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Engine break-in
The idea of Honda top end break-in is a myth, and unfortunately with some folks it has provided an excuse for poor workmanship. When using correct rebuild techniques, professional machining, and factory parts, the Honda engine will have the highest cylinder compression it will ever have mere minutes after the first start, and certainly by the end of the first test ride. This is partly because Honda rings are designed for instant seating. The very early ones are soft enough to make this happen, and the later ones are tin-plated for the same reason.
Honing
Honda cylinders should not be finished to a rough surface. Everyone talks about “cross-hatch” and that is good, even necessary. But all I see on YouTube and in numerous molested engines I disassemble is extremely rough finishes. If quality rings are being used, the finish should be very fine, from a 600 grit stone, not a 250 or 320. Even major aftermarket piston providers such as Wiseco and JE have ultimately conceded the superiority of Japanese-made rings, and now supply them with their pistons.
Determining piston clearance
The factors that go into piston-to-cylinder clearance are these: 1) The material the piston is made from. 2) The method of manufacture of the piston. 3) The reputation for overheating that some engines might have. 4) The material the cylinder is made from. And 5) The diameter of the piston. The biggest difference between a vintage Honda cylinder and a pushrod Brit bike cylinder is 1) The material the piston is made from and 4) The material the cylinder is made from. Thus 0.003" or more is proper for a low-grade aluminum piston in a cast iron cylinder. But it is very wrong for a high-silicate (low expansion) aluminum alloy piston in an aluminum cylinder. They have similar expansion rates, and moreover, in Hondas especially, the same alloy is used in both the piston and the cylinder casting, so their expansion rates are even closer. No matter how much experience an automotive machinist might have, nearly all are steeped in cast iron V8 car technology and have very little understanding or experience in Japanese motorcycles. They simply can't accept the 0.0003" clearance that some Honda cylinders are fitted at. Their 40-50mm pistons are fit so tight you must push them through even when there are no rings. Their 55-65mm pistons are put in at 0.0005" to 0.0007", and when you reach 70mm and up we're in about 0.001" territory. How can they do this? Three reasons. One, the pistons are cast in incredibly carefully designed molds whose object is rigid control of mass, i.e. thickness. The piston expands at its thickest spots. Two, the alloy used in Honda pistons has the highest silicate content of any piston made, resulting in unbelievable expansion control. And three, Honda uses the same alloy all over the rest of the engine, including its cylinders, meaning the two--piston and cylinder--expand at almost the same rate.
Measuring piston clearance
Piston-to-cylinder clearance in a Honda motorcycle cylinder is not properly determined with a feeler gauge, despite the advice of Internet "experts". The curve the gauge will necessarily undergo distorts the feel, and besides, where do you find a 0.0003”, 0.0005” or even a 0.0007” feeler gauge? The proper way is to measure the piston with a micrometer, then transfer that measurement to a special tool called a dial bore gauge. Once the bore gauge has been calibrated (“zeroed”) it is then placed into the cylinder and the difference between the piston’s size and the cylinder’s reads directly out on the dial. The gauge is oriented in the cylinder at three different heights—top, middle and bottom. This is how cylinder taper is determined. Then the tool is turned 90 degrees and the same routine is repeated—top, middle and bottom—which measures cylinder out-of-round in addition to offering more taper information. Some advocate simply measuring the piston and that’s all. This is useless and may be a misunderstanding of the official manual. Both piston and cylinder accrue wear in use, but the cylinder wears many times faster than does the piston. Fact. You must measure both to accurately determine piston-to-cylinder clearance and the use of the dial bore gauge is part of the industry standard, best practice method in that process.
Assemble spec vs service limit
Some argue that Honda’s “service limit” allows for greatly loosened-up clearances before rebuilding. That is true—if you live in the outback hundreds of miles from civilization. And even then, that's "make-do”, not rebuild. Honda’s service limit is very different from its assemble spec. Pistons and cylinders reassembled near to or even beyond the larger end of Honda’s as-manufactured clearance range are two-thirds worn out already and will exhibit blowby, piston noise, loss of power, and oil consumption. That is not a rebuild. Unfortunately, Honda has traditionally made you work at finding their engines' piston-to-cylinder clearances—only a few official manuals for vintage models give the clearance as a stand-alone spec. In most cases you must determine the factory specified piston clearance by doing the math with the as-manufactured piston and cylinder diameter figures. It's always been this way.
Cast vs forged pistons
It is very easy to distinguish between the cast and the forged piston. Look for material above the pin boss. If there is extra mass there, the piston is forged. If not, and there are no tool marks to indicate that metal was removed after manufacture, then the piston is cast. A forged piston bears the right-angle shape of the manufacturing ram. Most modern pistons have expansion-limiting silicon in them, but cast pistons have the most. Silicon can make the piston brittle, so manufacturers are careful with its use, especially forged piston manufacturers. Cast pistons are light. Their multiple-piece steel casting molds allow intricate contours inside and out. But die-casting is costly. It requires huge, expensive tooling that does only very specific jobs. Thus mainly vehicle OEMs use cast pistons, so there aren't many appropriate cast pistons to chose from if you are modifying an engine. Cast pistons are very OEM-proprietary. Forged pistons by contrast characterize the performance aftermarket.
For various reasons, forged pistons have more mass than cast pistons. Again, the mass in a piston is where it expands the most. Also, the same silicate-induced brittleness that can make the cast piston crack when dropped could result in defects during the forging process. The forged piston’s extra mass and less silicate result in that piston having to be fitted looser. More silicate is used today in forged pistons than was possible before thanks to newer alloys containing nickel, which offsets the brittleness. But they still need extra clearancing. Forged pistons also tend to have overly-thick skirts because piston forgers make piston blanks—essentially ingots—from which the piston wholesaler carves out several different sizes and shapes of pistons. If the piston for a particular model happens to be the largest the blank supports, that piston ends up with the thickest skirt. This piston is heavy and prone to the most uncontrolled expansion. The best forged piston makers such as JE and Wiseco often mill the interior of the piston to ameliorate that.
Forged pistons have a more highly-compressed metal grain structure that resists the damaging effects of detonation. However their main popularity is that they can be had very quickly, and in virtually any configuration desired—even custom ordered. Moreover, the forged piston's added crown thickness is welcomed by engine builders as it allows extra valve relief and combustion chamber customization. There's a lot of extra material there. The forged piston was also the first piston type to adopt the modern high-rpm, ultra-thin piston ring. Cast pistons on the other hand are lighter, less expensive, quieter, and longer-lasting. Each piston type came from, and is suitable for, a different part of the industry. More... Think of them in these terms.
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