[NoDOHC]

Wow! This is awesome information. I have seen the same marking on several sets of housings and I never made the hot plug area connection.

It makes sense that the housings would grow the most next to the spark plug, both because there is a more extended high temperature condition in that region than elsewhere and because the housing is less restrained (threads tend to grow more than smooth holes).

I have known spark plugs to gall in place on rotaries and that the plugs show signs of high combustion temperatures. I have also seen two housings where the apex seal left a large mark in the housing 1/2" to 3/4" after the leading plug hole (looked as if that was where the seal actually failed).

I wonder if a very small bevel around the spark plugs would cause a significant loss of cold compression. This may save apex seals from the spark plug speed bump when under boost.

Fooling with the coolant passages would be difficult, as they go through the intermediate iron and both end plates. Maybe a little porting on #5 (without getting too close to the plug).

Maybe some experienced builders can give a better number on how many housings have this damage.

I can add this as a good reason in the causes of rotary engine failure section once we have more information.

Thanks for the input.