effective engagement ?? no disrespect and don't take it that way but with only a 1/4 inch effective engagement it would be tough to draw down a valve cover with gasket with only a 1/4 inch depth of thread in the head. You need more effective thread depth to pull down the valve cover. Compression of the gasket and the bolt will bottom out. Also for example when talking about effective thread depth with a certian diameter you have to take the torque of the bolt into consideration in certian applications, for 3/8 intake bolt tapered on the starting threads and sticking below the gasket surface I wouldn't torque the bolt to 25/30 ft pds and expect the threads to hold in the heads with a 1/4 inch of bolt thread taking the load. I would be afraid the the threads would pull in the heads.

MLR426