Thickness of Mercruiser 140 Manifold at Drain Plug Hole

[myoldboat2]

1974 Harris Flote Bote
Mercruiser 140 4-cyl 181 cu.in. s/n 3888148

Hello all,

We've been doing a lot more fishing off the pontoon boat. This coming spring 2025, we're hoping to take it out earlier than usual. Here in Wisconsin, it will still drop well below freezing on some nights, so I will drain the block and manifold after each early spring outing (in 25 years, I've never gotten a drop out of the riser elbow drain).

The boat came with a petcock engine block drain plug valve, and I've never had a problem with it. The manifold drain plug is item 3 in the parts diagram, "22-68307 PLUG, PIPE - EXHAUST MANIFOLD (1/8-27)". Because the bilge is tight, the manifold drain plug is a pain to put back in after draining, so I'm going to replace it with a full-port brass quarter-turn ball valve, which will still allow me to probe with a wire. The manifold casting has a pad for the drain hole. Does anyone know the casting thickness at the drain hole? If the ball valve threads extend into the manifold water volume, then some water won't drain and will lie on the bottom of the manifold. Maybe that small amount of water is nothing to worry about; I'm not sure. The attached photos show the story.

 

[Bondo]

Does anyone know the casting thickness at the drain hole? If the ball valve threads extend into the manifold water volume, then some water won't drain and will lie on the bottom of the manifold. Maybe that small amount of water is nothing to worry about;

Ayuh,..... 'bout 1/4 to 3/8",.....If you probe the hole, 'n it drains, any bit of water left, won't be any problem,.....

 

[Candy Gramformongo]

You could remove the plug and measure the depth of the hole. Then you would know for sure.

I have two classic brass radiator petcocks on my 1976 120. I can never remember which way to turn them to open, then a wire won't go straight in, so it is hard to know if probing is doing anything. I love the idea of replacing them with a ball valve. If you wanted to go high tech, you could use a Fumoto valve and have the security of a safety clip.

 

[myoldboat2]

Thank you both.

Bondo, the ball valve threads are 1/2" long, so 1/8" to 1/4" of water could be left after draining, but the manifold is shaped so the drain is sort of a well, so it would be only very local to the drain hole. So it makes sense that even if that little bit freezes, it's not being restrained and won't cause any damage.

CGforMongo, I plan to probe the hole with a wire with a tiny hook on the end so that I'll know the thickness, but the boat is wrapped up for winter now (Wisconsin, and it's -8 F right now). I'm just getting all my parts ready to go fishing early in spring. I think the ball valve will stay closed through any vibration. Even if it did open, it'll drain more slowly than having the drain plug out, which I've done a couple of springs without issue. The seal in this ball valve is operational from -4 F to 370 F, so I don't think it'll degrade at all.

 

[Candy Gramformongo]

Where are you getting the ball valve from?