Posted by Brian in Oregon on September 24, 1999 at 19:09:29:
In Reply to: Flathead a knockin posted by CJ Meier on September 24, 1999 at 16:50:18:
If it is a hollow sounding knock:
Take a plug wire off one at a time and run the truck.
If the knock goes away, it is most likely a bad wrist pin.
If the knock remains, it is most likely the crank end of the rod.
If it is a wrist pin, you may be able to get away with dropping the pan, pulling the head, and removing the piston and rod and having them repaired (assumming the rod and piston are not damaged.) The rod can also be replaced. If the piston, you need to be aware that when measuring a replacement, the pistons are OVAL when cold. So be sure you measure the replacement in the same exact direction as the original. (A severely bent, or rather twisted, rod can cause noise also, but usually long after the damage is done.)
If it is the crank end of the rod, most likely the crank journal will need some attention. This means a total rebuild is likely in order. Unless the journal is not damaged, which if that is the case consider yourself lucky. It is very difficult to measure a crank journal for roundness in a vehicle.
You might also make sure that you are not getting piston slap from being to far advanced with poor octane fuel. Back the distributer off a wee bit (retard it closer to zero) and see what happens. The large open combustion chambers of flathead do not like too low of octane nor too much advance.
If it is more of a tap, like tapping a hammer on the block, then it might be a valve needing adjustment. Or it might be a lazy valve that is slow to keep up due to a combination of a weak spring and gummy fouled guides, or possibly a very slightly bent valvestem.