From: jws@billy.mlb.semi.harris.com (James W. Swonger)
Subject: Re: How to use blue silicone RTV
Date: Mon, 27 Jul 1992 20:30:24 GMT
Distribution: usa
Organization: Harris Semiconductor, Melbourne FL

 RTV makes a good sealant but has poor mechanical strength. It can't stand 
on its own. It sounds to me like the instructions are bogus. Here's my 
tried-'n'-true gasket goo how-to.

                            Gaskets that don't leak

 That's the point of gaskets, after all. But most of the time it seems that
replacement gaskets do not seal as well as the originals. Here's how I go
about making a nice, tight gasket job.

 First of all, the sealing surfaces must be clean, dry, oil-free and flat.
This means every little bit of the old gasket must be removed, rust and
crud scraped off. Carburetor cleaner often works well at softening old,
hardened-on gasket material. A razor blade may be used on cast iron or steel
but something softer is advisable for aluminum pieces. After the solids are
gone, solvent cleaning will take off the oil and grease. Clean enough of the 
surrounding area that you can maintain a perfectly clean sealing surface
without new stuff getting on it. Acetone and a white paper towel make a
good finale - if it comes up clean then you're done. If you cheat, it'll
pay you back later. Do it right.

 Next, or maybe first, attach the gasket to the piece that's off the engine
(valve cover, water pump, what have you). Start by cleaning it as above.
Then coat the sealing surface and the mating face of the gasket with the
gasket goo of your choice. I prefer Permatex Liquid Copper for this phase,
because it's good and tacky and dries very strong. The object is to get the
gasket bonded tightly to the cover. If you use an RTV sealant, first coat the
surface and the gasket face with a thin, even layer of RTV (I smear it thin
enough to see through in several passes; the surface should be wetted, not
just have RTV sitting on it). Then put a thin bead down and press the cover
and gasket together. You may have to keep pressing them together until it
skins up, since RTV is not tacky. This assembly should dry fully, and meanwhile
is a good time to go clean the engine sealing surface. Before it hardens,
though, be sure all of the holes are in the right place. Also remove any goo
that has squidged out from between the gasket and the piece; it's useless and
may wander off later.

 If the component is not likely to be removed for a long time I use the 
copper Permatex on the second surface too. Otherwise, an RTV sealant makes
for easier removal. In either case, coat the two faces as with the cover and 
assemble them together. Having the gasket already bonded to the cover makes 
it easier to control things and leaves only one interface to mate at a time.
I will usually smear the cover back and forth a tiny bit to make sure the
RTV/Permatex has no voids and the surface is wetted. Then bolt the cover
piece down to the specified torque. The sealer does not replace the gasket,
it augments it by providing a chemical bond rather than just the clamping
pressure of the bolts. The sealer should be just thick enough to do this
job. Any conformal properties ought to be provided by the crush of the gasket.
In my opinion gasketless sealing is a cruel hoax. It may work for the 
factory but not once things are opened up. Gaskets can be bought for any
application and come with most parts that need them. A crush seal with an
adhesive is about as good as you can get, better than either alone.

 On thin cover pieces such as valve covers, do not overtighten the bolts.
This distorts the cover, which is the most common cause of valve cover and
transmission and oil pan leaks. If the cover is badly warped, buy a new one.
Mild distortion can be banged out against a straight surface with a flat
face.        

 

 
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