Ask Sarah: O-ring a block, AEM water/methanol kit, importing a Tsuru


Dear Sarah, i would like to know what's 'O-ringing' a block?  I know it basically replaces the headgasket with an o-ring on each cylinder but could you elaborate more on the whole process, method, advantage/disadvantage, variation (i heard some still incorporate a headgasket but some don't?) and the 'type' of o-ring used….thanks very much 
Amaury Tan Lip Han

Got clamping pressure???

Centroid CNC machine
A CNC machine like this one from Centroid uses a heavy duty rotary axis and rigid block fixturing to handle everything from o-ringing a block to digitally blueprinting the block including the diameters, locations, and angles of cylinder bores.

Most stock head gaskets are not designed to seal against 3-4 times the power the cylinder heads were built to hold. For the most part, modern composite gaskets can handle the cylinder pressures most racing engines endure but when supersizing to a powerful forced induction component, machining o-rings around the cylinder bores may improve gasket seal and help prevent blowing out the stock gasket. O-ringing a block involves machining a shallow groove around the cylinder bores and using a rawhide headed hammer to tap a wire into the groove.  To do this, the block is bolted to the table of a (CNC) milling machine. The center of the bore is located and a tool set that can provide the right diameter and groove cut to the precise depth is used to machine the groove. Depending on the block, a vernier caliper can be used to to measure the bore and depth of the steel insert overlap and the groove can be hand cut, or rotated with a manual o-ring cutter to score the groove. Surprisingly, cast iron is soft enough to allow this to be done accurately.

The groove depth and size of the wire is calculated precisely so the wire will extend slightly above the deck. The intent is for the wire to be located in the exact center of the gasket. The ends of the wire should be hand filed so they line up perfectly square-,no gaps, no overlap! The wire forms a tight fit in the groove and bites into the head gasket to prevent blowout. Since the o-ring machining process can cause the cylinder bore to close up a bit, it should be completed before the final honing of the bores is performed.

Blown head gasket
Don't blow it!  The head gasket, that is…

The advantages of o-ringing is increased clamp around the cylinder bore to seal combustion pressure managing higher compression ratios and effective combustion pressures. Catch 22- it does lead to additional cylinder head deflection which contributes to (go figure…) bad cylinder head sealing.  Don’t run a gasket with a built in o-ring with an o-ringed head or block. The overlap between the two works adversely.


Hylomar around o-ring
This is an o-ring around the rotary housing of Mazdatrix's 13B REW/MSP hybrid. Hylomar is used to form a flexible, plastic seal. It is capable of sealing in situations where exposed to coolant, oil, or fuel- perfect for o-ring applications.

Copper gaskets hold more boost pressure so they are often employed in high horsepower drag racing applications with a stainless steel o-ring wire. A small bead of anaerobic sealer like Hylomar can help prevent coolant from leaking, more often a problem with copper gaskets. The block can also be o-ringed to use a composite head gasket with soft annealed stainless steel or copper rings. MLS composite gaskets are more forgiving and seal better and pretty much make o-ringing obsolete except in instances of more specialized or unusual builds or when pushing power and pressures to the limit. High performance head gaskets offered from companies like HKS, Cosworth, Tomei, and A’pexi include stopper layers which provide higher sealing forces around the cylinders. These head gaskets are die-stamped so they can provide much more precise tolerances. The folded layer design increases the gasket by 0.15mm around the cylinders which provides greater and more consistent clamping loads when torqueing down the cylinder head, improving ring seal considerably.

Cosworth FSL Head Gasket
The Cosworth Folded Stopper Layer head gasket provides a uniform, tight seal around the cylinder bore.

There’s only so much power engines can hold regardless of how much you modify them, at least if longevity is your goal. You could weld a head gasket on but it’d only redirect your point of failure to another issue such as broken ring lands or the demise of your rod bearings. If you continue to blow head gaskets, your only other option is to adjust your tuning or run less power.  But where's the fun in that?  Go fast or suck!



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