One thing we’ve come across on the forums of old (and now, Facebook groups) is a constant conflating of two extremely critical figures. Those are piston-to-wall clearance and ring end-gap. At first glance, it might seem silly, but with the number of times we’ve seen the two conflated and misunderstood, it can be classified as a common problem.
One of the reasons, potentially, for this misunderstanding, is that both figures change with bore size, piston material, and usage application. However, the line on the chart moves in different directions for different materials between the two. Generally, if you can read an instruction card and follow directions precisely, you’ll be good. But here at EngineLabs, we believe it’s incredibly important to understand the “why” when it comes to these specs, especially if you want to deviate from the guidelines with your specific combination.
Piston-To-Wall Clearance Explained
As the name suggests, piston-to-wall clearance is the amount of distance between the skirt of the piston and the wall of the cylinder. The diameter of the piston is measured at a specific location on the piston skirt, called the datum point. Most pistons will come with a spec card in the box that will tell you where the datum point is for that piston, but as JE Pistons points out, the datum point is usually .500 inch up from the edge of the skirt, 90 degrees from the wrist pin centerline. That measurement is subtracted from your finished bore size to get your piston-to-wall clearance number.
Ideal piston-to-wall clearance is usually provided as a tolerance range spanning .0005 to .001 inch (so, say a clearance range of .0030 to .0035 inch). There are several factors that affect your ideal piston-to-wall clearance: piston material, bore size, engine type, and use case are all factors. Looking at piston material, a forged 4032 piston requires significantly more clearance than an OEM cast piston. Moving to 2618 aluminum requires even more piston-to-wall, because of the thermal expansion properties of the forged pistons versus cast. Conversely, a hypereutectic piston will call for comparatively tight piston-to-wall clearances — clearances that would absolutely destroy a forged piston. (This is one of the reasons for confusion with piston ring end gaps, as we’ll discuss in a minute.)
Your bore size plays a large part in the clearance needed as well, as a larger piston has more material and will expand more than a similar piston of a smaller size. For example, on the small end, JE Pistons lists the minimum clearance for a small-bore 2618 piston as low as .0030 inch, but lists up to .0050 inch of clearance for a large-bore piston of similar construction.
Your engine’s environment and use can also alter how much piston-to-wall clearance you need. For applications that generate a lot of heat in the cylinder, like heavy forced-induction or even high-endurance naturally aspirated applications, you’ll want more clearance between your piston and cylinder wall, as the amount of heat in the piston will cause it to expand more. Getting into marine applications, even more clearance is recommended.
Piston Ring End Gap Clearance
Like piston-to-wall clearance, piston ring end gap is a balancing act between maintaining enough clearance to operate, but minimizing any leak paths. A lot of the same factors that affect piston-to-wall clearance also affect the ideal piston ring end gap. There are general sizing specification guidelines for piston ring end gap, but for any newer or specialty ring materials, there will be a sizing sheet included with your ring set, as different materials will expand at different rates.
In general, the gap size is determined by two factors: Bore size and application. This is far more granular than calculating piston-to-wall clearance, and as such, the bore size is a direct component in the calculation. The second factor in the calculation is a multiplier that is based entirely on use. More specifically, how much heat the intended use will put into the rings.
As a rule of thumb, the more heat the application will generate, the more ring gap you want. That is solely to allow for proper expansion of the piston ring under operating conditions, without the ring ends butting up against one another. You’ll see this in the ring gap chart with multipliers ranging from .0045 inch per inch of bore size, all the way up to .009 inch of ring gap per inch of bore size.
A Big Source Of Confusion
So, likely this all seems simple enough, and you’re probably wondering where any confusion comes into play. The biggest point of confusion comes with the use of hypereutectic pistons. As we said earlier, hypereutectic pistons run significantly tighter in the bore (less piston-to-wall clearance) than typical forged aluminum pistons. However, when it comes to piston ring gap, the opposite is true.
According to UEM — one of the leading producers of hypereutectic pistons — a flat 40-percent increase to whatever end gap the piston ring manufacturer recommends is recommended with their hypereutectic pistons. Another of the big names in hypereutectic pistons, KB Pistons, has its own gap calculation charts. When compared to a standard ring-manufacturer’s chart, the multipliers average about 40- to 45 percent more than similar gap recommendations for a forged piston.
And therein lies the biggest source of confusion we’ve seen. Hypereutectic pistons require LESS piston-to-wall clearance but MORE piston ring end gap. We get it; logically, that seems backward, since it goes against the trends seen in piston-to-wall clearance calculations. However, it’s been proven time and time again. So now that you understand the “why” of these calculations, it makes it easier to follow the manufacturer’s instructions (and not give out incorrect “advice” on the internet).