Another equipment mechanic for Loctite 515 or 518. It has structural properties that keep the surfaces from moving against each other while remaining flexible. It is used on tractors to seal and secure transmission to differential housings. I use it on all the covers - primary, timing, gearbox, rocker box. Copper spray for head gaskets. Permatex Aviation Form-a Gasket for misc gasket coatings. Rich, I also used 515 as a thread locker/sealer on occasion.
Loctite 290 wicking grade threadlocker - amazing stuff. I've sealed some different things with it besides nuts and bolts. I had a rocker box oring leak. After flushing the joint with carb and choke cleaner or acetone or lacquer thinner - and compressed air - I apply the wicking sealer which soaks in by capillary action. I also have applied it around gas tank cap fittings where the cap part meets the tank to seal the joint. One of my bikes has a slight crack on a sealing plug thread. After changing the oil, I dry it with the above solvents and compressed air and apply wicking Loctite to the crack and the plug.