Correct. The only way to correctly set the timing is
a timing light.
As to WHY #3: Make sure a thermostat, or at least a
restriction plate, in used in the thermostat housing.
You need this to insure proper pressure in the engine,
otherwise with too fast of coolant flow and localized
hot spots, it will leave an steam pocket against the
engine. When ever I see #3 or #4 piston burned, check
the cooling system.
#3 tend to run hotter than the rest of the cylinders.
With lots of corrosion in the system, typically you see
#4 fail first. Those cylinders just run a little hotter
than the rest.
ALWAYS confirm the timing does not exceed 35 advance
BTDC @ 4,000 rpm (all advance should be in). The number
is not set in stone, but will vary with all sort of
factors (as listed in the distributor FAQ). Most important
is to realize that engine compression, and water temp play
a big part of this maximum setting. Higher compression,
and hotter water temp, both require less TOTAL advance.
Also check the total advance on #3 as well as #1. A bent
distributor shaft will have different timings as they will
open the ignition points at different times. Little things,
but could easily be off by a few degrees per cylinder.
If you notice your timing mark, with a light, is jumping
around. Sign of a worn out distributor. Also something to
be addressed before doing any more work.
You can easily set the old engines by ear for knock. Chevy
and Chrysler motors run just fine with this method. Sadly
the Datsun's will have incipient knock long before it becomes
Typically it is not just one item, but a series of little things
that all add up to burn a piston.
FOR RACING: Cheap insurance is a EGT (Exhaust Gas Temperature)
measurement of all four exhaust pipes. The temperature reading
will give you an idea of each cylinders performance.
Then again, I am one to talk... as Rodney is still hiding in
the garage. ;-)
Tom Walter '68 2000 Rodney