In the diesel performance world, O-ringing or fire-ringing the heads is a very common upgrade. Common, because, in high output situations, it’s all that will keep the fire contained in the combustion chamber when pressure is trying to launch the cylinder head into low synchronous orbit.
The head gasket is a weak link in any performance diesel. Some engines start off a little better equipped than others but, sooner or later, power levels will rise to the point where the fire can no longer be contained with the stock head gasket, with or without head studs or other high-tension fasteners.
Boost pressure is most often touted as the marker for head gasket improvement, but it’s more complicated than that. Timing, compression ratio and “drugs” (that is, nitrous or propane) all add their own complications.
A shop might tell you, for example, that a stock 24-valve Cummins can tolerate 40 psi boost before it needs to be O-ringed. What they really mean is that when you add the fuel and timing to go with that boost, the survival of the stock head gasket is in doubt.
We could get real complicated here and talk about boost versus discharge pressure, dynamic versus static compression and thermodynamics, but we don’t need to. The general recommendations based on boost hold water—as long as you remember that it’s all about cylinder pressure, or mean effective pressure, which is the amount of pressure in the cylinder when the fuel/air charge fires. That can be raised simply by changing the injection timing a few degrees. Just remember the other factors, if you’re thinking, “I’m safe under 40 psi boost!”
What Is O-ringing?
Basically, you’re cutting a groove into the cylinder head around the combustion chamber (the block deck on some engines) and inserting high-temperature stainless steel wire into the groove so that it protrudes from the fire deck. The wire more completely crushes the steel fire-ring built into the head gasket for a tighter seal.
Fire-ringing is similar, but the groove is deeper, and the ring is larger. The original fire-ring in the head gasket is removed, and the new steel ring is crushed between the head and block. Sometimes, grooves are cut into both the head and the block.
Along with the O-ring or fire-ring, it’s mandatory to employ better head-retention hardware. That usually means a head stud kit that uses studs of a high-grade steel that don’t stretch much.
When you ask a diesel performance shop about how much boost your engine can carry after it’s been O- or fire-ringed, you may find them hesitant to give you a number or, if they do, it will come with lots of conditions. It’s highly variable between engine types, but a few degrees of timing or a little extra nitrous could put you right over the top. They don’t want you coming back saying, “But you told me … !”
On Cummins engines, the closest consensus (admittedly conservative) we could find was to O-ring above 40 psi and fire-ring at 70 psi. However, there are other considerations.
Fire-ringing is not generally tolerant of the heat cycles of a daily driver. They may not seal so well when the engine is cold, for example. You are also married to constantly retorquing the head.
However, because O-ringing is bolstering a regular head gasket, it works well in the daily driver environment. Daily drivers who are weekend warriors report O-ringing holding up to 100 psi in occasional spirited driving. Most everyone agrees this is possible—but pushing the envelope.
Ringing In the 24-Valve Cummins
We had the opportunity to follow along as John Downard, of Preble County Diesel, in Camden, Ohio, O-ringed an early 2000s Cummins 24-valve head. He used a BHJ ORG-3 O-ring groove-cutter. This tool can also install fire-rings and, with the right tool blocks, O-ringing and fire-ringing can be done to a variety of heads and blocks.
The cutter machines a circular groove in the head. In this case, a 0.041-inch stainless wire was used, and the groove allows for a loose interference fit. The depth of the groove, which controls how much wire sticks out above the fire deck, is variable. Stickout is most often chosen according to the head gasket and how much crush is available in the fire-ring. Too much stickout, combined with not enough available crush, and the gasket around the fire-ring isn’t held tight enough. Not enough stickout, and you lose the whole reason for O-ringing in the first place.
In our investigation, we discovered a wide variance of preferences out there for stickout. It’s often quoted that you don’t want more stickout than about 10 percent of the gasket thickness.
Because we were using a 0.065-inch 12V marine head gasket (as opposed to about 0.045 for the stock), John opted for a 0.011-inch stickout. Lenny Reed, at Dynamite Diesel, likes to see about 0.004-inchstickout with stock gaskets and not a thousandth more. Both of these hold to near that general credo, but we’ve heard of shops preferring more stickout for a variety of reasons.
The 24-valve head in question belongs to the University of Northwestern Ohio’s Diesel Club, which is building a pulling truck as a project. It was getting an overhaul (with Sealed Power parts and will appear in a future issue) and some upgrades for an upcoming competition.
Sources
ARP
800.826.3045
BHJ Products
510.797.6780
Dynamite Diesel
360.794.7974
Fel Pro
Preble County Diesel
937.452.5505
University of Northwestern Ohio
419.998.3120
This is in the November issue.
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- The combustion pressure inside your diesel engine is on a mission … to escape. The OE builds a good enough “jail” to keep it confined at stock power levels, but when you let those gasses get “pumped up” by power-adders, the stock jail may not be enough to hold them. One of the first mods the UNOH Diesel Club put on its list was to stud and O-ring the head of their 24-valve Cummins (with the help of Preble County Diesel, Fel-Pro and ARP). The engine now has the beef to handle some upgrades without another teardown, and the Diesel Club has some useful boost limits to build around.
