NOPE, don't do it. Years ago I had my engine "cleaned" professionally using plastic media. Shards of hard, brittle, thin triangular plastic. The stuff isn't abrasive and can not harm the metal. It just removes old paint, grease, stains and old gasket material and anything that's on the metal surface. It won't remove heat discoloring on steel, and isn't very effective on iron oxide (rust). I made the mistake of glass-beading a valve cover, and the stuff absolutely, positively without a doubt MUST be thoroughly cleaned out of any threaded holes! Same goes for that black beauty stuff which is only steel slag in powder form. If you're not going to disassemble the engine, the plastic media will work as long as you mask ALL possible ingress points, shaft seal rubber, and any place where the media can get past the covers, like the starter cover, points cover and valve cover. Leave the covers in place when blasting, and get yourself some rubber stoppers to plug up the intake and exhaust ports too.
