I'm pretty sure these are my block drains, but what is on them? A temp sensor, or a knock sensor? I'm assuming just pull the brass plug to drain my block, on each side? I'm going to winterize later on this month, but I'm trying to get everything all set up so I know exactly what to do.

I'm not positive, but that looks like an oil pressure sender based on the location.

Yeah that should be your knock sensor. To winterize it do just like you said pull the brass fittings off and let the water drian out. While it is out might be a good idea to probe in there w/ a small screwdriver to see if there is any rust/gunk stoping the water from flowing out. No need for a cracked block.

Knock sensors are usually up on or near the heads, but??? My guess is that it's the temp sensor that makes your beeper beep if your too hot...

I agree with FR: that's a temp sensors. It's definitely not a knock sensor.

Thats what I thought, a temp sensor....but why would my 5.7Gxi have three? One on each side of the block, and one on the thermostat housing?

Blockage on one side could overheat one side. Help shut down before too much damage is done. Also, the ECM might be monitoring temp differences which could help determine how much fuel is delivered to help control combustion temperatures.

True. Who knew my boats ECM would be more advanced than my trucks. As far as I know I only have one temp sensor on my truck.

Im speculating. I don't know for sure. Also, I bet the truck has more than one temp sensor. But, what kind of company uses a MAP system instead of a more efficient MAF???? wtf?

Back to boats now.

On my 1997 Mercruiser, it has one of those on the starboard side. And it is most definately a Knock Sensor. Not a temp sensor, sender, oil sensor, or oil sender. You know how I know? Cause I unhooked it, ran the boat without it, and oil pressure guage operated, so did the temp guage, and the motor reached full throttle and ran normally.
It's a knock sensor.

I know there are multiple knock sensor designs...but the ones i've been around fit flush.

Without a shadow of a doubt, it's a knock sensor.

The experts have spoken!

Why would a knock sensor be mounted so low on the block and have a plug in it as if to drain a fluid?

The cylindrical shape of the body looks like it might be holding a spring loaded diaphragm to operate a set of contacts.

Look at parts 64 and 26.

The diagram shows the connection for the knock sensor up by the manifold/head area. The sensor in the photo is down by the oil pan.

the plug is there to drain the block, the knock sensor is mounted there as a matter a convenience it just needs to be attached to the block to work. It really doesn't matter where.

Dumb question here..... not to hijack the thread, but what is a knock sensor?

It allows the ECM to advance the timing as far as possible until microscopic pinging start, then it retards it just enough to stay at that critical, most efficient point.

Thanks D!

Here more info on how the actual sensor works: http://en.wikipedia.org/wiki/Piezoelectric_sensor

And that fitting makes it a little harder to drain the water from the block, as the wire you poke in there might need to have a curve in order to get the trapped sediment out. Glad I only have one on my Mercruiser.
FC, I worry about that brass plug. Hope it comes out easy, or you are gonna unsrew that whole fitting when you go to drain the block. That is one area where Mercruisers little blue or black plastic wing nuts are very handy to have.

Knock sensor makes sense. Thanks guys. I'll be spending some time today poking around my engine and figuring it all out, it sure is different than my carbed 4.3 V6 I had on my Sea Ray. And this one really is down in there, talk about a PITA to reach anything!

I miss those little blue plastic wing nuts I had on my Mercruiser. This one will be a PITA for sure. The knock sensor/drain on the starboard side can't be seen. I will have to feel around to find it.

Weird place for a knock sensor, but that's what it is! They DO also put a temp sensor in the same place, but it's much different looking. From Marine Parts Plus website, edited for better detail:

Brass plug came right out. Just tried it to be sure. This motor won't be hard to winterize at all. Do I need to coat the brass plug with antiseaze or loctite so it won't stick?

I would not recommend any kind of anti-seaze or loctite for a brass fitting or plug.

I always wrap some Teflon tape around the threads of any plug or fitting that threads into the water jacket of the block or manifolds as protection against leaks.

Thanks Admin. I did not think of teflon tape. I should have though, I have a 10 pack in the garage!

I'd buy one of those blue Mercruiser plugs!

The merc plugs are nice until you overtighten them or they stick in there and you break the wingnuts off. Then you have to get in there w/ a screwdriver or something of the sort and hope you can back it back out. But other then that I like them.

Part #? I haven't had any issue with them breaking off on my last boat.

FC, when they break, they lose all clamping/holding force, so removing them with your finger tip is possible. Ask me how I know! But if a screw driver is available, it makes it very very very easy to remove them, when they break.

I believe it. I take it they should thread right in? Have an idea on cost or a part #?