Cylinder Scoring

[mbruce333]

I've got a 79 85hp motor with a melted piston and other damage that I'm working on.

I'm gonna swap the power head with a 75hp block that runs great and has good compression-125lbs on all 3 cylinders. I originally bought the 75 to use for parts, so I don't have a history with it. Last night I decided to pull the cylinder head (on the 75) and found some scoring on #3. I'm not sure of exactly what I'm looking at in terms of fixing it up.

From what I've read here and what makes sense to my little brain, I think I can hone the cylinder, (maybe with one of the good ones from the 85hp head?) and put new rings on all 3. Does this sound about right?



I can feel the scoing, but my finger nail does not catch; it seems very light compared to other pictures I've seen. Are the lighter patches aluminum from the piston rubbing? I'm assuming it is and I'll be able to see more damage once I get it apart.

Let me know what you guys think...

Mike

 

[Frank Acampora]

Re: Cylinder Scoring

Yes, that scoring is not bad and a light honing should bring back the finish. There are probably stuck rings in those affected cylinders so removal of the pistons is required anyway. Then, while they are out, soak the pistons and free the rings by hand. It is a long tedious job and if you try to rush it you will break the rings. Soak in solvent then heat with a propane torch while pulling the rings out by the ends. Again, be careful. Wear good thick gloves and work carefully--do not apply undue force on the rings or they will snap. Take your time and worry them free. Once they are free, remove them from the pistons and clean the grooves. A broken ring does the job nicely. Reassemble with a good dose of TCW-3 oil.

Note in the first photo how the two center exhaust ports are round. They are smaller than the 85 ports abnd while the block is the same, that is how horsepower is reduced. No matter what parts of the 85 you put on or in this engine, it will still only deliver 75 HP.

 

[tater76]

Re: Cylinder Scoring

Looks like aluminum to me. If you are careful.... you can scratch off the aluminum, then clean the cylinder with a hone. Most likely that piston has skirt damage and needs to be taken out. So yes, replace with a piston from the 85 and you should be ready to go! I have an 89 85hp leg with a 76 75hp powerhead. The swap was simple, but you do of course lose that 10 ponies and definitely feel it.

 

[mbruce333]

Re: Cylinder Scoring

Is the size of the exhaust ports the primary (only?) difference in the blocks?

I've been wondering how much difference there will be dropping to 75. But, the 85 that I'm used to is a 1968 75hp with an 85hp block. My dad had that motor and made the power head switch back in 1973...been runnin great ever since! The boat is a 16' GlasPlay and with the old motor it would top out about 36mph at just under 5k rpm with just me and 6-gallons of gas in the boat. My hope is that even though I'm losing some ponies, I can gain something back with the 1pc lower unit and SS prop and maybe end up close to even. I'm mostly running around on lakes/pulling tubes with this boat, btw.

Do you guys think it would be worth it to enlarge the exhaust ports? 36 feels pretty fast in this boat and the water is rarely calm enough to stay at that speed for very long...I can only imagine sitting next to Frank doing 55+! Holy crap! haha

Either way this is becoming a fun rebuild. The motor should look pretty much new inside and out when I'm done. The motor is in great shape, I'm workin on some new graphics for the hood (nothing as fancy as a semi-naked mermaid, though). For a guy who works on a computer all day, the rebuild is very therapeutic...nice to do stuff with your hands besides type and click!

Mike

 

[Frank Acampora]

Re: Cylinder Scoring



Yes, the primary difference is in the exhaust ports, although the 75 has 1 3/16 venturi carbs while the 85 has 1 5/16 venturis. Some engines did have "packing" in the bypass side but I don't think any 75s did. These were cast aluminum blocks shaped to match the crankcase shape and inserted into the bypass passage. I know that Force discontinued using them.

Look: You already will have the engine apart! Search out my posts on port work and port both the bypass and exhaust sides. You will not regret it. You can gain at least 5 HP per cylinder and if the 75 exhaust ports are enlarged properly, you will gain more. The engine will deliver an honest 90 and you should be able to keep up with Merc 90s.

Now, this doesn't sound like much, but the 85 you have is relatively weak, was rated at the crankshaft, and really only delivers about 70 HP to the prop. With porting you should see a 5-8 MPH gain. Done correctly and by hand, porting should take about one hour per set of ports so figure six hours labor on your three cylinder engine.

The 90 in my avatar tops out at 45 on a 15 foot Glastron. Originally it topped out at 38.

I also did a comparison on the one piece lower unit to the two piece. On my test hull (Glastron) The one piece was about 2-3 MPH faster. A stainless prop will usually give about 2 MPH more than an aluminum prop