you are relating to 'flexible or
non-flexible rotors'
Typically Auto cranks are balanced by drilling the counterweights a little
deeper or what ever.
I can't say about Franklins, but Lycomings and Continentals do not have
counterweight molding in
the casting because the arrangement is a 'balanced' engine in the 1`st order
by design.


In the 'approved' after market, Lyc and Cont cranks are further balanced
the same at the mfg does but to a finer degree.
in my experience, it hasn't been whether is the balance is better than '3
mils' as the factory print specifies, but rather it has been making sure
you do have not a sloppy part with some 40- 80 gram-inches imbalnce that
slipped out the door!

good luck

Kent Felkins