Be careful buying used distributors as those old parts could have worn housing bushings, causing the shaft to be sloppy. This can cause erratic timing and dwell,,, or those settings bouncing around and not being stable. If you can wiggle the shaft in the housing and feel the clearance, it's too much. Installing electronic conversion kits may very well make the shaft slop a moot point though......
Also, be aware that Mercruiser had a 165HP 4 cylinder engine, and of course that distributor won't work for your engine(s).
