I found some info while searching the forum on spark plugs but it
didn't really answer my question. Mostly heat range related.

Has anyone used Denso W25EDR14's? They look very similar to the
NGK's that I am replacing (four electrode) but the threaded part of the
plug appears to be a bit longer. I don't want to ruin my engine by hitting
the apex seals.

Comments?

 

if they are the same ones you can get from autozone then yes they'll be fine.

 

I think I found those in a used FB once, and replaced with NGKs

B

 

The W25EDR14 is the same plug as the stock NGK,just a different brand.The Clymer manual lists them both as plugs intended for the 1st gen.If they are indeed longer,thats good as it gets the spark closer to the combustion chamber.I never had any contact problems running them in my 12A and 76 13B.

For the record,I had very good luck running the Denso plugs.They lasted many times longer when I ran my MSD direct fire setup on those old, carbed engines.With stock NGK's I was replacing badly eroded plugs every oil change.When I put in Denso plugs,after 2 oil changes they still looked fine,good edges and no porcelain damage.

 

Those are the stock plugs that came from the factory, standard heat range. This is per the 85 Owners Manual.

 

Your right.

Mazda Factory Service Manual lists the NGK and Denso plugs and their common heat ranges....

NGK BR7EQ14=Denso W22EDR14
NGK BR8EQ14=Denso W25EDR14
NGK BR9EQ14=Denso W27EDR14

 

anyone run the IRE01-27 Denso Iridiums?
im looking at these for my next spark plug change