Frequently Asked Powerhead Questions

How do I break-in a new/rebuilt powerhead?

It varies and you should always check your manual for model-specific details. It usually breaks down to varying RPM's and running double-oil in the gas for the first tank or two. In most cases, this means run 25:1 in pre-mix applications and 50:1 in oil-injected models, which works out to be 25:1. Avoid sustained full-throttle and heavy loads for a while as well.

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Is rebuilding a powerhead much different than a car engine?

Yes, very.

Most importantly, a two-stroke powerhead is far more sensitive to mistakes and a dirty work environment. One of the first things you should notice, if you're only familiar with 4-stokes, is the difference in bearings. The soft, flat metal bearings in automotive engines have a surface that often allows dirt particles to embed, without harm.

That's not the case with roller/needle bearings, which can be destroyed by one piece of debris--including a piece of hair falling from your head during assembly. Once a needle/roller bearing slides instead of rolling smoothly, there will likely be trouble.

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Do I have to bore all cylinders, or just the ones that need it?

It's fine to only bore the cylinders that require it. Try that with your car engine and you'll definitely run into problems. A two-stroke's cylinders are sealed off from each other, and have equal crank case pressure on the other side of the piston. Since a four-stroke doesn't have crankcase pressure, you would end up with one high-compression cylinder fighting the others.

One advantage to only boring as needed is that you leave more metal in the other cylinders if there's ever another failure. That could mean the difference between boring a cylinder and having to re-sleeve it.

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Can I have an automotive machine shop bore my block?

Sure, if they're equipped and qualified to do the job. Some of them might not be equipped to do outboard V-blocks, though. Inlines shouldn't be a problem for most of them, unless there's a clearance problem with their tool at the bottom of the cylinder.

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Do I really need to use a special grease on needle bearings?

Yes! It's a special grease that pretty much dissolves as soon as gasoline hits it. That's very important because it's what keeps the needles from sliding instead of rolling. As far as required lubrication goes, you could just use two-stroke oil, but the grease holds the needles in place, making the job a lot easier. I believe Mercury simply calls theirs needle bearing grease and OMC calls theirs Sealube.

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Uh-oh, I stripped the spark plug threads. Do I need a new head?

No, the threads can easily be repaired. Just go to your local auto-parts store and pick up a spark plug thread repair kit. There are a few different ones available, but I prefer the one that reams and re-taps the hole with the same tool. It comes with everything you need to do the job. The inserts are stainless steel and if the job is properly done, the repair is actually an improvement over OEM.

This should go without saying, but you have to do this with the head removed or you'll fill the cylinder with metal shavings.

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Disclaimer: Use information on this page at your own risk. Seiler Marine is not responsible for the mechanical ability of others. Never attempt a repair without the proper tools and service manual. When it doubt, it's best to take it to a pro.