I am approaching the moment of using chemicals... Wanting to use Bleach as I am slightly more familiar with it, and guessing it is less reactive to the head gasket.. But I can be re-directed, if told that bleach is a bad idea, go straight to lye..

Any opinions as to how long to let the solution of bleach 'cook' in the block and cylinder head?

Lye is my go to parts cleaner (except for aluminum or other soft metals). It removed grease, oil, paint, rust, sealer, and other crud. I immerse the part to be cleaned in boiling water and add lye. Safety equipment is a must! It gets super hot. Don't use it on anything that will come in contact with bearings, pistons, or anything you're nervous about.

FYI- lye is used commonly to make pretzels.

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If it was easy, everybody would be doing it.

I'm more nervous about lye than I am the head gasket... so I would rather use Bleach.. (which I am not real excited about ether, but it seems the less nasty of the two substances... and ether one is better than a plugged cooling system..

If bleach will do anything helpful in a case like this, it's front-page news to me.

(Hey, raspberry juice! Triaminic™! Blue tempera paint!)

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Gotta try that molasses method. Thanks Dan.

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

Molasses works great (post 1, video, post 2) for removing rust. Won't do a thing for organic matter. What this present case really calls for is the OP to take apart his engine and do a proper cleandown job, but properly-selected chemical cleanout might stand a chance of getting things at least somewhat less clogged…though he may very well wind up taking it apart anyhow.

Ever play that game "Operator" or "Telephone" as a kid, where you sit in a circle and the first person whispers a message to their neighbour, who whispers it to the next neighbour, and so on and on until the last person speaks the message out loud, and it's always way different than the original? Near as I can tell with some cursory googling, this idea to use bleach is like that. Looks like the original message might have been that wood bleach, which is an oxalic acid solution, can be used to clean some kinds of corrosion out of cooling systems.

I could certainly be wrong, though. Will someone speak up who has actually used chlorine bleach to clean out severe cooling system blockage, with good results and without causing new problems?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

My resident chemist, Mrs wjajr, got me some Ph14 (strong) alkaline liquid to remove rust from some dash parts used on my old IH truck project. Can't say what the solution was brewed up from however, but it's the same stuff her company sells for boiler cleaner. Alkaline eats rust and not iron she explained. Acid on the other hand eats rust, and iron.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

I have not but I Googled 'Chlorine and Cast Iron' and got multiple hits that discussed the effect of chlorinated water on cast iron pipes in older houses and apartments.

But if I really wanted to remove 'organic material' from the water jackets of an engine block, I would be looking for a machine shop or powder coating provider that offered a true

hot box cleaning process.

Prior to seam welding the OE K frame from the 68 Barracuda I wanted to get the K frame thoroughly cleansed outside and in of oil and weld contaminating debris. I found a local powder coating shop that has a oven baking process, they slowly heat the part up to 500 F then hold the part there for several hours then slowly bring it back down followed by a series of agitated detergent and clean water rinse baths. They have an oven and tank big enough to do a complete car chassis. They had several sprint and dirt car chassis in process, they were stripping off the powder coat to allow an inspection of the frame for cracks. The K frame that I had processed came back to me absolutely clean outside and from what I could see clean inside, I know it welded without issue.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8