Further question on vacuum pump

[KSMITH1@mailgw.sanders.lockheed.com]

Hi fellow Audiphiles:

While lubricating the infamous vacuum pump push rod on my 1988 90q, in 
expectation of reducing/eliminating the well known rapping, a question 
occurred to me, which I pose to you:

The push rod seems to run on a cam lobe, and drives a hexagonal shaft 
on the vacuum pump.  This shaft works against a spring that is at 
least as hefty (technical term) as a valve spring.  It would seem that 
in order for rapping to occur, particularly at idle, a clearance gap 
would have to exist between the push rod and the shaft.  Otherwise, 
the stiff spring would force the cam-following push rod to follow the 
cam contour without any slamming.  If such a gap does exist, why does 
lubrication have any effect?  If not, what is causing the rapping that 
the lubrication cures, at least temporarily.

I used GM engine oil supplement (used for bedding cam shafts on 
rebuilt engines) on the push rod and the noise seems to have gone 
away.  However, after reassembly, it occurred to me that the flat that 
exists on the push rod might want to be "up" to allow the engine oil 
that undoubtedly impinges on the rod inside the head to more easily 
cover the entire rod.  Anyone have knowledge about this?  The new 
o-ring should be good for another assembly if the rod orientation is 
important.

***                 ...Kirby    (Kirby A. Smith)                 ***
***                      2 X 1988 90q                            ***
***              ksmith1@mailgw.sanders.lockheed.com             ***
***              [=]   kirby.a.smith@lmco.com                    ***
***  Opinions expressed herein are entirely those of the author. ***