Cylinder Blow-by

Understanding Cylinder Blow-by

While this industry continually runs on tighter and tighter engine clearances, in today’s world of engine building, no one has yet to figure out how to run engine parts on zero clearance. There has to be some amount of space available or an engine simply won’t run. Thus, when it comes to pistons and piston rings, there will always be some amount of blow-by occurring at any given time.

Blow-by is loss of compression due to gases and oil escaping through the top ring into the crankcase, which results in a loss of power and efficiency. It is an engine builder’s job to understand the effects of blow-by, why it is happening and find a balance for the given application.

Excessive blow-by doesn’t just happen. There is always a reason, and too much blow-by can mean serious engine issues are on the horizon.

So what do you need to know about blow-by and how to fix the issues that can cause it or result from it? There are several important factors at play.

“Certain levels of blow-by are unavoidable, acceptable and are helpful in providing a mechanism to move oil that is on the cylinder walls back into the crankcase rather than allowing the oil to move upward into the combustion chamber and get burned,” says Justin Keiffer, director of applications engineering at Hastings Manufacturing Company. “Blow-by is instrumental in reducing oil consumption through pressure balancing of the piston rings to keep them stable in the grooves during certain portions of the operating cycle. Blow-by does, however, represent lost power, so there is a limit as to how much is acceptable.”

Pressure balancing will result in the upper compression ring sealing 85-90% of combustion gases with the lower compression ring making up the other 10-15% while preventing pressure buildup between the two rings that causes the top ring not to fully seat against the bottom side.

A fresh engine should have 1 percent or less of blow-by. However, when you get over 10 percent blow-by there is an issue. “It may not be critical right then, but it won’t get any better from then on,” says Matt Dickmeyer of Dickmeyer Automotive Engineering. “It’s the progression of a problem. Some racing applications just beat the snot out of engines literally. So from the time it’s fired up until the time it’s rebuilt it’s on a downward decline of longevity and ring seal. When you get to that 10 percent blow-by you start losing power.”

There are a couple of ways to measure blow-by. First, you can use a compression gauge to measure the pressure the piston exerts on the fuel/air mixture before combustion.

“This gauge is threaded into the spark plug hole,” says Gary Nebel, a senior product engineer for Federal-Mogul Motorparts. “If the cylinder holds consistent pressure over a span of four or five cranks, there isn’t a blow-by issue. If pressure drops, further investigation is warranted. You can also use a flowmeter, or a manometer that can calculate flow, to measure the rate at which gases flow through the crankcase.”

Cylinder pressure is what makes power. Superchargers, turbochargers, larger cams, larger heads, carburetors, and intakes are all ways you get the air into the cylinder. But it’s not necessarily about getting the air in – it’s about cylinder pressure. It’s trapping combustion pressure – getting the air in, keeping it in and compressing it to maximize the desired operating cycle/range.

“Sealing and controlling cylinder pressure is of paramount importance in any engine, especially one intended for increased performance,” Dickmeyer says.

Blow-by Misconceptions

The general understanding of blow-by is that it’s simply bad ring seal, and that’s the misconception. “It’s more than just the ring,” says Keith Jones of Total Seal Piston Rings. “Pistons, rings and cylinders all have to work together. You’ve got to get them all right, and if you do that it’s going to seal really well. If you get one of them wrong, it’s not going to be as good as it can be.

“You also have to make sure your pistons are good. Guys will try to reuse pistons and they’ll throw a mic across the skirt and if the size of the skirt is good they’ll say, ‘Oh, it’s good.’ But they haven’t run a set of gauge pins through the ring grooves. If you do that you might find there is tapering in the lands, which means the piston is shot. The ring won’t be able to fix those kinds of mistakes. If the piston or cylinder isn’t good, the ring won’t work either.”

To easily explain piston and ring set up, if you look at the profiles of rings you can tell what it is meant to do. “The top ring is not intended to control oil whatsoever and the second ring is not intended to seal compression,” Dickmeyer says.

It used to be that guys would gap the top ring wide because it sees more heat and the second ring would be tighter. “Guys often run the top ring gap much too wide for what they need and they end up losing power they think they have,” he says. However, the opposite is true. “The theory is to gap the top ring as close to touching without it touching to make it seal better,” he continues. “Builders need to look at rings as though oil is never to get above the top ring and compression is never to get below it, and good things will start to happen.”

