Import performance engines produce more power per cylinder than ever. On the street or track, it’s common for these engines to have four to six times the original horsepower. There are now four-cylinder engines with over 1,500 horsepower and six-cylinder engines with over 2,500. Power levels magnify an engine’s design flaws. The weakest links are the factory connecting rods and pistons. Therefore, most tuners upgrade these components during engine build. The engine’s head fasteners are sometimes overlooked, resulting in a leaking head gasket. You have to question the sanity of any engine builder who uses factory head bolts on a high-performance engine. Quality aftermarket head fasteners in various alloys can seal any application. To help you choose the right head fastener, we’ll discuss cylinder-head sealing factors and head fastener science.
Head Gaskets
Head gaskets and head fasteners hold the top of the engine to the bottom. Certain engine designs are better at maintaining a head gasket seal than others. The number of head fasteners per cylinder, the diameter of head fasteners, and the quality of head fastener material all affect how much power can be produced before the head lifts and the gasket seal is compromised. All other factors being equal, engines with five fasteners around each cylinder are less prone to head gasket failure. Engines with 12mm fasteners seal better than those with 11mm or 10mm fasteners. The fastener material quality is also crucial.
Better Or Bigger?
For most high-performance builds where engine torque is doubled, ARP 8740- or ARP2000-alloy head bolts or head studs will do. If you want to increase the torque output further, you may need to increase the size or material of the fasteners (or possibly employ a combination of both strategies). A 12mm stud has 10% less clamping force than a 1/2-inch stud. A higher-strength alloy can be torqued for more clamping force. An ARP 2000-alloy fastener has a 10% higher clamping force than the ARP 8740 alloy. ARP can produce custom head fasteners from L19-alloy or CA625+ if more clamping force is needed. The L19 or CA625+ alloy fastener increases clamping force by 35-40% over the 8740 alloys or 20% over the ARP 2000 alloy.
Considerations
Off-the-shelf CA625+ or custom L19 head fasteners cost three to six times as much as standard ARP 8740 or ARP2000 fasteners. If your engine block isn’t going to the machine shop, larger head fasteners may not be practical. As nearly every enthusiast is on a budget, you must be realistic when choosing high-performance head fasteners. Always supply the same head studs, head gasket, and torque specs used for engine assembly to the machine shop honing the cylinders to ensure round cylinders.
Bolts vs. Studs
Do you buy head studs or head bolts? In the past, the answer was, “Use studs whenever possible and bolts when you can’t remove the cylinder head(s) with studs.” When a stud can’t be removed from the cylinder block, the cylinder head must be lifted for the entire length of the stud. On some apps, this can be an issue. ARP now makes head studs that can be easily removed with the head still on the block.
The higher cylinder pressures, the more clamping force Barra head studs need. The larger diameter or better alloy fasteners can deliver more clamp load to maintain the head gasket and block compression seal. Since engine torque output is directly related to cylinder pressure, increasing engine torque will require improved head fasteners. Not all engines are the same; thicker block and head decks reduce clamp load.
