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I have started to reassemble my M38 engine. Where should I, and where should I not use gasket sealant?
What type(s) of sealant are recommended?

I have worked on vehicles for as long as I can remember, I also teach automotive technology at a vocational school and I only use sealents at junction points such as at the corners of the oil pan and timing cover . Most gaskets are designed to be installed dry unless other wise stated in the instructions, I have worked on some vehicles where someone has used sealents and found that the gasket has slipped and caused an oil leak. I hope this helps.

Most gaskets are designed to be installed dry unless other wise stated in the instructions,

Hill is absolutely correct. I have been a mechanic since 1960. The bottom line is follow the manual. When a sealant need exists it will be published in the manual.

A word of caution on head gaskets. Unless you know for a fact you are using the brand and PN gasket the factory originally used then follow the gasket manufacturer's directions for their head gasket.

Gaskets like the pan gasket and side valve cover gasket must be dry and both surfaces of the block and cover must be clean and dry or these gaskets can easily walk out as you tighten the fasteners.

Some don'ts:
Do not use tape type sealing products on fuel or oil line fittings. These can work their way into the system and plug screens up. There are thread sealants available in the paste type that are meant for this purpose.

Do not use RTV type adhesives and sealants where they are exposed to fuel or oil. Again they find their way into the systems and plug things up.

Ok, all understood. I'll be careful with the sealant, that's for sure.

What about the crankshaft oil seal in the timing cover? Does it need sealant to keep it leakproof? I can't find any text about this seal in the engine manual.

But don't forget you need sealant on bolts & studs that go through to the water jacket.
Head bolts/studs, exhaust manifold studs, t'stat housing bolts etc.
Tom

Try to remember the factory service manuals are designed to be used by experienced mechanics. There are many steps in a job that are not explained in detail in factory service manuals because they are an industry standard and it is assumed the mechanic is trained and experienced and familiar with the maintenance standard procedures.

To answer your last question, first lets see what type seal you are referring to. It is a Garlock type seal often with a metal support or a rubber coated metal support frame or a sealant coated metal support frame. The seal is a lip type and often has a tension spring included to hold the lip seal tighter to the shaft. These seals must be installed with the lip pre-oiled and if no rubber coating or sealant coating on the metal support frame then apply a very very light film of Permatex #1 or 2 to the outer circumference of the metal support ring before you install it.