1981 Mercury 7.5 2 stroke

[chrisgt]

A slight leak in a crank seal doesn't usually damage these smaller outboards. The scoring is probably caused by being started up cold and run straight to full throttle without a warmup.

Have you run this thing under load to see how it actually performs??

You can only see a small percentage of the rings, they tend to stick more on the exhaust side, especially if it's a crossflow engine (you can tell if it has a weird shaped piston)

Not all engines came with thermostats in them, it could have been like that from the factory.

 

[Texasmark]

A slight leak in a crank seal doesn't usually damage these smaller outboards. The scoring is probably caused by being started up cold and run straight to full throttle without a warmup.

Have you run this thing under load to see how it actually performs??

You can only see a small percentage of the rings, they tend to stick more on the exhaust side, especially if it's a crossflow engine (you can tell if it has a weird shaped piston)

Not all engines came with thermostats in them, it could have been like that from the factory.

Smaller engines probably so, with only 2 cylinders. On larger engines a thermostat forces cooling water over the top of the top cylinders. Without the stat, they can overheat where there is no cooling water at the top as I read herein and what makes sense. Same thing on a car/truck/tractor engine.....engines that have them are designed for them. Take them out, like in the summer when your equip. overheats, you are doing it a disfavor...find the restriction in the cooling loop, fix it and keep the stat.

 

[Texasmark]

the 7.5 and 9.8 only differed by the carb.

PSI was 120 ish on my 9.8's

run some ring-free thru the motor first to see if the compression comes back

There are skeptics everywhere that condemn snake oil. I maintain all my equipment and have over my 83 year lifetime and that includes cars, PU trucks, OTR trucks, Boats through 125 hp, tractors through 110 HP, all types of 2 and 4 cycle equipment that are required on a farm, and just recently a Kawasaki Prarie 360 that sat for several years with fuel in the carb. Snake oil doesn't have to prove itself to me. The right kind, in the right place, at the right time, with supporting equipment cycling as needed works......no brag, just fact!!!!

 

[racerone]

Issues with these 7.5 / 9.8 HP motors---Poor impeller design.-----They do not have an overheat warning system.-----They do not have overheat shut down.----Engine must be taken apart to replace the lower crankshaft seal.---------Replace crankshaft seals when you have it apart.--Replace upper and lower ball bearings.-----Check reed valves and labyrinth seal--Replace plastic washer at the top of the water tube.

 

[keithb7]

I removed and tore down my 7.5 tonight. These things are so simple. Easy to work on. The whole experience was a pleasant. One of the easiest engines I’ve ever pulled and opened up.

The cylinder walls look great. No scoring seen. Some mild scoring on the rings.




The crank seals were pretty hard. The bearings, I can feel wear when spun on my ginger. Not a ton but enough. They too will be replaced. Does anyone know the generic bearing number for the crank bearings?

I found corrosion at the lower seal area seen here:




I didn’t stop and smell it before wiping it up. Could be old gas, but I was running the engine last weekend. Maybe the engine was incorrectly stored before I bought it? Water weeped into the crank and sat there? Pistons appear decent. I’m planning to hone, re-ring, new seals, bearings & gaskets.






Reeds appear tight and decent. Any comments are welcome.

 

[racerone]

No surprise that seals were crispy.----Bearing is a 204 I believe.----Good on you for taking it apart.----Now you know it will be good for many years of service again.

 

[Texasmark]

Before you reassemble it, get a cylinder hone and run it up and down at a slow rate making X designs on the cylinder walls....just enough to rub the glaze off....be sure to wash out the dust. The purpose of this is to allow the new rings to wear off slightly and seat against the cylinder walls. I'd use Lubriplate 105 to grease up things prior to assembly. 105 is an engine rebuild lube that is thicker than motor oil but thinner than GL2 in the tube. Most auto parts stores around here stock it.

My first engine rebuild was a '49 Mercury V8. Not knowing any better, I put it together "DRY"........you just don't want to do that!!!!!!!!!

Lube all moving and to be contacted parts (like cylinder walls) using a lube that the running engine will use to lubricate itself until enough fuel mix enters to take over with the lube process and will wash out the reassy grease.

 

[keithb7]

My local marine dealer had everything I needed stock. I’m impressed. Clean up time and reassembly next.

 

[racerone]

These older motors are far better quality / easier to repair than newer 4 strokes.

 

[keithb7]

@racerone earlier mentioned changing the upper water tube plastic washer. I was wondering where this was. Found it. Thank you!

 

[keithb7]

I have a couple questions:

The small holes in the tops of the pistons. Are they possible lube holes to piston pin? I’m doubtful but want to be sure. I can’t seem to spray aerosol lube down thru them.


Next Q. What’s suggested method to get the upper main crank bearing out of the housing?
A micro side hammer type tool? Heat outer housing? Theres a cages needle bearing in behind it, each bearing impeding the others removal. Thx.





Thanks folks! Going good so far. It’s cleaning and degreasing day.

 

[racerone]

Those " holes " in the piston are machining centers.-----Absolutely nothing to do with oiling of this motor !----Lube holes are clearly visible on the piston.---Wrist pin has needle bearings.-----Heat helps remove bearings.----Perhaps a review is needed on the concept of 2 stroke lubrication.

 

[racerone]

Place bearing housing on a tin can.-----Ball bearing down.-----Heat housing evenly with propane torch till bearing fall into the can.-----Gently tap needle bearing out while housing is hot.

 

[keithb7]

MHeating from crank bearing housing worked perfectly. Thx. I completed powerhead reassembly yesterday. Today I’m sourcing some material to make the exhaust block-off plate. I’ll pressurize the assembly and monitor for possible leak down.

I noticed less sprung piston ring movement when testing rings thru intake inspection plate, after reassembly. To me, a likely good indicator the old rings were well worn. Or gummed up behind rings. I could have measured ring gap too I suppose. I did not.

 

[racerone]

This test would be overkill in my opinion.

 

[keithb7]

Leak down test shows the new intake port inspection cover leaking. Tightened it again still leaks a small amount. I’ll remove the gasket and spray some Permatex Copper Gasket aerosol on both sides. Reinstall install it again. Minor issue but might as well do up right.


Another interesting point. Air will bleed out the spline end of the crankshaft center. Where the drive shaft inserts. Covered it with rubber and a hose clamp. It sealed up. The crank seal here is fine. Seems odd to me. Is there an explanation for this? Second guessing myself as it is my first time doing this. Will go back, research and review.

 

[keithb7]

Leaking air out center area here.

 

[racerone]

On some of these that hole is sealed by the driveshaft and the O-ring.-----Look in there.----Turn the crankshaft.----Tell me what you see.

 

[keithb7]

I’ll take a closer look in there later this evening. Looking at the Mercury parts web site, they only show a felt washer in there. I do have that. Its in place. #4 in the image below. #2 is the lower crank seal. My engine does not utilize #3, some type of spline adaptor.

Perhaps a little mixed gas/oil is pushed into this spline area to keep from it corroding? There’s numerous little internal passages on the block. Just grasping at engineering possibilities.

 

[racerone]

????----I can nor see or touch your motor.-----I have numerous rebuilt and parts motors like yours.----On some years there is a hole straight into the crankcase.----Plugged by the driveshaft and sealed by an O-ring.