When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I have my 455 on a test stand and had run it good and well up to temperature not so long ago (over the summer) with no coolant leaks...I then drained it of fluids so as to fix various things (oil plug leak, custom stainless water neck studs for electronic cruise, leaking trans pan - but _not_ leaking freeze plugs though).
Last thing I did (August) was clock that mini starter to fit - and it wasn't leaking coolant then either.
But SINCE then, "seemingly just sitting there" (without coolant in the radiator and only residual coolant in the engine) the freeze plug in the head directly above the starter has leaked quite the little bit of coolant out of it.
The freeze plug installation is probably 15 years old. Absent banging on one of them with a punch and hammer (or similar), or running the engine through some sort of calamitous temperature event (which didn't happen), or a rust-through of the plug - what would cause a freeze plug to just start pissing coolant for "no reason" (and while not pressurized)?
Is there a superior replacement freeze plug part available, or is one freeze plug just like any other?
What sealant should be used between the head casting and the plug?
Although only one freeze plug (in only one head) has "sprung a leak" I guess I should replace all the head plugs...yes? Should I plan to replace block plugs too (also 15 years old)?
I use steel, thinking they are mechanically stronger, with a coating of ultra black RTV on them and on the holes in the block/head. Even better would be NPT plugs if you want to tap the holes. It is hard to get the tap started in the block holes, and the head has bosses in the way internally so you have to cut off the end of the tap.
You wondered why the plugs decided to leak after the block came back from vacation.
Antifreeze absorbs carbon dioxide from the air and becomes acidic. There are additives that delay the onset of acidity but they get used up (that's why you have to change anti-freeze periodically).
Anti-freeze in the cooling system has is little air contact, but when you drained the block the anti-freeze then had a large surface to absorb carbon dioxide. The antifreeze became acidic and corroded the block or the plug or both, causing the leak.
In the future, don't leave a residue of anti-freeze in anything that will be stored. Rinse it out with water. It the part is cast iron, do a final rinse with water having a tablespoon of baking soda in it, drain what you can, and allow the rest to evaporate.