Add O-rings to increase combustion seal in high-pressure applications
By Mike Mavrigian
CYLINDER HEAD/BLOCK O-RING
In order to obtain a superior seal around the head’s combustion chambers/around the block’s cylinder bores, a stainless steel or copper wire O-ring can be installed to increase sealing in extreme cylinder pressure applications (high compression ratio, use of nitrous injection, high boost forced induction).
Actually there are two approaches: O-ring or fire ring. An O-ring is placed into a machined groove (in block deck or head) and is located directly in line/mating to the head gasket’s compression ring. A fire ring (often called a hoop) is a slightly wider insert that is placed into a machined groove that takes the place of the head gasket’s compression ring. An O-ring creates a pinch-effect at the gasket’s compression ring to supplement the sealing ability of the head gasket. O-ringing is generally applicable for builds up to around 750 HP, with 70 psi or less boost. Fire ringing applies more to higher HP builds where boost is 80 psi or higher. Some fire rings are made of phosphor bronze, which expands under heat for a superior combustion seal.
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Installed O-rings should protrude no more than 25% of installed head gasket thickness. A protrusion height of about 0.008-0.010″ is acceptable for head gaskets thicknesses of up to 0.050″ gasket thickness. Fire rings may be installed with about 0.025″ protrusion (the goal is to obtain about 30% more installed height than the head gasket thickness on dry block (no coolant) applications and about 25% protrusion for wet blocks (coolant style block).
According to SCE Gaskets, if the combustion chamber or cylinder bore size dictates that the O-rings will be placed less than 0.200″ apart between cylinders, it is advisable to use a figure-eight groove pattern, which allows for more even clamping load over the head deck surface. If a figure-eight pattern is required this should be done on a CNC machine.
The following information was kindly provided by Ryan Thompson of Rottler Manufacturing:
O-ringing is perhaps the simplest and most cost effective way to improve sealing, Cylinder heads are cheaper and easier to replace if anything goes wrong during the engines life. This is the first choice for applying O-ring grooving in engine building. Very common on light -truck diesel cylinder heads such a Powerstroke, Cummins, and Duramax. For any customers looking to perform light builds or run tunes on their engines this is a great option when increasing power beyond stock levels.
The standard application is .041″ diameter piano wire, knife edge cut at a 45 degree angle so it lays on top of itself at termination. Typically, I recommend between .005-.010″ of protrusion for most applications, albeit this depends on your gasket thickness. Another good rule of thumb is using no more than 25% of your gasket thickness for the protrusion of the O-ring.
The machining of this can be done with manual devices such as the system sold by BHJ, Boring machines such as Rottler’s F10XS, or full CNC machines such as Rottler’s F69A. The latter being the most accurate given CNC and probing.
For a cylinder head O-ring groove using wire only, the tolerances for position of the ring only needs to be within a few thousandths as you aren’t attempting to install a full hoop or match between block and head.
The second option would be to place the O-ring of a similar type and shape in the engine block, This option is less common as engine blocks typically aren’t out of vehicles as often as cylinder heads. The benefit of this option is it’s a permanent solution for the engine and if a head cracks or warps, the customer doesn’t have to worry about paying for the machining again and again. Beyond that, this operation is the easiest to perform from a manufacturing standpoint as it can be done on just about any boring machine whether its CNC or manual. The machinist simply needs a grooving bit and holder that can be placed in the machines spindle.
The final options/improvements would be to use Top-Fuel style hoops and “split the groove depths between the block and the cylinder head. This is the method used for extremely high horsepower applications upward of 2000 HP.
To perform a top fuel hoop with a matched block and head groove this work has to be done on a Full CNC machining center such as the Rottler F69A. The tolerances are much tighter, sub 1 thousandth of an inch, which is why CNC is required. To perform this work the dowel pin location is used as the reference point for the grooves on both the block and cylinder head.
For the protrusion in this scenario typically you will sink the hoop into either the block or head so that the hoop protrudes by .010-.015″ above the surface. After that the “receiver groove” can be machined on the mating component (block or head). The receiver groove should allow the wire to protrude into it and then compress within the groove. For best results the receiver groove should be slightly larger in width than the wire, around .010″ and should have around .005″ less depth than the protrusion set for the hoop. This allows the hoop to compress and fill the void around the groove to create a very tight a matched seal.
MANUALLY O-RINGING THE BLOCK
As noted, cutting the grooves in the block deck can be done on CNC. If this equipment isn’t available, a block deck can be O-ring grooved manually by using Isky’s Groove-O-Matic O-ring groove cutting tool, which features a carbide cutter, a twin-arm operating handle and self-centering in the bore.
Determine groove location: The wire should be located to align with the head gasket’s fire ring. Do not create the groove where it will intersect a water port. Also, pay strict attention to final wire protrusion above deck. If the wire is too high, while this will seal cylinder pressure, excess wire height will not allow the head gasket to seal water ports. Aftermarket cylinder head gaskets will/may provide advice for O-ring applications, based on head gasket thickness. If in doubt, contact the gasket maker for advice.
