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Outboard Technical Discussion This is our outboard engine related technical discussion forum. Topics include engine repair and maintenance, modifications, gearcases, electrical and electronics, etc. For technical assistance related to topics not outboard related please use our other Engine forum.

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Old 12-10-2009, 10:35 AM   #1
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82Diplomat will become famous soon enough82Diplomat will become famous soon enough

Arrow Outboard Powerhead Rebuild questions - Piston Type Choice

Sorry about the long post, but it's winter here in Ohio - what else is there to do....(boating wise...)

I'm in the middle of a rebuild project of a 90hp Mercury Sportjet (3 cylinder) Powerhead.

The motor was purchased on Ebay (rebuilt by a Merc certified tech) and I only got 8 hours out of it after a proper breakin. Needless to say, I've got way to much money in this engine, but I'm trying to save it and make it a reliable engine for fishing and some tubing with the kids.

The machine shop bored my block 0.030 over top and bottom cylinders and the middle at 0.020 over. Several folks have said that it's OK today (maybe not not 20 years ago) to have different sized holes in a 2 stroke. I'm far from the expert and the machine shop didn't want to take off good metal that might be needed for future rebuilds. I can still do all 0.030 over as the final bore hasn't been done yet, I still have to match pistons. Thoughts anyone on the best approach for cylinder size (all the same or odd hole OK)?? Which brings me to my question about type of piston.

I'm debating about Wiseco pistons (forged from what I understand) and a hyper-eutectic cast piston (Vertex) or others from what I understand.

My cost to replace the pistons with Wiseco and the gaskets I need is around $350. The freshly rebuilt engine had a whine sound to it immediately upon starting it - (but it wasn't too bad and I've never broken in a new engine) so I thought all was OK . Upon disassembly, my bottom piston was melted and rings were crushed. Fortunately, I could save the block and the carbs will be totally rebuilt as well since I've heard this is most like a lean fuel condition that lead to this problem.

Because of the whine, I concerned there may be other parts that should be replaced (maybe all the crank bearings). I want this engine to BE RELIABLE, not for racing, just back and forth to the fishing hole and occasional tubing with the kids.

A company on Ebay Tri-State Marine makes a TOTAL REBUILD KIT (minus carb kits which I will do also) for $750.00. Here's the link, if it doesn't work search Ebay for item 380027276464.


I wrestling with how to get the best benefit for the time and money since I was so dissappointed in the last rebuild done by someone who should know what's up. For $400 more, it seems like I'm getting alot of valuable new parts that should lead to reliability??

I asked Tri-State Marine (Ebay seller) some honest questions about their kit (they have 1200+ positive feedbacks ) to compare it to factory parts and Wiseco or Vertex. Here's what they said:

In our opinion Wiseco, being a forged piston is more suited for race applications. A forged piston has a greater expansion rate than a hyper-eutectic cast piston. This expansion rate is so much more that if not fitted properly it can lead to seizure. therefore you should fit the piston with more clearance than the factory outlines in the service manual. In some cases the piston must be 'looser' in the bore than even Wiseco recommends. The added clearance can promote greater cylinder, piston and bearing wear not to mention piston noise. The Vertex piston is a good choice as this is a hyper-eutectic cast alloy, although we are not a fan of the piston ring manufacture Vertex chose as a vendor. Our pistons are hyper-eutectic cast alloy and our piston ring vendor is RIK, a Japanese manufacture who supplies Yamaha. This is the best ring made hands down. The piston is manufactured in Italy to our specifications.

We are in agreement with your machine shops good advice and (1) odd-sized 0.020 piston is OK with (2) 0.030 pistons.

Lastly, most engine failures are related to cooling or fuel supply. Be sure to rebuild the fuel system and pay very close attention to jetting for restrictions and double check float levels. Check for intake leaks as well as good constant fuel supply to the engine. Lean air to fuel ratio will cause high combustion temperatures which will cause failure. Be sure to set carb linkage and adjust the ignition timing as outline in the FACTORY service manual. Renew the water pump complete and not just the impeller. Thermostats should also be replaced. Please keep in mind most outboards when run at 3/4 throttle or around 4000 RPM is actually peak load for the motor. These motors are made to run at full throttle as there is less load on the engine. There are a lot of Marine Technicians out there that do not address the cause of failure when making the repair. If you do address the cause the repair should give you years of service.

Can anyone share some thoughts or opinions about the best approach for me?

Again, I'm not racing, just wanting reliability from this engine.

Thanks in advance for everyone's 0.02.

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