Perhaps in the perfect world, O-rings always make good seals on their own.
I have used plenty of them in scientific instrumentation as well as in
various repair jobs around the house.

I would also agree, that if the mating surfaces are in pristine condition,
the O-ring alone, if made of the proper elastomer for the liquid and/or gas
in question, will hold tight if the joint is properly tightened.

In the case of putting back the original cooling system components on a new
long block, the mating surfaces on the parts that must be reused are not in
pristine condition.  Therefore, there is a possibility that the O-ring will
not make a perfect seal, and there will be a coolant leak.  Thus, it makes
sense to guard against failure with the proper gasket adhesive/sealant.

Since the adhesive/sealant I chose has, once cured,  high heat resistance
and high resistance to the solvent properties of engine coolant, and high
adhesion to the mating surfaces, where is the problem?  If the O-ring make a
perfect seal, the adhesive/sealant is along for the ride.  If the O-ring
does not make a perfect seal, the adhesive/sealant saves the day.

Also, please note that earlier in my career I was one of the sr. R&D
managers at a major adhesives/sealants company.  I know first hand what
happens when such products fail.

John

----- Original Message -----
From: "George Goff" <THX0001@AOL.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Saturday, September 03, 2005 10:08 PM
Subject: Re: o-ring cooling crossover tube

> In a message dated 9/3/05 9:27:24 PM, rrecardo@WEBTV.NET writes:
>
> << Caterpillar uses, and suggests their own high tack on every o-ring
> joint
> on any of their engine's. >>
>
> Esteemed Sir,
>
> Is this not only to keep the o-ring properly seated during assembly rather
> than to act as a sealant?  Inquiring minds want to know.
>
> George
>