Re: Umbrella seals

["Bill Thebert" <bthebert@domain.elided>]

> How many miles have you put on your motor since you put 
> the umbrella seals on?

My guess (SWAG method) is that I've put on about 40-50K since the rebuild.

> I suspect you have heard the stories about excessive 
> stem wear from the use of the umbrella seals just as I have.

Indeed, I have.  Everything I've heard has been very anecdotal, however. 
No real "science" or facts in support.

> I'm not sure what bad experiences in his many motor tear downs prompted 
> son, John, to indicate that he thought that the umbrella seals would be 
> counter-productive with IH V-8's but I know for sure that that is what he

> said and meant.  

I'd love to hear more from him when you get the opportunity.  Perhaps he
could shed some additional light on this for us.

For folks unfamiliar with the stock setup, allow me to recap:

The "valve guides" are essentially bronze "tubes" pressed into the cylinder
heads.  The valves (shaped like lollipops, but with the "candy" part
attached to the "stick" in the center of one of the flat "faces", rather
than by an "edge") are insterted from the underside of the heads with the
"stick part" extending upward through these valve guide "tubes".

A spring goes over the top of both the valve stem and the guide tube.  A
flat, washer-like "retainer" goes over the pencil-sized valve step tip, and
spans the larger-diameter spring.  Two small "keepers" (each like half of a
small cone) nestle down inside the "retainer" and lock into a small groove
in the end of the valve stem.

The stock "valve stem seals" aren't really valve *stem* seals at all. 
They're tiny little rubber O-rings (square-shouldered) that fit into
another groove in the tip of the valve stem.  They reside just about where
the "retainer" and the "keepers" meet.  Their only apparent function (to my
eyes) is to prevent oil from dripping directly down the valve stem.  The
oiling originates from the rocker arms/shafts -- atop the valve stem tips.

Oil from above is forced to overflow the *edge* of the retainer, and
dribble down the outside of the valve spring -- safely away from where the
stem and guide meet --  where it then drains back into the crankcase
through holes in the heads.

This "valve stem seal" [sic] design performs no actual "wiping" of the
valve stems.  And in reality, all the moving parts of the valve train
create a fair amoint of "oil splash" -- at least a fraction of which must
land on the valve stems.

The Chevy "umbrella seals" are little cup-shaped affairs, with a hole in
the bottom.  I don't recall precisely which part number I bought -- I just
asked my trusty parts guy for "Chevy umbrella seals".

The seals install in an inverted orientation OVER the valve guide tubes. 
The valve stem passes through the hole in the bottom of the "cup".

Here's the interesting part:

I don't recall which guides were which -- but I remember that the seals fit
snugly over the guides on EITHER the intake guides OR the exhaust guides,
but not both.  On the other, they were loose enough that the seal would
"ride" up & down on the valve stem, rising up & down onto & off of the
valve guide with each stroke.  On the snug-fitting guides, the seal
remained firmly on the guide and the valve stem was "wiped" in both
directions as the stem passed through.

I performed no machining of the valve guides; and I figured that either
"mode of operation" provided more "shielding" of the stem-to-guide meeting
point than the factory seals.

I apologize for the length here, John, for I know that you did not require
the "recap".  I just wanted to assure that anyone who cared could follow
along.

I'm curious whether your son noticed any "tendencies" regarding whether it
was the intake or the exhaust guides that experienced accelerated wear in
his view.

I'm also curious (possibly enough to remove a valve cover <g>)  which
guides were the "tight fitting" ones and which were the "loose-fitting".

My hunch (and my first hypothesis) would be that the intakes were the
tight-fitting ones -- which see the most benefit from the improved wiping,
and which thereby do the most to prevent oil from being sucked in by engine
vacuum.  I would further suspect that the exhaust guides (being subject to
more heat) would be the first to fail from oil deprivation.  If these
happen to be the "loose" ones, perhaps there's still enough splash to
prevent their premature failure.

All speculation here, but curious nonetheless.

> Good to hear from you. I envied Tom M. his opportunity to wheel with you 
> in Colorado. Wish I had been there. 

Likewise, John.  Maybe next year we can get you out to Colorado!  I've
begun thinking in earnest about assembling a very small group of "hard
cores" (3 - 4) to attempt Surprise Canyon next Spring. Think you
could/would "pave the way" with George for us?  I'll even offer to let you
ride shotgun . . . <g>

Bill Thebert