[Bob Carriere]

After Googoling and other web sites plus Stovebolt.....It seems that the culprit to melting a hole in a piston is a lean mixture that creates more "HEAT"..........lots of it....... if sustained the aluminum gets soft like putty

A lean mixture combined with the ignition being to "early" as in in not enough advance....possible defective vacuum advance diaphragm/ poor timing ....but this would also affect all cylinders/piston......

....... but a lean mixture could also be cause by a manifold leak........

...a drop in vacuum by a leak OR by the vacuum connection on the manifold which location would be below the carb in the intake manifold roughly centered on the siamese/twin ports to cylinder 3 and 4.........

Not sure about the 216 and the location of the vacuum outlet on the intake manifold but on MY 261 it was right smack in the middle inline with cyl/pis. 3 and 4....... however when I installed my 261 in the confined engine compartment of the cab 11, the manufacturer vacuum outlet was sticking out so much off the side that the engine sheet metal cover could not be installed.

My solution was to remove the outlet, plugs up the manifold with a glued pipe plug...... and to fabricate a 3/4 thick spacer from a solid piece of steel.....spacer is same size as the phenolic spacer with another gasket. The spacer was then sandwiched between carb and manifold but removes it higher and further away from the 3 and 4 port intake..... the spacer was drilled and tapped for the wipers, vacuum gauge and one vacant for possible brake booster....... it raised the carb slightly but not an issue.

Once the sheet metal engine cover is installed. the passenger side cover is only 3/4 in. away from the red hot exhaust manifold....engine cover was eventually lined with 1/4 inch ceramic felt.

I hope this will help my no 4 piston........

Pic