Dave

  Thanks. You mentioned perhaps the biggest reason a housing with leading
  dual plugs should be designed from the get go. That is the force of the
  combustion event against the threads. This explains why the current
  housing plug boss design is so robust. It also explains why there is a
  little half moon ridge near the plug holes on the outer housing.
  Beefing up the housing to account for loss of strength near the large
  breach no doubt. I suspect but haven't confirmed yet that there is a
  stainless boss welded to the outside of the liner near the plug hole.
  That would make sense because the flame propagation starts there. Nut,
  screws and bolts !%#`!

  Next idea. Common ground Y shape Siamese twin plugs that fit into the
  present 14mm hole.

  One less hurtle or one more concern

  Doug in Japan

  Sorry Doug but wrong! The robustness of the plug boss isn't to protect
  the threads. It is because you need a castable boss there and to
  reinforce the side of the ROTOR HOUSING against the HOLE being put
  in it. The threads are fine, in fact engines seeing higher combustion
  pressures than we will EVER see, AA fuel dragsters running 90% nitro
  use standard plugs, just very cold ones.
  Bill Jepson

Hi Guys;

Are you really sure that the boss is there for structural reasons?
Then, why not complete the boss circle?

BTW, some people run the RX7 just fine on leading [lower] plugs only.
I may give that a try this summer.

Cheers
Cary

There is a possible cooling problem with side by side plugs.
The lower plug region is the hottest part of the rotor
housing.  You could over temp the material between the two
plug bosses due to inadequate water flow. The NSU spider engine
cracked excatly where the spark plug boss was.

Paul Lamar