Re: Cylinder Boring
Hi Nathan.<br /><br />First, make sure you bore both cylinders. This will ensure that the engine will be balanced when you are completed. Not all over-sized pistons will be equal in weight to the standard, stock piston. <br /><br />Some machine shops get in a hurry and forget to lightly hone the bore and smooth the piston port chamfers. Make sure they do this. Also make sure you replace the wrist pin, wrist pin bearings, wrist pin retainers, and rings.<br /><br />Next, I could be wrong, but you might not find parts for a .015 over-sizing, especially with non-oem parts. Most over-sizing is done in .020, .030, .040, etc sizes. A good machinist will need the actual pistons when doing the boring so he can match up the transfer ports with the skirt design and take actual measurements.<br /><br />Yes, oversizing will change the HP. All things being equal, HP increases with displacement and you are increasing displacement. In short, there is more air/fuel charge volume available to make power. Although in a small outboard like this the difference may not be felt seat-of-the-pants. Here are some calculations for displacement:<br /><br />.7854 (a constant) X the bore" squared X the stroke" X number of cylinders = cubic inch displacement.<br /><br />cubic inch displacement (cid) X 16.39 (a conversion) = cubic centemeters (cc)<br /><br />For example:<br /><br />.7854 X 2.56" squared X 2.38" X 2 = 24.5 cid or X 16.39 =
402 cc.<br /><br />increasing the bore .020:<br /><br />.7854 X 2.58" squared X 2.38" X 2 = 24.9 cid or
408 cc.<br /><br />Compresion ratio is the relationship between the cylinder volume when the piston is at TDC (top dead center), compared to the cylinder volume when the piston is at BDC (bottom dead center).<br /><br />Should you go bigger? Well, unless you are boring strictly for performance (hot-rodding), the object is to remove only as much material as necessary to smooth and straighten the cylinder walls. Again, this is usually in steps that relate to the over-sized parts you will be installing.<br /><br />One additional thing....from my experience I would recommend you increase your main jet size (and possibly other fuel circuts) when over-sizing. A larger bore means increased air intake volumes. You will need to match this increased air volume intake with fuel or you could find yourself running too lean. And that means another rebuild.<br /><br />Hope this helps.
