In this article, “cylinder” refers to the 1960s through 1980s Honda cylinder. It has cast-iron sleeves more or less permanently-embedded in an aluminum casting. 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 widely (wildly?) disseminated and entrenched.

The cylinder sleeve fit
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 would expect from cast iron. However, 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 a certain aftermarket company's piston rings for this model having a famously-poor reputation, though it is documented 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. Oddly, this interference fit is 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 bolted on top prevents the sleeve from moving. Honda engine builders consider this a defect, and it 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. But note, "at running temperature". If a cylinder sleeve is loose at room temperature, the entire cylinder assembly needs to be replaced.

Because of this tendancy for Honda cylinder sleeves to be loose, 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 flange ends of the sleeves and hold them on with tape and wrapping. And beware. 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 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, and in addition, can make sleeve reinstallation challenging. If repainting of the cylinder assembly is planned that includes time in an oven, Honda sleeves must be clamped down against their aluminum host 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 other engine builders, and virtually all machine shops, are not.

Honing cylinders
There is so much error about honing online. Honing is the final step in the boring process, it’s not 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 of course make the cylinder even larger. Even before that however, it's too large for proper piston clearance. The cylinder must be bored.

Even lightly honing non-rusted cylinders is problematic. It is not necessary to hone a cylinder just because new rings are being fitted. That's an Internet myth. As a lifelong industry professional, trust me. Can it hurt? Potentially. Vintage 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, much less--some only 0.0005” clearance and most 40-50mm four-stroke cylinders only 0.0003”. That’s not much! 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. If you must do it, use a Flexhone, not a straight hone. And lightly. But why hone? There is no such thing as "cylinder glaze". Just one more of many Internet myths. Machining is always preferred over honing. Show me an instance 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 the best Honda engine rebuild solution. I have seen honing in a lot of engines I got in for rebuild. I disassembled them and there were very recent hone marks, yet here the engine was 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 these 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, and some, such as the venerable CBX, three times.

Part 2