3cyl 65-70-75 OMC Rebuild

[slackwater67]

Hello ,
My ’89 70 HP johnson is suspect to have broke a ring after a recent duck hunt and locked up. I am going to rebuild …..again last time was in 2005 . my question is what can be done to greatly improve performance. Any advantage to 75 stinger heads ,intake ,carbs etc. I just got the motor removed today , survey damage and get ready to put back together .What can I do to get all I can out of this motor. Thanks guys any info would be greatly appreciated.

[dan-in-tn]

Your ’89 model is a 56 cu in powerhead. The (60,65,75) motors you are referring to are 49 cu in powerheads. Different engines. When your engine was rebuilt was it modified? A service bulletin was written with steps to improve durability after rebuilds to this motor? Starting in 1986 when this motor became 56 cu in it picked up a larger exhaust port with a bridge. The bridge could give problems with ring breakage in some circumstances. I can try to get the number or a copy. The ’86 60hp engine did stay 49cu in until ’89. As far as modifications I can’t think of anything I would do except make sure it is running as well as it can. WOT should be as close to 6000 RPM trimmed out as you can get. That will give you a decent hole shot and keep the engine cool at WOT. Your machine shop needs to know what mods to do to the exhaust port. Hope that helps!

Dan in TN

[slackwater67]

Thanks Dan, and yes sir ,it was ported when last built. What about shaving the head and gaining compression.? Any advantage.? And I have a’ "72 short shaft 65hp evinrude will the 75stinger carbs , heads , improve the performance ?.

[dan-in-tn]

In particular was the bridge rounded and relieved to stop the sawing effect on the rings? Some of the improvements included, but may not be all:
Move the overboard elbow to top of block to make sure block is full of water at all times before it overboard indicates.
.002 larger HS jets
Can’t remember if they dropped timing a degree or two.
#432955 water pump kit with thermostat spring, etc. Update kit included different impeller that pumps more water (6) leg impeller instead of (3).
I don’t think any of the 49cu in stuff can be used on this engine to advantage if that was what you were asking.

Dan in TN

[jerry-ahrens]

Interesting reading. Dan, if I remember right, didn’t OMC put out a service bulletin that tells you how to do the tell tale hose modification ? If you ever come across that bulletin, I’d like to have a copy of that. It’s amazing to me how many of those effected 3 cly. motors are still running without the modification. I use to have a 1992 70. When I bought it, it was on the back of a pontoon boat and had a scuffed center cylinder. It had never had the above mentioned modification performed. After a rebuild, it turned out to be a great motor.

[John Schubert]

Dan,

Good suggestion regarding water outletYes there was a bulletin. When I built SST 60 56 cube engines for guys. they loved to "jack them up" & as a result stuck # 1 & sometimes #2. When the re-routing of the water outlet came out, they wouldn’t allow us to do it. The project leader for the SST 60, whose name I forget, just stated "tell them to lower their motor".

[dan-in-tn]

The motor finally picked up the revised sleeve in 1994 I believe. It was an oval exhaust port at that point and corrected the problem. All of the fixes were in by then. The water pump kit was the main fix, but moving the overboard elbow was a good way to make sure the block was full of water at all times (vs air locked). They later came out (by 1994) a thicker exhaust side cover. That allowed deeper support ribs which in turn allowed more water to pass by the exhaust chamber. This stopped the scuffing of the piston skirts. Especially the top pistons which combined with the block air locking. You have to think of combined problems. John I feel for you on problems/updates that can be seen from outside in regards to running motors in stock classes. Doesn’t matter if the engines don’t run well that way or won’t last. Especially racing! Maybe my engine builder will have a copy of the bulletin. We sent it out with every motor block we modified so the dealer could do the rest of the stuff to the motor.

Dan in TN

[slackwater67]

Hey Dan, sometime when you have time could you give me a call , I would love to visit with you a little more extensive on the subject of 3cyl.OMC motors. I can be reached at 501-281-8002. Thanks

[r-c]

One thing we have done on exhaust bridges for racing is to take your Dremal drill with a sanding disc and relieve it a few thousands in an arc from one end to the other. Finish smooth by hand with some wet and dry. A ball hone does a nice job to finish off all the edges smooth and leaves a nice finish to the entire bore. A 240 grit is fine.
So when the bridge expands from heat, because there is no way to cool it, it doesn’t expand into the bore. If it is expanding into the bore you will see rub marks on the piston, especially near the ring area. Bad enough it will stick the rings right there.
Also, take a round stone and radius all the edges of the port, especially the top, so the rings don’t catch an edge. The radius helps to move the rings back in the groove without catching an edge.
If I remember right these engines have an angled bridge. So you can’t put oiling holes in the piston to help lube the bridge. If it is a straight bridge, you can drill two oiling holes on the piston in line to the middle of the bridge. About 1/4" below the bottom ring and the other 1/2" below the first. A number 1 center drill works nice as it leaves a counter sink bevel and the hole is about 0.070" diameter. Of course don’t drill so deep that the bevel is wider than the bridge width. Take a round cutter tool and radius the inside holes of the piston.
As far as cutting the head, you can. But then you are a slave to higher octane fuel. Also if you are stuck with ethanol gas you may need to go richer on the high speed jets. The gain is minimal compared to more future problems. You can do more working props than modifying the powerhead and asking for problems there.

[slackwater67]

Thanks RC for the input. I am waiting for machine shop to get motor disassembled to see what happened. I am game to shave head , do all I can to get all I can. Or buy one, we have some duck boat races in May and Sept….I need to build a motor that will scream.! Any additional info would be greatly appreciated.

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