Mike---I don' think the valve springs would've been too strong. And the
likelyhood of them bottoming out would only happen if the head was
milled to where the assembler lost all clearance when setting the lash.
Other symptoms would've showed up before the pushrods bent. And why only
three of them?
The mechanic who is doing the work has the advantage over those of us
who can only offer ideas on possibilities of what happend.
You reported that the "compression is good all around, and no valve
damage". To get a reading, then the valves must be operating, unless he
used a leakdown test, where all valves must be closed.
"One cylinder blowing back thru the carb says that at least one intake
valve was open! There's a missing piece here somewhere.
In discussing this with other listers, it would appear that something
must've galled, either in the guides or in the rocker shaft at pivot
points, enough to bend some pushrods. Let us know what the tear-down
shows!
Dick
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