RE: Roll pins

["Mark Battley" <m.battley@domain.elided>]

>I have 2 points about roll pins.  Friction loss in fluids have 2 basic
>components.  One is from the size of the restriction.  The other is the
>length of the restriction.  The length will cause the greater losses. 
>The roll pins are short, even if they were doubled thsi would not effect
>the oil supply to much.

Actually there is another significant component which is that due to the
entry/exit losses at the change of section, i.e. as the flow enters and
leaves the roll pin. As the second roll pin will be hard against the first,
the entry/exit pressure losses will still be (more or less) the same as for
a single pin. The relative magnitude of these losses compared to the
frictional losses in the roll pins depends on the flow rate, diameter of
oilway and roll pin, surface finish, oil viscosity etc., but it seems likely
to me that the losses (pressure differential) due to two roll pins will be
less than twice that of one.

I fitted roll pins to mine following a head gasket failure: and there was
definitely a significant increase in oil pressure (about 10psi more when
cold, on a motor which already had good pressure). Is this typical? Probably
not a bad thing for the bottom end of the motor, but it did make me slightly
nervous about how the supply to the cams might be affected. But if Alfa
Romeo approves, who are we to argue!

Mark Battley
Auckland, New Zealand.

1974 Alfa Romeo 2000 GT Veloce

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