If your ring gap is too wide and you are losing ring seal you then push oil into the combustion chamber, and when you burn oil it is more likely to detonate, so you’re blowing the engine up. “That often gets misdiagnosed,” Dickmeyer says. “They think their pistons are getting so hot from the amount of power and the compression that the ring ends have to be butting, so they make the gap wider and wider. What they don’t realize is they’re making the problem worse and worse because the ring gaps weren’t butting. That is how things can go bad really quick. Correct diagnosis is very important.”

Common Causes

One of the problems is the ring face needs to remain in contact with the cylinder bore at all times. Depending on the machining methods, the cylinder roundness will vary at a given condition. Typically, machine shops achieve a round bore at a room temperature free state similar to assembly cnditions of the engine, however, when you add fuel, air and ignition, it creates heat and pressure that changes the geometry of the cylinder. This is why you see some machine shops offering torque plates and “Hot Honing” to attempt to simulate running conditions. Otherwise, the once perfect round bore becomes a geometric problem that the ring has to deal with. A given ring material and design can only compensate for so much out-of-roundness.

“Once the blow-by gets past the top ring, we need to quickly get it past the second ring,” says Bill McKnight, team leader/training at MAHLE. “If it gets trapped between the top and second ring, we cannot load the top ring for the next firing cycle and blow-by goes way up.”

Measuring blow-by is a bit difficult, at least by scientific methods. Top fuel dragsters and Nitro funny cars measure oil pan pressure (blow-by) in percent using a pan pressure monitor. “When pan pressure reaches 10%, the engine is automatically shut off (new rule for 2016), indicating that engine condition at that point is quickly heading south,” McKnight says. “In a normal, naturally aspirated engine, 2-3% would be a desirable max. When you have to tie the dipstick down to keep it from blowing out, you’ve got too much.”

The bottom line is blow-by is more than just about piston rings. If you approach the whole scenario of blow-by, you have multiple leakage paths.

“You can have leakage paths between the ring and cylinder wall face itself, and inner face of the ring to the ring groove, because you need clearance in there, and there’s going to be leakage across the end gap of the ring,” Jones says. “What we work on is how to get as close as possible to fix those three leakage paths. Less blow-by is more desirable because any amount of blow-by is a measure of inefficiency in the engine.”

Other common issues seen are poor cylinder finish and excessive clearances in the ring land.

“Today, there are a lot of people stroking engines,” Jones says. “When you stroke an engine you change the geometry of the engine. You can run into a phenomenon called excessive rock-over in the piston, which is common, meaning when the piston hits top dead center and bottom dead center there is going to be some rotation or rock when the piston is changing direction as the connecting rod is changing the sweep of its arch. You want to keep that to a minimum. If that is kept low, the piston is stable in the bore, and if the piston is stable the rings are stable. Stable rings are happy rings.

“If you do have a lot of rock-over, you’re literally picking the rings up off the cylinder wall and they’ll lose ring seal. That can turn into a blow-by problem and an oil control problem. A stable piston is a happy piston. A piston is a ring holder. If the piston isn’t doing its job, the rings can’t do their job. It’s not the rings’ job to fix the piston, it’s the pistons’ job to make the rings happy.”

A lot of times when you have a loss in compression or a mechanical issue right after start up, its an installer error.

“The last person to touch the engine is solely the one responsible for quality control, whether you did it or not,” Dickmeyer says. “The biggest problem is installer error, incorrect machining for the application, and incorrect rings. I also see people buy pistons before they have the rings. Often times you have to widen the bore to release the tension on the ring in those cases.”

The bore diameter establishes the tension of the ring because it either compresses it in or it releases it out.

“It’s a slippery slope because if you lose too much tension you can lose sealing, the ring can flutter, or if you have too much tension you’ll cause excessive drag and reduce engine output while potentially causing wear to the cylinder and you’ll wear the ring and you’ll cost yourself power because of the resistance,” he says.

According to manufacturers such as Hastings, a ring is designed with a given tension for a given application and bore diameters should not be sized to target a compression ring’s tension to a prescribed value. Tension is set up and targeted during the machining/manufacturing of the given compression ring for the final bore diameter.

