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 blamed 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. This requires a special head gasket, whereas a-ringing utilizes a stock head gasket.

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

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. Stick out is most often chosen according to the head gasket and how much crush is available in the fire-ring. Too much stick out, combined with not enough available crush, and the gasket around the fire-ring isn't held tight enough. Not enough stick out, 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 stick out. It’s often quoted that you don’t want more stick out 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 stick out. Lenny Reed, at Dynamite Diesel, likes to see about 0.004-inch stick out 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 stick out for a variety of reasons.

Stainless wire being installed for o-ringing:


Fire Ring head gasket kit: