You can enlarge any of the images above by clicking. Silicone sealer (or "RTV") may have some historical legitimacy on old (particularly pre-O2 sensor equipped) American V8 exhaust manifolds and oil pans. I don't know much about cars but observation tells me it's common. In powersports however, it is among the handful of chemicals considered anathama, verboten, something never used anywhere on the engine. Instead, so-called non-hardening sealers are preferred by all the Big Four manufacturers. For very good reason.

There are only two exceptions I can think of. One is personal watercraft jet pump "ride plates", which have traditionally been sealed to the composite hull, in more or less standard boating construction practice. No harm done there, but they are a bear when they need to removed. Another legitimate use is the dirt bike guys of years ago (still maybe) used to put blobs of silicone sealer on the wheel spokes, precisely where they crossed each other. Supposedly this kept a spoke that broke or came loose from stabbing the inner tube. This is probably ancient history, but I have witnessed it. But if a career Big Four mechanic sees you use silicone sealer for any other purpose, expect to be corrected. It certainly should not but often is, used by some who don't want to replace a gasket or an o-ring and use sealer instead. Silicone sealer is hack.

There are many kinds of sealers used around engines. BMW even had one (Dirko) that was nearly epoxy-like, and actually came in the package with their cars' replacement head gaskets. And then there's Rolls Royce specified Hylomar, at the other end of the soft/hard spectrum in its never-hardening, forever tacky, chewing gum-like consistency. Different products for different uses, right? Naturally. Hylomar however has very limited applicability on motorcycles. I use it on carburetor slide diaphragms and SOHC four cylinder head wafers.

That's decision number one: the right product. But, you ask, don't Honda and the other Japanese manufacturers use silicone sealer at the factory? Yes, unfortunately, it appears they do or did, very oddly despite their specifying non-silicone sealers in their literature. Countless times I have uncovered the presence of silicone sealer on new bikes during warranty jobs.

But there is a real problem with silicone sealer, and that is it has no surface tension. The bit that squeezes to the edge of the joint won't stay there. It will fall into the oil supply, and that little bit will be there no matter how sparingly you use the stuff. Because another issue with silicone sealer is its lack of body: it is impossible to avoid squeezing it to an almost nonexistent film between the cases, with the inevitable squeezeout. Not that squeezeout isn't wanted, it is. Look at a never-disassembed Honda crankcase.

Proper sealers intended for use on engines, including the one Honda recommends, have excellent stay-thick characteristics, and the squeezed-out portions stay attached because the material, unlike silicone, is non-hardening and exhibits great surface tension. It stays tacky and stays in place, in other words. Hondabond #4, made by Threebond and actually the same as their #1104 product, is the real deal. I won't use anything else where sealers are called for.