Manual 23

^^^ will have to add as important info, Even thou the manual above was written 22 years ago and some of it is correct - but things do change overtime.. Hence why it should be looked at for the current updates from the OEM's and the spark plug manu's.

A good example of this was the updated spark plugs for the Merc ECM 555 engines thru OEM service bulletin, Plus the other thing was the Merc 496's original came with Denso spark plugs. Well that changed very quickly as well.

Another example is the listing above post 31 - for champion listing RV15YC4 - but currently RV9YC is the recommended Champion spark plug cross referenced in some of the marine big blocks, Again that manual was written 22 years ago not all always applies

The long reach plugs are for the 7.4 L MPI L- 29 engines as I mentioned before in a past post.