Blueprint Series: The How And Why Of Setting Piston Ring End Gap

Good engine builders have to be detail freaks. The slightest mistake can have tragic and expensive consequences in a world where leaving horsepower on the table because some minute specification wasn’t followed can be considered a tragedy. In this month’s episode of the EngineLabs Blueprinting series, we’ll dive into setting piston ring end gaps.

The concept is simple enough; piston rings help seal combustion pressure in the cylinder and the top and second rings must have the proper end gap when the engine is cold to accommodate expansion as the combustion temperature increases. The idea is to create a gap between the ends of the top and second rings to prevent them from butting when expanded. This may not seem like a big deal, but it is.

While the focus of this story is aimed at setting ring gaps for forged pistons, it’s important to note that the Keith Black brand of hypereutectic pistons requires a much wider top ring end gap than traditional forged pistons. The KB spec is 0.0065-inch per inch for a street engine. For a 4.00-inch bore piston, this is a top ring end gap of 0.028-inch. For mild nitrous, the spec moves to 0.0080-inch per inch of bore.

Mind The Gap

An example of what can happen, took place a couple of years ago when we delivered some parts to our machine shop. On the shop counter was what appeared to be a brand new piston – except that it had broken in two right below the oil ring, leaving the entire piston crown and ring package intact. As we picked it up, the machine shop owner told us the story.

“A customer brought this in yesterday. He claimed we machined the cylinder wall clearance too tight and that broke his piston. I knew that he had assembled the engine, so I asked him ‘What did you set the top ring end gap at? He said he set it at 0.018-inch,” said the shop owner. “That’s when I told him these were hypereutectic pistons that needed much more end gap. You should have set them at more like 0.028-inch. So when the rings got up to temperature, they stuck in the bore and that’s what broke the top right off the piston.”

Top-ring end gap is affected by heat from the chamber, which can change significantly with the ring’s proximity to the top of the piston. If the top ring is moved closer to the piston crown, expect to widen the gap slightly to account for the increased heat.

While this is an extreme example with a somewhat brittle piston material, the point is made with alarming clarity. Piston ring end gaps are critical.

We will look at not only how to check end gap but how to custom set the proper clearances. Standard ring packages are set with somewhat wider end gaps, since the manufacturer has no way of knowing how the rings will be used. Most performance piston ring manufacturers offer ring sets in 0.005-inch larger diameters so the engine builder can custom tailor the top and second ring end gaps.

We’ve reproduced a ring end gap recommendation chart from JE Pistons that lists multiple top-ring and second-ring end gaps based on how the engine will be used. These are based on different user applications. For example, a mild street engine gap recommendation of 0.0045-inch per inch of bore would put a 4.030-inch bore small-block at a 0.018-inch top ring gap. However, if nitrous or a serious turbo boost package was in this engine’s future, the recommended gap would change to 0.006-inch per inch of bore which would produce a top ring gap of 0.024-inch – a significant increase to compensate for the higher cylinder temperature.

These are the minimum recommended ring end gaps, per inch of bore, as recommended by JE Pistons. Other piston or ring companies may offer slightly different recommendations. Per this chart, a 4.500-inch-bore big-block Chevy that will see moderate nitrous would need a (4.50 x .005) = 0.0225-inch top ring gap and a (4.50 x .0055) .02475-inch second ring gap, rounded to .025 inch.

You may also notice that the second ring gap is recommended to be slightly greater than that of the top ring. Not all that long ago, accepted practice was to tighten the second ring gap over the top ring’s. However, testing has revealed that increasing the second ring gap relieves pressure buildup in between the top and second rings, which can partially unload the top ring and induce leakage.

Keep in mind that the primary job for the second ring is not to seal compression but instead to squeegee any remaining oil off the cylinder walls left by the oil rings. It’s is the second ring’s secondary  job to help seal cylinder pressure.

Once a proper gap has been calculated, there are several necessary steps to performing this job properly. In the early days, engine builders used a flat file mounted in a vise to file the rings. If you are stuck on a desert island with no tools, that’s not a bad idea. But today there are far better ways to accomplish this task.

The least expensive ring grinder is a manual tool much like this antique Childs & Albert version (left). Always turn the wheel counterclockwise (to rotate the grinding wheel from the face of the ring, inward) to prevent potentially chipping the moly coating from the ring face. Our electric ring filer (right) is much faster than the manual, but experience has shown that we still have to work up to the final gap, involving multiple filings and measurement with each ring. This delivers the most accurate results.

Nothing To It But To Do It

The least expensive ring grinders are manual units where the operator turns an abrasive wheel by hand. We’ve even seen photos of one enterprising shop that adapted a small hand drill to spin the abrasive wheel. For the occasional engine builder, the manual grinders are inexpensive and will do a great job. Of course, there are also more expensive electric grinders from Proform and a really nice one from Total Seal for the professional engine builder.

There are several key points to grinding the end gap. First is to always establish the starting point of the ring. Many times the 0.005-inch oversize ring will have almost no gap. This is important in establishing how much material to remove. Measuring the end gap is also critical in that the ring must be square to the bore, using the tools and methods illustrated in the photos and captions.

To accurately measure piston end gap, you will need a piston squaring tool to accurately place the ring in the bore. This particular tool (left) is similar to the ProForm tools that offers double sided steps for a wider range of use. For our small-block Chevy builds (right) we use an old 4.00-inch bore piston with a used second ring in place that squares the ring 0.500-inch down from the deck. This should be a flat or dished piston to square the new ring properly.

Once grinding starts, we prefer to start with a 0.003 to 0.008-inch thinner feeler gauge and work up to thicker gauge to establish the final clearance. We’ve found that this progressive change loads the ring into the bore and makes measuring the true gap more accurate.

When positioning the ring on the grinding tool, make sure the ring is perfectly flat and parallel against the grinding wheel. If it is not parallel, the gap will become tapered which should be avoided. Always grind only on one side of the ring and initially remove only a small amount to work up to the desired clearance. Always lightly de-burr the ring every time it has been filed to prevent nicking the cylinder wall. Also de-burr as lightly as possible, as excessive work can also taper the end of the rings that essentially increases the working end gap.

Once the ring is squared in the bore, a feeler gauge will indicate the gap. We like to work up from a smaller feeler gauge to help push the ring into the bore.

As mentioned, working up to the final end gap for each ring is the best approach. Using the hand-grinder, it’s best to keep track of the number of revolutions, as that will help dial in the remaining rings to be filed. Another important point for manual machines is to always turn the handle counterclockwise since that will grind the ring inward instead of outward. This prevents potentially splitting or peeling the inlaid moly in the ring face.

While a small flat file or even sandpaper can be used to de-burr, we prefer to use a light whetstone that is often used for sharpening knives. A couple of light passes with the stone is all that is needed.

Lightly de-burr the end of the ring to remove any sharp edges that could scratch the bore. Do this after each filing session to prevent damage to the bore. You can use a small file or sandpaper but we like to use a small knife sharpening or honing stone.

There are other measurements related to ring blueprinting like back side clearances, vertical ring clearance to the ring land and others but we’ll save those for a later story. Setting the ring end gaps can easily take the better part of four or more hours to complete.

We like to make sure the ring is square to the grinding tool by using a small flashlight under the ring to ensure the ring is perpendicular to the grinding wheel.

Yes, the process is tedious but necessary. It’s the price that has to be paid if you’re looking for every last bit of power from that engine. So the next time you wonder why it’s so expensive to have a custom engine built, at least you now know what it takes just to set the rings. It’s called craftsmanship.

Article Sources

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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