This is about 1960s through 1980s Honda cylinder with cast-iron sleeves permanently-embedded in an aluminum casting. And I say “truth”, because there is more misinformation about cylinders and heads in the public’s mind than almost any other part of the vintage Honda motorcycle engine, and thanks to social media these falsehoods have become entrenched.

Cylinder sleeves
Honda’s cast-iron cylinder sleeves are interesting. For one, they have a highly ductile nature—they can withstand relatively rough treatment and not crack, a huge bonus and much more than you should expect from cast iron. At least one vintage Honda model’s cast-iron sleeves are harder and more durable than all the others, the four-cylinder 350F’s. I don’t know why. But anyone who has machined these cylinders knows this. And this extra hardness may be at least partly to blame for aftermarket piston rings for this model having a famously-poor reputation, though it is documented that most of the problem in those cases is that these Chinese-sourced rings are themselves not very high quality. Honda’s cylinder sleeves are of course an interference fit in the cylinder casting and are not intended to be removed—they do not have part numbers. Strangely, the specifications of this interference fit are not consistent across all vintage Honda models. In all of the SOHC inline fours the sleeves fit pretty tight, and in most of the singles and twins also. In a few vintage Honda engines however they are much looser, in fact so loose that at running temperature only the presence of the cylinder head prevents the sleeve from moving. Honda engine builders consider this a defect, and it certainly is. But it can be worked with. From experience I would identify the Honda CBX1000 and SOHC 400 twin cylinder assemblies as the worst offenders.

Because of this vulnerability, when shipping cylinder assemblies it is imperative that the bottoms of the sleeves be protected from the inevitable shocks of transportation. I use discarded packing tape cardboard rolls placed over the bottom ends of the sleeves and hold them on with tape and wrapping. When you find a sleeve that is starting to protrude upward from the cylinder deck, it must be removed. You can’t simply force it back down. Carbon from between the sleeve flange and the casting will have fallen into the machined recess, making full reinstallation impossible without first cleaning. There have been folks who have simply machined down a protruding sleeve, only to find that later the engine failed its head gasket because the sleeve dropped into place and left a gap between it and the head gasket. To remove a sleeve the casting is heated in an oven to 400 degrees F and the sleeve will fall out (Honda’s problem sleeves require only 250 degrees). Then the sleeve and cylinder casting are thoroughly cleaned, involving wire brushing of the sleeve and scraping and Scotchbrite cleaning of the recess in the top of the cylinder casting. Then the casting is heated a second time, the sleeves shrunken in a freezer, and the sleeves plopped into place by gravity and afterward held down by a heavy weight to keep them from creeping upward during cooling. It is not advisable to apply any kind of adhesive to the sleeve. That will interfere with the transfer of heat between sleeve and cylinder casting. If repainting of the cylinder assembly is planned that includes time in the oven, the sleeves must be clamped down using specially made clamps to avoid them coming loose while being heated. This is true at any time, whether or not the sleeves have previously creeped. Similarly, when boring a vintage Honda cylinder, how the cylinder assembly is held in the boring machine must include clamping the edges of the sleeves to prevent them from rotating during machining. All of these cautions are things career Honda mechanics are aware of, and unfortunately many engine builders and machine shops are not.

Honing
Honing is the final step in the boring process, it’s not really a legitimate technique when separated from cylinder machining. The common Internet suggestion of honing an in-service cylinder is very misleading. At the end of the day, this practice is not what professionals do and in most cases should be avoided due to several potential pitfalls. For example, if the goal is rust removal, understand that significant rust in the cylinder by definition means there has been appreciable change in the cylinder’s dimension, due simply to the metal decomposition. The cylinder is already larger due to its having lost mass, and removing the rust will make the cylinder even larger, too large for proper piston clearance. The cylinder must be bored. Even lightly honing non-rusted cylinders is problematic. Honda pistons are fitted to very close tolerances. Honda engines with pistons up to 70mm assemble right around 0.001” and all of the smaller cylinders get less clearance, some only 0.0005” clearance and several 40-50mm cylinders only 0.0003”. That’s not much. Consider that 0.001” is a third of the thickness of a piece of notebook paper, 0.0005” is half that and 0.0003” only a third of that. An awareness of this should cause you pause when thinking of honing an in-service cylinder. Rarely is it good practice. It is not necessary to hone a cylinder just because new rings are being fitted, though it won’t hurt if the right kind of hone is used, the so-called “ball” hone or flexhone, at a grit of 320 or finer. However, show me a case where someone wants to put in new rings and I will show you a cylinder that needs to be bored. Simple re-ringng is not a common motorcycle solution. I see honing a lot on engines I get in for rebuild. I disassemble them and there are very recent hone marks, yet here the engine is in my shop to be rebuilt properly. What does that tell you? The fact is, excessive cylinder wear is virtually a given on 50 year old Hondas. High engine heat makes their cylinders wear more quickly than in other kinds of engines, with many vintage Hondas typically exhibiting more than double their assembled piston-to-cylinder clearance after less than 15,000 miles.

