Page last updated July 17, 2008



Mercury Outboard Powerhead Removal

I have several pages entailing the gasket replacement process for these particular Mercury Outboard Models (650 - 700).

In order to replace some of those gaskets, you need to remove the powerhead. I will try to lead you in the right direction, but please realize, some engines no longer follow this design. If you need better instructions, you may be better off purchasing an owners manual / maintenance manual.

NOTICE - THIS PAGE IS NOT COMPLETE BUT A WORK IN PROGRESS - I will add more pictures in a few days.

WARNING - older outboards tend to have frozen headbolts. Should you really be messing with headbolts if you don't need to? Well.........if you absolutely need to replace blown head gaskets or repair a thermostat, then you have no choice.
If your engine is old and you have no reason for opening exhaust covers, then you should stay away from that. If you break a bolt, then the repair could get expensive and timely. If you have a slight leak on an exhaust cover, it is only water; just make sure you shield all electrical from the leak, make sure the moisture or leak has an exit hole or drain and wipe up after each outing. These engines use water jackets and moisture gets into bolts and causes corrosion.
If you have a newer engine and you don't know when the last time the exhaust covers were removed, you should replace those gaskets and make sure you place anti-seize on those headbolts to prevents problems in the future. That would probably be your best bet.

First off, you want a workspace available for the powerhead once it is removed.

Some manuals also indicate to have a powerhead puller / lifter. Basically something to lift the engine up. I was able to lift the power head off the lower unit, and I way 170 lbs. You may or may not be able too. It is probably best to have a friend available to help you. If the motor is still mounted on the boat, it will be hard lift powerhead up since the engine is already high off the ground.

I removed a 70 hp engine off the boat and placed onto a motor dolly with a friend helping. It was old, difficult and heavy but can be done. Newer, more powerful engines may be heavier.
If you have doubts, don't attempt it. Get an engine crane or rig one up. Higher HP or newer engines may weigh more, so attempt at your own risk. If the engine falls on you, it will seriously injure you. MAKE SURE

You may also purchase an engine crane from

Have a plan how you will position the powerhead once it is removed. All plugs should be gone. You can safely lay the powerhead on the plug side assuming it is flat.

If it is not flat, you can rig a jig out of scrap 2 X 4; build the jig so that it will support a certain side flat so that you may work on it. You can build it before you remove so that it is ready when you remove powerhead.

This will also allow you to rotate the powerhead to remove exhaust covers, fuel pump, etc. You can also remove those items before you take powerhead off.

Removal of lower unit- You can get the engine a little lower so you don't have to lift engine so high if you remove the lower unit. The removal is not required on the Mercurys I was working on but if you have a different motor, just in case, it is probably best to remove it. (Prop is attached). You should be able to observe a few bolts holding it in place. Make sure you remove trim tab as it also holds the driveshaft housing and gearhousing together. There is also a bolt underneath trim tab that also holds the pieces together. Remove all other bolts and watch out for falling pieces as you remove the lower housing / gearcase. There is a plastic piece that guides the gear shifter onto the lower housing. It is important that you know how this piece is mounted in case you need to re-assemble.

With lower gearcase gone, and spark plugs gone, proceed to removal of the power head.

If you plan on doing a complete breakdown, might as well try to remove as much weight from the powerhead before you attempt to lift manually. Take starter off, carburetors off, anything that will lighten your load.

To use the crane, you will also need to get a flywheel nut with ring in order to secure the engine.

Once the engine is on an engine dolly, you can loosen the nuts in the mid section, inbetween the upper and lower sections. They surround the engine. Underneath the cowl, you will see two more, make sure you remove those.

Keep in mind, the gear shifter and water tube will remain attached to the powerhead as you lift so you will need to make sure you don't damage those parts as you lift the engine. The gear shifter is attached to a rod by means of a cotter pin. If you carefully inspect the shifter, you can see and remove the cotter pin, but you will need a new one to replace it with. Use a punch to get it started going in one direction and then grind a nail down flat and use that to finish the removal of the cotter pin. Once the cotter pin is gone, the rod shuld slide out of the driveshaft housing. NEVE RE-USE COTTERPINS.

