OS plugs are overpriced.  Fox, K&B, McCoy, and others make plugs that are as good (unless the heat range is wrong) and cost a bunch less.  Not saying OS plugs are bad, just overpriced.  For example a Fox Gold will replace an OS #8 in most engines.  K&B 1L or Fox RC if a slightly cooler plug is needed.  Might want to try the Thunderbolt plugs from Hobby People as well.   The Thunderbolt four stroke plug is hotter than an OS F.