The cutting should be done in a two-step process, first at a depth of 0.025″, followed by a final fine cut at 0.005″. Using a spare 0.025″ feeler gauge, cut the gauge in four pieces and place each piece under the tool’s four support legs (at 12, 6, 3 and 9 o-clock). Tighten the tool’s center screw. Allow the carbide cutter to drop and rest onto the deck surface and tighten the cutter’s set screw.
Applying downward pressure, work the cutter around the entire perimeter until the groove has been cut at 0.025″ depth. Remove the tool and clean. Clean all debris from the groove and from the cutter and four support legs. This is critical.
Loosen the cutter set screw. Install the tool in the bore again, placing four sections of 0.005″ feeler gauge under all four support legs. Tighten the tool’s center screw. Allow the cutter to drop and contact the bottom of the groove and tighten the cutter set screw. Rotate tool along the entire groove until it stops cutting (this tells you that you’ve cut the additional 0.005″ of depth. Remove the tool and thoroughly clean debris from the groove. Test fit a wire into the groove, tapping it into the groove using a brass hammer or hammer and PVC or brass plug (or simply use the plastic grip end of a gasket scraper or screwdriver) and verify that the wire protrudes 0.008-0.011″ above deck. Perform this at each cylinder bore. Once machining is complete, wash the block thoroughly to remove any debris. Using a 0.041″ wire, if you cut the groove a total of 0.030″, you should have 0.011″ above deck.
Installing the wire: It’s common to use 0.041″ annealed 304 stainless steel wire for O-ring applications. Cut a section of wire longer than needed to obtain an initial wire overlap. Use a fine file or a honing stone to flatten the end of the wire (the end to be installed first). Install the wire placing the flat end near/adjacent to a head bolt hole. Seat the wire using as noted earlier. Using a brass hammer or a brass or PVC plug, this avoids flattening the wire. It does not require much pressure…simply tap the wire into the groove to secure it. Using a pencil or a Sharpie, make a mark on the block deck at the first-installed wire end. Lay the remainder of the wire atop the first end and mark the wire to reference the cutting location. Remember that we want a no-gap fit (wire ends butting against each other…with a relaxed fit, avoiding a forced interference fit). File a flat at the end of this cut.
Note: some recommendations involve finishing the wire ends at mating 45 degree angles, as mentioned in information previously provided here by Rottler.
Cutting to length needs to be done carefully to avoid cutting the wire too short. If a discernable gap results, scrap the wire and start over with a fresh wire.
MANUALLY O-RINGING A CYLINDER HEAD OR BLOCK
When it comes to quality-engineered solutions, BHJ always comes up with the answer. Their ORG-3 O-ring groove cutter allows those who don’t have access to CNC equipment to accurately cut O-ring receiver grooves in cylinder heads or blocks. A precision-machined register plate (dedicated plates available for specific cylinder heads) secures to the head deck, providing accurately placed template guide bores. The cutter assembly, with crank handle and adjustable tool block, then secures to the register plate. The cutter, equipped with a carbide insert, allows easy and precise groove cutting.
The ORG-3 kit includes the cutter head assembly and tool block, a graduated adjusting screw, adjustable tool holder, carbide insert size of choice, insert screw, O-ring groove depth gauge, 1/8″ hex wrench, insert Torx wrench, 5/32″ hex wrench, big-bore adapter ring, Allen-head adapter ring screws and four cutter head hold-down knobs. Again, the register plates are application specific and sold separately. The carbide cutter size must accommodate the O-ring wire diameter that you plan to use.
The cutter system can be used to cut grooves for conventional stainless steel O-ring wire for use with copper head gaskets, copper O-ring wire for MLS gaskets Fel-Pro Loc Wire gaskets, as well as fire-ring installation in diesel and fuel drag racing applications.
Before the machining work begins, verify that the deck surface (block or head) is clean and flat. Make any required deck resurfacing corrections beforehand. The deck must be clean and free of any high spots.
When installing the register plate, do NOT torque the register plate clamping fasteners to duplicate head bolt torque. A high degree of fastener clamping can cause the plate to warp, which could result in uneven groove depths. Simply snug the fasteners evenly to about 10 ft-lb, just enough to secure the plate from movement. The goal is to simply snug in place to prevent the plate from moving.
Note: In extreme cases (hard spots in cast iron for example), begin with light cuts (as little as 0.002″ of depth per cut), until reaching the desired groove depth. This can be done using the tool’s depth-adjustment knob. Avoid dialing in the final desired depth at the beginning to avoid over-cutting the groove. While turning the cutter handle clockwise, apply rapid and smooth motion, using a steady downward palm pressure. Small amounts of a light lube such as WD40 will aid in cutting. Vacuum the area as often as possible to remove debris. Avoid excessive downward force. The supplied O-ring depth gauge allows precise monitoring of the cut depth.
Do not blindly move to the next cylinder location and start cutting. It’s advisable to double-check the diameter and depth settings of the cutter and to verify that the depth-locking knob is securely tightened before continuing. When all grooves have been cut, it may be necessary to gently flat-file the deck’s adjacent groove locations to remove any burrs or raised edges.
Note: BHJ strongly recommends practicing on a scrap cylinder head or block, to familiarize yourself with the technique involved. If you don’t have access to a CNC machining center, the BHJ system is the best way to go.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2026EPQ1/#p=38