I have been using Permatex Ultra Black and Ultra Grey for the last ten years sealing both 2 and 4 cycle engines. The only problem I had was when I first started using it and that was my fault. I wasn't allowing the product to cure properly before putting into use. Many of the Husqvarna/Poulan clam shell chainsaws are put together with the Permatex Ultra Black which is exposed to 2 cycle fuel mix when in use.
As I said at first even the 4 cycle OHV valve covers would leak within a few months when I didn't wait for the product to cure. Now I wait at least 12 hrs before final torquing and the adding of oil or fuel mix to the system. I know the label say 30 minutes but the 12 hrs seem to more inline.
Yes Max Honda and Yamaha do have their own Hondabond and Yamabond products for sealing the crankcases but I haven't tried either yet or had a need to.
