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.