Another problem some builders run into is when they estimate an engine’s potential power without running it on the dyno. According to Dickmeyer, you can build your clearances for what you wish and hope the power is, but if you’re not there or you’re over, those clearances can cause a major problem, and cause it fast.

“Round, properly honed cylinders, clean bores, proper fitting rings and pistons is what controls blow-by,” McKnight says. “None of this is something you fix quickly, so when you have too much blow-by the engine has to come apart.”


To fix blow-by, you need to look at what the potential sources of excessive blow-by are, and there are many. These factors may exist individually or in combination.

“Piston rings are the gate keepers when it comes to blow-by, and play a big role in controlling the phenomenon,” says Pim van den Bergh, manager of business development at NPR of America. “The reality is that simply replacing piston rings is a short-lived solution. An engine that shows excessive blow-by will likely need to be bored and honed to the next oversize.”

You’ll want to address excessive blow-by because the effects of it can lead to serious engine issues.

“Perhaps most damaging is oil contamination caused by migration of fuel from the combustion chamber to the lower block,” says Hoover Oliver, a senior product engineer for Federal-Mogul Motorparts. “You can also commonly see oil fouling the rings and leaving harmful carbon deposits on the piston crown, valves, valve seats and spark plug, in the valve guides and on the valve stem seals. This can cause localized hot spots on the piston and valves, in particular, and potentially lead to pre-ignition.”

Cylinder bore straightness, roundness and surface finish with a correct hone pattern, are key to the success of any piston ring. Also important is the maintenance of a correct ring gap.

“A ring gap that is not within tolerance will alert the engine builder to possible bore dimension issues, or incorrect size rings,” van den Bergh says.

If you have a stable piston, good cylinder finishes, good ring to ring groove clearances and the right end gap for the engine, it will have good ring seal. It’s when any one of those or any combination of those gets away from us that the numbers go flying through the roof and you’ll see loss of power, loss of fuel efficiency, loss of oil control, etc.

According to Keiffer, the cylinders can contribute to blow-by if you have poor cylinder finish, poor geometry or they are oversized, resulting in poor ring fit and excessive end gaps on compression rings.

“Cylinder honing and finishing is an art form,” Jones says. “It’s not just a matter of running a hone through it. As we put those scratches in the bore we’re creating a volume or VO figure, which will hold oil, but will also create leak paths. You have to be careful to get the right amount of volume and the right hone for the application. Another aspect you need to look at is bore geometry and how truly straight and round the bore is. Really sealing an engine up well is all about attention to detail, not just throwing it together.”

As far as rings are concerned, you want to make sure you use a quality ring that is truly flat and parallel on the piston, meaning there are no parts of the ring not touching the cylinder bore to eliminate leakage paths. Then you want to look at the ring to ring groove interface and how much clearance there is between the ring and the ring land.

“If we have an excessive amount of clearance between the ring and the ring land, that creates an area for additional or excessive blow-by,” Jones says. “You’ll want to run a reasonable amount of ring to ring groove clearance – about .001 – .0015 generally.”

The next thing is controlling the end gap. When you’re gapping the top ring you’re trying to create as tight a running clearance as you can without butting the ring ends together. If you butt the ring ends together, you’re in that zero clearance area, which is not operable and will destroy the engine.

“You want to run it as close to zero as you can get it, but the problem is that’s a big guessing game,” Jones says. “Everything affects that clearance because expansion occurs due to temperature, fuel, timing, block material, cooling system, heat exhaustion, etc., which all affects clearance and how tight you can run that ring gap. In a perfect world you would test and test your engine to figure out that perfect amount of clearance and perfect amount of end gap. However, most of us aren’t in that position so you make an educated guess. Sometimes that guess is really good and sometimes not.”

Figuring out why you’re losing compression is really dependent upon the application and what the problem is. And a lot of times its not that there is a progressive lose of compression, but that there was a loss from the get-go due to too wide of ring gaps, incorrect parts and applications or something done incorrectly.

“A lot of times it’s a quality control issue,” Dickmeyer says. “You have to assume that it wasn’t done correctly and verify that it is.”

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