The original plugs are Denso. there is no difference from the ones installed in the 8.1 truck motors at the local GM dealer. I do not recall the part #'s but there are other alternates, and the part #'s listed in the previous posts sound like the same ones I had come accross a while back when I changed my plugs. I chose to stay with the Denso plugs and got them through a local GM dealer at less than half of the Mercury price. These plugs generaly do not need replacement often, however if you experiance the same problem with the boost pump inlet screen being clogged causing the high rpm miss you may find the plugs will be carbon fouled, and want to chane them. I would also also suggest adding another fuel filter in line before the boost pump. Mercury has incorperated an inline filter to resolve this problem, but the price, as I recall is about $45, and it is a throwaway unit. We installed Racor canister type filters and mounted it to the coolant bracket on the starboard side of the engine. It fits and looks good, and it is a better filter, replacement cost for the filter element is about $10 after the initial price of about $60 for the whole unit.