ive had a 228,248 and two 235's that didnt over heat but ran warmer than i felt they should have. i have pulled the pet cocks out and nothing came out with the block full of coolant. i was able to drive a 3/16" rod in as far as it would go which was about 5-6". when i pulled the rod out nothing followed it. it took some doing but with persistance i was finally able to open it up and get it to flow. with numerous flushings it finally ran out clear with no chunks. the 59-235 that i just had rebuilt and intalled in the dump didnt flow from the hole wit the plug removed. after two weeks of trying i finally pulled the head. it was solid all the way across and one third of the way forward. it took me another two weeks to finally get it all out of there with some secret boiler cleaning chemicals. yeah they can hold a lot and still appear that the cooling jacket is clean.
for simplicity i dont put the petcock back in. instead i plumb it with a short 1/4" pipe nipple with a full port ball valve with a pipe elbow on the end of that topped off with a pipe plug in the elbow should the valve should come open by accident. it makes it real simple to drain it into a bucket without it making a mess plus you capture all the stuff that comes out for inspection. its not original looking but it very functional.
