Thread size on the blue plugs?

https://cimg6.ibsrv.net/gimg/www.off...ec7b09206e.png
https://cimg0.ibsrv.net/gimg/www.off...de73465f88.jpg
the blue plugs have an o-ring seal don't think its a pipe thread, the adapter they screw into is npt into the block,

I mentioned before a couple of times however removing plug and installing 1/4" pipe to 6 AN push loc and run hose over top of enge and secure somewhere convenient with a coupler. Makes iit extremely easy to drain once installed. If you do get some blockage just put a little air to the hose. Nice if you wanna flush your blocks from time to time also. About 25.00.

can use small socket to help for reach to undo and a very easy install as well. . These are the second most drain plugs I have installed next to the T handles I posted above. IMO really need nothing else to have for drain plugs and they are all brass. I also install these in tranny water side drains. https://cimg7.ibsrv.net/gimg/www.off...433295d6e0.jpg

BUP - do you know anything about the "season extender" setup on the newer Merc I/O's? Could it be adapted to older models?

Have experience with them all -- every single one from all marine manus. Volvo Penta has the best in the industry. The Merc air actuator / blue hand held air pump --- down the road the drain points leak and at times does not drain all the way. Anyways here is Volvo s most current one called EASY DRAIN Its been out for a tad over 2 years and won industry awards

Here's what I did. I installed a AN style fitting into the block and then attached about a 2 ft. length of hose to it. The other end of the hose is secured where I can easily reach it and has an AN style fitting with a cap on it. My thoughts are that I now have the ability to suck water out of the block or push antifreeze into the block if I want. I can also hook up an air compressor and blow air through the block. With this setup you don't have to reach any valves nor are there any valves to rust, break or stick. It's less likely to clog because the orifice is bigger and if it does clog I can clear it with air although a wire will likely still go down the hose as well. I have no idea if it will work as I haven't tried it, but it is installed and ready to go. Here's the oil drain and block drain next to each other at the top of the engine.https://cimg2.ibsrv.net/gimg/www.off...0885a3e2d9.jpg