Engine oil consumption
Many people think of engine oil consumption in only one way, burning oil in the combustion chamber. What they are not aware of is that powersports engines also consume oil without burning it. And it’s normal. In fact, motor oil designated specifically for powersports use has a vaporizing metric, and it’s among the highest of all engine oils. Higher-revving smaller capacity motorcycle engines froth their crankcase oil, which results in some of it vaporizing and escaping out of the crankcase breather as fumes. Not all oil consumption happens at the spark plug.

Torque wrenches
Most people know they should assemble an engine using torque wrenches. We don’t have to beat that drum very hard. But notice I say, “wrenches”—i.e. plural. The plural message still needs to get out there. For Hondas you really need two different ones. Like many other things, a torque wrench is the least accurate at the extreme ends of the instrument’s range, both low and high. Consequently, using a foot-pound torque wrench—despite Honda’s listing of a 6mm bolt’s torque in foot-pounds—is bad practice. You’ll be at the extreme low end of the scale, and the reading will not be reliable. For this reason, your toolbox must include not only the usual foot-pound torque wrench for the larger bolts on head and crankcase, but also an inch-pound unit for the 6mm bolts elsewhere on the engine. If you want to do it right. And there is another, not often considered problem. Though not found in every manual, Honda has published in its service manuals a foot-pound value for 6mm bolts that is way too high, typically as much as 12 foot-pounds. This is equal to 144 inch-pounds, over 50 inch-pounds more than the 90 inch-pound maximum for a 6mm bolt in aluminum. Watch out here.

Precision insert bearings
Honda’s plain type powersports crankshaft and rod bearings are not similar to what is found in car engines. Honda crankshafts are made in one area of the plant, the engine cases in another. They are made to very high tolerances. When the two are joined together the tolerance range of the crankshaft and that of the crankcase intersect by the use of five different thickness bearing “shells” that allow joining the parts while maintaining the target 0.001” clearance. The shells are not marked on their backs but on their edges with five paint colors. These colors naturally fade from time and engine heat, and of course the bearings wear and the crankshaft wears, so although the manual includes a matrix table of size codes, and the crankshaft and cases are marked with some of these codes, the bearings must be Plastigaged to determine their actual oil clearance. When a bearing has too much clearance, the manual’s table is referred to in order to find the next smaller bearing (i.e. thicker shells). After it is installed, the new bearing also must be Plastigaged to ensure the resulting clearance is correct. The connecting rods are handled in the same way. As part of this process the crankshaft should be inspected for wear and measured to ensure its journals are not out-of-round. Unlike in car engine practice, Honda does not offer undersize bearings to facilitate regrinding the crankshaft, and neither does the aftermarket. So although crankshaft regrinding was widely practiced at one time in the auto trade, in Honda powersports it has not been. A bad crankshaft has to be replaced.

Valve adjustment
If I had a $100 for every time a customer told me his valves must be adjusted correctly because they don’t make any noise, I would be rich. I heard it again yesterday in a phone call. It’s silly. Just as a carburetor’s float level is not all about carburetor overflow, a valve clearance specification is not there to keep the engine quiet. Whether or not your valves make noise has absolutely no bearing on whether they are adjusted correctly. In an earlier article I explained that a manufacturer determines the valve clearance by how much “ramp” a camshaft has. If this transitional zone of the cam lobe is generously provided by its designer, then only a very small valve clearance is needed. You can see this on many early vintage Hondas that specify clearances of only 0.002” to 0.003". Look at their lifts—most are barely over 5mm. If on the other hand the camshaft is so designed that its transition zone is very small, then the engineer specifies a larger valve clearance to make up for that. The NX250 and many other modern bikes’ high-lift cams require five times a CB550’s valve clearance. Valve clearance has nothing to do with heat, or engine size, or engine quietness, and is instead directly related to a cam’s aggressiveness.