If you have the engine supported by an engine crane, then the lower unit should slide down and damage to the shifting rod and water tube may not be of concern.

When all nuts have been removed, you may need to lightly tap lower unit with a rubber mallet to loosen corrosion that has built up.

Lift the engine either by hand or crane and if the lower unit does not seperate, and you are sure all nuts have been removed, lightly tap with rubber mallet.

You can also use a chisel or screw driver to seperate the pieces (see 2nd pic). My engine had so much corrosion, it took some prying to get the pieces seperated. When you

have an opening, try to use something made of plastic or wood to jam in-between the pieces and apply pressure evenly to front and back to seperate. You have to lift a little to remove nuts underbeath front of engine

Once 2 nuts are gone, the two pieces may just seperate without any additional work.

Lift it up watching out for water tube and shifter. Be careful with those two items, they are easily bent and would expensive to replace if you crack them. I lifted my engine up in the tilt position and removed it while I was inside the boat. It was not terribly heavy and I was barely able to transport the powerhead to my work table without any assistance, but have someone just in case (this was a 65 HP model). Watch out for the flywheel, try not to grab the flywheel in order to transport it; it moves!

You can place the powerhead on table and go to work. If you decide to replace gasket that surrounds the camshaft, you will need to pull the flywheel off. For that, you will need a flywheel puller. Make sure you mark the flywheel and ensure the timing decals are in place. Once it is off, you will be hard pressed to find the correct way to put it back on. More to come on flywheel later.

Exhaust cover gasket - This area has 2 gaskets and two plates; one gasket inbetween each exhaust cover. If you have a thermostat, this has a rubber ring that needs to be

replaced as well as the housing that it sits against. Those may or may not come with your gasket set.

There are 3 bolts that hold the exhaust cover in the center that may give problems (see pic to the right). Both Mercury Outboards that I worked had the same problem. Bolts were solidly corroded in place and broke off easily. Be careful, apply some WD-40 or similar item and maybe you'll get lucky. Water circulates around these bolts and so they are exposed to lots of moisture.

Be careful. If you break them, you will need to bore them out with a press or better yet, take to a professional to bore out. They have the proper equipment for the job.

Also, inspect all parts that are removed for hairline cracks. The exhaust cover to the right had a hairline crack

on the upper left bolt hole. It was over torqued and cracked the cover. Luckily, I already had a replacement cover. You may also want to sand the cover until it is flat with some sandpaper. Glue a sheet of sandpaper to some thick particle board (the smooth type) and slide cover over it until all high spots are gone. Through the years, the covers warp due to heat. If the cover is warped, you may or may not get a good seal when you try to install a new gasket .

The fuel side of the Merc 700 has 3 gaskets. It requires that you remove all electrical parts to get to them, and if that area leaks, might as well replace gasket do be on safe side. The hardest part is removal of all components in order to get to the gaskets.




Try to replace all bolts, especially if it is an older engine. If not all bolts, then any bolts that seem to be weak / corroded and have cracks on threads. I have found Carquest to have the best inventory and selection when it comes to replacement bolts. Some stores limit you to packaged bolts and are not marked with their strength. Carquest sells them individually and about 25 cents per bolt vs. $4.00 for a package of 4 at other automotive shops. Make sure you get the same SAE grade bolt. If you have a 5, replace with at least a 5 or stronger; try to avoid generic bolts you may find at Lowe's and Home Depot and replace any bolts that are not marked with at least a 5. Remember, these are prone to corrode...stainless steel bolts might be a better option. The engines I was working on had a mix and match of stainelss steel and regular bolts. I prefer the non-corrosive type. The bolts do not require much torque to do their job.

The hardware store bolts will have them in the specialty racks and they will packaged and expensive for only a few bolts.


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