I am working on reviving a 455 in a new to me 73 gmc motor home, it has been sitting for 20+ years, so far i have disconnected the fuel line and connected a new hose going to a gas can, i have changed the oil and filter, (looked ok) i have put marvel mystery oil in the cylinders, and i am now able to turn the motor with 1 3/16" socket on the front of the crank. (previously it was very hard to turn and felt rough)
next is the cooling system, i have found it is all completely dry, and i am finding allot of rust and this hard substance:


what would be the best way to get this stuff out of here? i do plan on a new water pump and have the radiator rebuilt but i would like to get this thing started and find out what condition its in. (compression, oil pressure, etc)

 

Welcome,
Others will comment about the best products. As far as procedure, flush before radiator work to avoid clogging a new radiator, consider replacing the heater core, if that isn't feasible flush it independent of the cooling system and flush it in both directions. Hot water cleans better than cold, flush it with hot water from a washer spigot.

There are 2 9/16" hex head pipe plugs on both sides by the motor mounts, remove them for a good block flush.

Good luck and pictures please!!!

 

This looks like it could get ugly!! That’s a lot of rust. As stated before, remove the block plugs, get as much crud out of the cooling system before you attempt to flush it. Hopefully you don’t have rusted out freeze plugs or other corrosion issues. Good luck.

 

Thank you for the advice, I flushed everything the best I could, the 3 drains, heater control valve and port were clogged, and the water pump is seized.

I was able to take the belts off and get an idea on the cranking compression with marvel mystery oil in the cylinders, most cylinders are between 140-160, one cylinder was 120. These are numbers after not being run for 20+ years.

 

I've not done this; but, have read that citric acid flushes work well. They are common in MBZ maintenance.