Roll pins, last time around

[McNeon@domain.elided]

I stated my opinion on roll pins in v7#623. Then Jaap Bouma Emailed me thus: 
"Well, in MY humble opinion, it's not water pressure that would cause the 
collapse of the o-ring, but the squeezing action of the block and head 
surfaces as the head is torqued down. Without the roll pin the o-ring is 
unsupported on one side, which would allow the o-ring to slip out of its 
recess into the oil gallery.

Mr. Bouma brings up an interesting point, and John Katos in v7# 625 seems to 
support his theory. I've assumed that there is no fluid pressure behind 
(outside) the O-ring, and that may not always be a good assumption. 

Typical O-ring applications use the fluid pressure being sealed to force the 
O-ring to seal. This assumes that the O-ring is pliable and can be forced to 
deform enough to seal the opening (which they are when new). Once the O-rings 
become hardened through too many heat cycles, though, it seems entirely 
possible that the oil could find its way around the O-ring. Then, there is no 
pressure differential and no (or insufficient) sealing.

The other unresolved point is the amount of flow through the oil passages in 
the head. I believe it is very small and the restriction of one or more roll 
pins is insignificant. Too bad someone can't measure that, though we do have 
clues (see Russ Neely's posting in v7#625).

Bob McKeown

------------------------------