Benefits of overboring cylinders?

[BRzuki]

I saw that Wiseco made pistons that were .040" over stock for my motor. That is a little over 2 times larger than what the factory offers. What advantages or gains would I be seeing from having the cylinders bored and putting these new pistons in?

[JohnsonM50]

I saw that Wiseco made pistons that were .040" over stock for my motor. That is a little over 2 times larger than what the factory offers. What advantages or gains would I be seeing from having the cylinders bored and putting these new pistons in?Ive used Wisecos in an OMC, 31.8 they were lighter than the origonals & work fine.

[Mark75H]

There will be very little or no gain from simply boring to .040" over on an outboard.

You time and money are much better spent on props and testing time - and blueprinting your boat's bottom

[BRzuki]

Mark,

What do you mean by blueprinting the bottom of my boat?

[Mark Poole]

And to add to what Sam said, depending on the engine, shaving the head may be necessary to maintain whatever performance the engine had with standard bore. Many people assume that the slight increase in displacement means more power. Until you think about cylinder volume and compression. ;)

[BRzuki]

So...if the head was shaved to maintain the compression would the increase in displacement give me anymore power? I'm interested even if it's only .5 mph.

[Powerabout]

overbore increases compression and crankcase compression.
If you had tight comp before it will be way higher with a 040
But a better prop is probably more bang for ya buck as Sam Says

[zul8tr]

Mark,

What do you mean by blueprinting the bottom of my boat?

Getting the bottom straight and flat in the appropriate areas. That means removing hooks and rockers and squaring the edges where water leaves the planning surfaces like at the rear of the bottom at the transom and the rear of the sponsons. If these edges are not sharp they promote water creep up the sides via surface tension and it adds drag. On fiberglass hulls there are usually rounded edges at the transom and at the lifting strakes that needs to be squared off.

There are also possibilities with the surface finish of the planning surface, glossy, rough, satin, slick paints, dimpled, etc.

[Allen J. Lang]

Over boring is used to bring cylinders back into round condition. Should never go more than what it takes to get them round.

[BRzuki]

I appreciate the help guys...I'll check that off my list of possible things to do.

I'm running this motor on a 15 foot aluminum duck boat that gets beat around pretty good. Working on the bottom is really not an option. All the work would disappear after the first log jump.

[zul8tr]

All I said in my post above would not practical for aluminum hulls.

[Mark75H]

I appreciate the help guys...I'll check that off my list of possible things to do.

I'm running this motor on a 15 foot aluminum duck boat that gets beat around pretty good. Working on the bottom is really not an option. All the work would disappear after the first log jump.

That really changes things. I doubt there is ANYTHING that you can do to your motor (other than selecting the best prop) that will help at all.

A smooth straight bottom is the first thing for speed. If you can't keep the bottom smooth and straight if you want to go faster you need to consider if you have any possibility of using a much larger motor.

[BRzuki]

A larger motor is out. My motor is a 25 yamaha 3 cyl. I've seen noticable gains already from larger carbs, exhaust work, intake work, and even plugs. Just trying to exhaust all options to get the most out of this motor.

[fs5]

there must be some gain doing it.i've been thinking about it myself.
theres guys over here that fit the twin cylidner sleeves out of a 40hp 2cyl tohatsu into the 3 cyl 50hp.to up the displacement.
has anyone ever tried it on a stock motor?
you'd think it would have to raise the compression a bit?