Friday, November 24, 2006

VW - Gaskets II

A week or so ago we were discussing my recent success at making gaskets. You mentioned that it was easy to make gaskets if you had the proper material for the job. I guess you mean there are different types of material for different parts of the engine. Care to elaborate?

Gasket material comes in different thicknesses, a distinction obvious by inspection. You’ll recall that your oil pump cover uses an extremely thin gasket whereas your exhaust flanges use ones of metal, wrapped around asbestos (yes, you can still get asbestos gasket material... if in a kit manufactured in a foreign country). The object of the thin material is to provide the minimum possible clearance between the ends of the pump’s gears and the cover plate, hence the need for a thin gasket.

Most gaskets are meant to seal the joint between two parts for the life of the assembly. The best material for gaskets of that type is a kind of glue that hardens when exposed to pressure and heat. Any thermally-induced motion between the parts is accommodated by the compliance of the gasket material. The gasket not only seals the joint, it bonds to both surfaces and must be scraped away when the assembly is dismantled for overhaul. Gasket scrapers are standard tools in any mechanic’s kit.

Often times the joint in an assembly must retain oil yet allow for some degree of motion, induced either mechanically or by metals having different thermal coefficients, as in the case of the Volkswagen crankcase and cylinders. Such gaskets fall into the group of ‘elastomeric’ seals, as do shaft and tranny seals. Another gasket type is one intended to permit the parts to be dismantled frequenty, as is the case with your valve cover gaskets. For that task you want a material that will compress to form a seal but will not harden nor bond to the surface that must remain free. The usual procedure is to glue valve cover gaskets to the valve covers and leave the other surface free to form a seal with the cylinder head.

Your axle boots are another form of gasket, as are the boots on your tie-rod ends, although when the gasket is a molded elastomer, such as neoprene or silicone, we tend to call them ‘seals’ rather than gaskets, but as you can see the primary fuction -- keeping something in while keeping something out --grease and dirt in this case, gasoline and air in others, is the primary role of gaskets. And seals.

The gasket used under your carb should be made of a material that will compress but will not bond. But once compressed the stuff tends to stay that way, meaning you need a new gasket each time you dismount your carb.

Some gasket material is permeable, allowing oil to pass through it. The cheap cardboard gaskets for the sump, as found in oil change kits, are like that. Such gaskets have a place, but not in a Volkswagen engine. The only reason they are there is because they are cheap and everyone expects a VW to drip. It doesn’t have to be like that, as I pointed out in one of my sermons. In theory, it’s possible to machine surfaces to such a fine finish that they need no gasket at all. The VW comes close to that goal with its engine case but still requires a coat of sealant along the joint to keep the oil in and the dirt out.

Your window seals are gaskets of a sort, as is the seal around the windscreen or the bas of your radio’s antenna. Gaskets (or seals) exposed to ultraviolet light must be made of material that will withstand UV degradation. And so it goes. Different tasks call for different gaskets, some thick, some thin, some soft, some hard, some flame resistant and so on.

Just as there’s no on sealant that can do every job, neither is there such a thing as a universal gasket material, although paper, in all its forms, comes close. (Leather was once the most common gasket stuff, and is still used in a surprisingly wide variety of applications; some leather seals flew to the moon.) Oil impregnated wood is still used for many shaft-sealing applications. Indeed, it’s hard to find a material that hasn’t been pressed into service as a gasket or seal. I once patched the hull of a dory with pieces of cotton fabric and varnish. The cotton fabric came from a ladies skirt. (Okay, it wasn’t exactly a gasket but definitely a seal.)

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