Ron, you were right the first time. I Just went and checked some manifolds I have and the hole in the pass side manifold is for the EGR. Not all manifolds have this hole because not all SC's had EGR. However, all pass side manifolds have a flat boss cast into them that is drilled and tapped for EGR when required. I, like you assumed the EGR hole was tapped for a pipe thread. However, after looking closely I don't think it is pipe thread. The EGR adapter screws in and bottoms on a tapered seat in the manifold (similiar to a spark plug seat that uses the tapered seat and no gasket). If it were pipe thread it would be tapered threads and the plug gets tighter and seals on the threads the deeper it's screwed in. Now, the only question is what size are the threads, and if metric or standard. I don't have any way to check those size threads to find out for you. Take the manifold or the EGR adapter to the parts store and fit and try until you find the right plug. Even then you may have trouble making the plug seal if it doesn't contact the seat right. Alternatives would be to use the EGR adapter and weld the hole to make a plug, or retap the manifold to a pipe thread and use the proper pipe plug. I would think there are a lot of members here who have deleted EGR and plugged that hole. How about it any advice on the correct plug? John