During these quieter times, I am playing with this 1971 4.2L engine, I want to make sure I get all the crud out of the cooling passages. Any suggestions as to how I can remove the plug that is on the side of the block without breaking the spanner? It is 15/16, my next step is to buy a 1/2" drive impact socket - I do not want to break the standard socket in my kit.

 

I would leave it alone, as they are virtually impossible to remove after all this time in there. The plug covers the tapped hole for the block heater sometimes fitted by owners who live in very cold climates. It is perfectly possible to clean at the back of it, (I did one in the late 80s); just use a brush like one uses to clean out test tubes or whatever.
Something like this: -
https://www.toolstation.com/fischer-...SABEgIvbvD_BwE

 

If it is not leaking leave it alone, you run the risk of snapping the bolt head and then ?

the block's drain bolt is on the opposite side, near the rear of the block.

3.8 liter blocks and early 4.2 liter had a "faucet." where the bolt is. They used to leak so I suppose they went to the bolt with a copper seal.

 

Take all the core plugs out, they are worth replacing anyway as they are likely to be rusty.
When you buy a 1/2" socket set get a good one with 6 sided sockets.

 

It can be removed, but the process is brutal. And, as already said, not likely to do any good.

Remove the Welsch plugs, aka freeze plugs and scrape out the detritus that way.

Carl

 

Thanks all. If you look carefully at the photo you will see that all the welch plugs have been removed. I have had the pressure washer on all the openings to blast out the rubbish, but some will be hiding behind this plug. The problem being that when the studs are pulled, if any of that detritus falls down the stud hole, the new stud will not seat properly down. Also I am aware that removing corroded studs can sometimes be effected by working through the welch plug holes, but what if it is the stud behind this plug?
As Fraser says, I may have to leave it in. When the block is fully stripped I can turn it upside down and give it another blast.

 

When I rebuilt my 4.2 engine in the late 80s, I removed all the long studs, although I think I left them in when flushing the block out. I used wire and long thin brush to get the crud out. However, to make sure the tapped holes were clear, I had a tool made up of a plug tap on the end of a long T-bar that went down the stud hole through the coolant jacket and into the stud hole. I also made up a vacuming tool to vac out each hole first, then ran the tap down gently with some grease on it to grip the crud. All my new studs then went in perfectly.