Engine break-in
The idea of 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 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. Because of this, 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 previously molested engines are 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. By the way, Honda piston rings are so good that the Honda company was at one time the ring supplier for Toyota. 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.

Piston Clearance
One of the major challenges in rebuilding Honda motorcycle engines is finding a machine shop that will/can bore a cylinder straight, round, and—most importantly—to the requested piston-to-cylinder clearance. In fact, it’s a monumental task. The problem is ninety-eight percent of machine shops deal in cast-iron car engines, a context in which no one speaks in terms of tenths of thousandths of an inch. Yet vintage Hondas typically assemble at 0.0005”-0.001”, and many of the smaller-bore engines at quite a bit less—just 0.0003 inch. That's a third of a thousandth of an inch! Some argue that Honda’s “service limits” allow for much looser clearances before rebuilding. That is true. If you live in Paraguay. And even then, it’s a “make do” ethos being communicated there, not a rebuild. Honda’s service limit is not the same as its assemble spec. Pistons and cylinders reassembled beyond the larger end of Honda’s recommended clearance range are two-thirds worn out already and will exhibit blowby, piston noise, loss of power, and oil consumption. By the way, Honda has traditionally made you work at finding their engines' recommended 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 piston clearance by doing the math with the official assemble specs.

Piston Clearance
"Break-in” is not a thing with Honda engines. Not when speaking of the top end, and not when it is assembled correctly and with Honda or similarly quality parts. The embarrasing truth is, the highest cylinder compression the engine will ever have is reached at the end of the tuning session that follows the first start. If compression increases with miles it is because inferior rings were used or machining was not done professionally. Machining should be performed in most cases where people want to hone. Honing is the final step in boring, it is not used instead of boring. It smooths the boring machine's tool marks, imparts the necessary cross-hatch finish that will control cylinder oiling, and most importantly, determines the cylinder's finished dimension. A hone is not used to "freshen a cylinder”, or "break the cylinder's glaze”, or to remove rust from a cylinder. That last may seem intuitive, but a rusty cylinder is almost surely worn excessively from use; it certainly will be once the rust is removed. A rusty cylinder is one that needs to be bored.

Factory replacement pistons for vintage Hondas are getting very hard to find. More difficult is the rings. Even when you can find the pistons, often after waiting months on CMSNL, the only rings you’ll find are standard size, and you will wait twice as long for them or pay $100 a set on ebay. Aftermarket pistons can be pretty good, even if made in Taiwan or China. Can be. But the rings? Most are very low quality. The better known aftermarket piston suppliers such as Wiseco switched to sourcing their rings from Japan some years ago. They are good rings, appearing in every way to be made just like Honda’s, even to the tin coating. Good rings are made to go into cylinders that are highly finish-honed for long cylinder life and instant sealing. Junk rings need more aggressively-finished honing that results in faster cylinder wear and sealing that takes time.

Determining piston clearance
Piston-to-cylinder clearance in a Honda motorcycle cylinder is not properly determined with a feeler gauge. 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. 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, it’s a “make-do”, not a rebuild. Honda’s service limit is not the same as its assemble spec. Pistons and cylinders reassembled 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. Oddly, Honda has traditionally made you work at finding their engines' recommended 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 piston clearance by doing the math with the as-manufactured piston and cylinder diameter figures.

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 therefore very proprietary.

For various reasons, forged pistons have more mass than cast pistons. The mass in a piston is where it expands the most. Also, the same silicon-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 silicon result in that piston having to be fitted looser. More silicon is used today in forged pistons than was possible before thanks to newer alloys containing nickel, which offsets the brittleness. But they still need careful clearancng. 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, it ends up with the thickest skirt. This piston is heavy and prone to the most uncontrolled expansion.

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.