Mercury small outboard flush plug thread size

[iansan5653]

Does anyone know the thread size and threaded part length of the flush adapter kit / flush plug used with the following motors:

• 4 hp through 5 hp 2-cycle Mercury or Mariner outboards
• Mercury or Mariner 2.5 hp through 3.5 hp (F2.5, F3.5) 4-stroke outboards
• 4 hp through 6 hp 4-stroke outboards (serial number OR411837 and lower)
• 8 hp through 9.9 hp 4-stroke outboards (2005 and newer)

The adapter kit part number is Quicksilver 16841Q02 and the flush plug part number is Mercury 803592. They have the same threading.

I am trying to replace the plug with a thumb screw to make it easier to flush, but McMaster shipping is expensive so I'd really like to buy the right part on the first try.

[thomm225]

Why don't you just take the plug (bolt/screw) off the engine and match it up with a thumb screw at the local hardware store or Lowes etc?

I always worry abut losing mine when I remove it as the engine seems always be over the water when I remove it for the yearly engine fresh water flush.....

[iansan5653]

Originally Posted by thomm225

Why don't you just take the plug (bolt/screw) off the engine and match it up with a thumb screw at the local hardware store or Lowes etc?

That's my backup plan - I was just hoping someone would have that info easily accessible. I'm afraid that in the process of going to the boat to get the plug, taking it to the hardware store, trying it out, bringing it back to the boat, and reinstalling it, I'm inevitably going to lose the tiny thing.

Originally Posted by thomm225

I always worry abut losing mine when I remove it as the engine seems always be over the water when I remove it for the yearly engine fresh water flush.....

And that's the #1 reason I started on this quest! I actually want to buy a couple of backup plugs and washers for when they end up at the bottom of the marina. Especially because I flush mine every time I use it. It's such a terrible design.

[thomm225]

Originally Posted by iansan5653

That's my backup plan - I was just hoping someone would have that info easily accessible. I'm afraid that in the process of going to the boat to get the plug, taking it to the hardware store, trying it out, bringing it back to the boat, and reinstalling it, I'm inevitably going to lose the tiny thing.



And that's the #1 reason I started on this quest! I actually want to buy a couple of backup plugs and washers for when they end up at the bottom of the marina. Especially because I flush mine every time I use it. It's such a terrible design.

Yes it is.

I usually give my engine a "nice" saltwater flush several times a week and one every week or two in Winter plus the yearly fresh water flush. I dock my boat bow first so engine is far away from fresh water spigot

A guy across the way flushes his 9.9 every time after use.

My engine is 10 years old. 2011 5 HP 4 stroke

https://www.youtube.com/watch?v=cpZhq35etcw

[srbaum]

So, I have one of these outboards on my trimaran. I got tied of installing a flushing fitting, so I came up with a permanent plan. Now I can flush my outboard after each use, without extra hassles.
I purchased a Yamaha Jet Ski flushing hose ABB-FLUSH-JD, purchased on eBay and installed a plastic water hose ball valve to it. Open the valve, connect a hose, flush, then remove the hose and close the valve. I leave this assembly in place.
Do coat the treads of the brass flush fitting with Tea-Gel or Lana-Coat, to prevent dissimilar metal for contact as the engine is aluminum.
PS, the outboard is a Tohatsu and they rebrand (decals only), as Mercury, Evinrude and Nissan.

[thomm225]

I still think the easiest way is a nice salt water flush unless you are one of those folks that doesn't use your boat and outboard very often then you would need to do the fresh water flush.

For the last 5-6 years, I have done 5-6 fresh water flushes. I do that when I change the oil and every other year the impeller.

[iansan5653]

I may be misunderstanding what you mean by a saltwater flush - if you are literally flushing with saltwater, what's the point of flushing at all? The whole reasoning behind it is that you are clearing salt deposits out of the cooling system to keep it clear. When you run the motor in saltwater you're already 'flushing' it with saltwater.



Anyway, to answer the original question it's an M8x8mm screw. Unfortunately McMaster has no affordable 316 stainless M8 thumb screws (I passed on the $40 one), so I'm just buying a 5-pack of hex socket head screws instead. I'll still have to use a hex wrench but that's easy and at least I'll have spares.

For the washer, I found a ten-pack of fluorosilicone washers. I could buy a 50-pack of plain silicone ones for the same price, but I don't know what I'd do with 40 more silicone washers anyway so might as well spring for the fancy ones.


I like the permanent hose fitting idea but decided to keep it simple for now. Might do that later.