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- You have to start with a perfectly true head. John Downard, at Preble County Diesel, won’t touch a head unless it’s within 0.004 inch of being perfectly straight. The UNOH was off by about 0.005, so it was machined. Bear in mind that when you take material off the head of a Cummins, it affects injector-targeting, so you might have to adjust by thicker or thinner head gaskets or with thicker or thinner copper injector washers. You want the cone of the injector spray pattern well inside the piston cup. If it sprays outside the cup, it can have disastrous results on piston life.
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- After bolting the application-specific registration plate (a) to the absolutely clean fire deck of the head, the cutter head assembly is (b) attached. The actual cutters (c) are installed and calibrated in a separate fixture, according what size groove will be cut.
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- The depth of the cut is set right below the handle (arrow) and John likes to take about 0.020 inch off in the first cut. The BHJ tool is manual, so you spin the handle rapidly and smoothly until resistance is eliminated. How deep a groove is cut depends on the size of the wire and the stickout desired. Since John was using 0.041-inch wire and wanted a 0.011-inch stickout, he cut the grooves 0.030 inch deep.
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- After the first deep cut, the succeeding ones will be shallower and checked for depth with a depth micrometer. This is the “measure-twice-cut-once” part. You can’t put metal back on.
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- A closeup of how the cutter head slices in a groove. This process is repeated exactly the same for each cylinder.
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- The wire is an expensive Inconel stainless steel, the same high-temperature stuff used in hi-po exhaust valves. It’s cut a little oversized for the circle, and John likes to use a brass drift to tap it into place.
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- The end is cut off so that the two ends butt tightly together when tapped into place.
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- The only rule for placing the wire gaps is that they can’t adjoin a coolant hole. John has chosen a standardized aiming point toward the exhaust side of the head, which he demonstrates here.
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- A forest of high-grade steel! Head studs are necessary if you want to successfully contain the fire, and since ARP is well known for its stud kits, it was a natural choice for this project. If necessary, and ideally, chase the holes in the block before installing the studs. The studs are installed hand-tight only. They can be Locktited in place (not generally recommended) or sealed with thread sealer, if necessary, but you must make sure to get the head on and torqued before the chemicals are fully cured.
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- There are many choices for head gasket thickness. The factory replacement Fel Pro 24V gasket (top) is about 0.045 inch thick, compressed. The 12V marine gasket (lower) is about 0.065 inch thick, but you can get several head gasket thicknesses from Cummins or high-performance diesel shops directly. You would choose the gasket according the compression ratio desired (Dynomite Diesel’s Lenny Reed’s rule of thumb is that 0.010 inch equals a ratio change of about 3/10), and the injector targeting desired. The O-ring stickout would then be dialed in according to the gasket thickness.
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- Head studs are vital elements in any performance diesel and are more vital for a good O-ring result. ARP has been at the cutting edge of these for decades now, and this kit is the one used on our 24V project. The hardware is made from a variation of an SAE 8740 alloy called ARP-2000, which is a proprietary alloy developed by ARP. It has a tensile strength of 220,000 psi and a yield strength of 180,000 psi. We tried, but weren’t able to get material and strength information for the OE head bolts. We did find out from ARP that the minimum the company shoots for is a 20-to-25 percent increase in strength with its standard fasteners. ARP offers custom products that are much stronger.
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- Yeah, these heads are heavy. Watch your fingers as you carefully lower the head onto the studs. In addition, make sure you don’t drop the head and mash the gasket.
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- You want the most accurate torque measurements you can get, so lubricate the washers and the threads with ARP’s special Ultra Torque ™ assembly lube before installing the nuts. This stuff was especially designed by ARP and tested against a plethora of other fastener lubes. It reigns supreme and will allow the fastener to attain 95-to-100 percent of the applied torque accurately. Don’t think any of the goop you have is better, because it’s not. Save any that’s left for other fasteners or, better yet, buy it by the bottle and use it on everything.
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- According to the ARP instructions, the heads are torqued in three steps to 125 lb/ft in the factory specified order. Your arm will be sore! It’s best to let the engine sit a day and then retorque the nuts again. Once the engine has had a few heat cycles, and before you work the engine hard, the head should be retorqued again. Yeah, a lot of torquing, but your biceps will grow.
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- This closeup shows you how the O-ring (a) puts a carefully placed dent into the fire-ring of the head gasket (b), crushing it even more than normal for a better seal and more support against increased pressure.
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- For comparison, here is a fire-ringed 12-valve head. Note that the head gasket fire-ring (a) has been laser cut and removed. The new fire-ring itself (b) is larger and specially made of a mild alloy steel that crushes a little. The groove in the head (c) is larger to suit. There may also be a corresponding groove cut into the block. Fire-ringing will hold lots more pressure but is high maintenance—meaning regular retorquing of the head bolts. It doesn’t deal with heat cycles well and may leak in a cold engine. For those reasons, it’s not usually done on engines that will be street-driven.
Tags: 24-Valve Cummins, ARP, BHJ Products, Dynamite Diesel, O-Rings, Preble County Diesel
