Below the motor mounts right down by the block pan rail. They are 18 mm heads for a socket or you can use an 8mm allen that they accept in the center of the plug head.

Best Regards,
Ray @ Raylar

I think I have found the brass drain plugs and they seem to take a large hex drive to remove, 17-19 mm, so I picked up large sized set of hex sockets. Plan to drain, fill with water and a flushing solution from Napa, run engines, allow to cool, drain, then fill with DexCool.

Does anyone have a trick to preventing the coolant from being pumped into the lake by the bilge pumps? Space is a bit tight my 400 SS. I do have an electric water pump to possibly pump the bilge into buckets before bilge pumps kick in. Or I could try to capture the fluid as it comes out.

Thanks.

In a 400 SS I doubt the 5 or so gallons that come out will be enough to trip the bilge pump. Just run a wet/dry shop vac in the bilge after you drain the coolant of each motor.

Good tip, thanks. Almost done. But a marina sold me 10 gallons of dex cool for both engines. That looks to be enough for almost 100 percent dex cool versus 40 percent water solution. Just fill each engine with 5 gallons and top off? Really strong concentration will not hurt?

Strong concentration will not cool as good as a 50/50 mix.

Apparantly there is a merc brand that is pre mixed. But this is not. So i need to dilute with water.

BTW, the drain plug requires a 17 mil hex driver.

Be careful, the big 1-1/2" threaded plug on one side of the block is not one the drain plugs, but a big core plug for removing the sand from the original casting. Removing it will not drain all the coolant from the block passages. The actual drain plugs are right down above the oil pan rail joint at the bottom of the block on both sides and an 17mm socket or appropriate metric allen wrench in the internal hex opening will remove them. It takes about 4.5 gallons of coolant to fill each engine and the coolant bottle to the correct level. Use a 50/50 mix of Dex-cool if using full strenght Dex-cool. If you have been using Dex-cool or good mixutre of antifreeze coolant all along there really is no need to flush out the block as there will be no rust, residue or scale unless your system has been improperly filled or maintained in the past. Do not use descalers or caustic flushes in this system as they can damage gaskets and seals that should not be exposed to them.
Do not over engineer a coolant change in a Merc 496 closed coolant system engine. You should be able to just drain and install new coolant and be on your way. Try not to make this simple service more difficult than it needs to be.

Best Regards,
Ray @ Raylar

I got a question, how often should the AF be changed in the 496? I have a 2003, but it only has 80 hours on it. Is this something I should think about just because of the age of the AF in the block?

That's a good question...I have a 2006 (496 HO) with 60 hours.

5-6 seasons on the AF tops. If it's got a metallic looking color, (darker) than bright pink, it's time.