Performance Perspectives

Motor, May 2008 by Mavrigian, Mike

Studs offer many advantages over bolts in performance engines, especially at the main bearing caps and cylinder heads. Special techniques are required to achieve proper clamping loads.

In far too many cases, an engine builder focuses his attention on selecting the proper components for a specific performance engine build, and on determining proper clearances, intake and exhaust volume and flow, surface finishes and precision assembly. However, threaded fasteners, the vital link that secures everything together, are sometimes taken for granted. Here we'll discuss the advantages of using studs as opposed to bolts, primarily in main cap and cylinder head applications.

Studs are available for most popular engines, especially in terms of cylinder head applications. The use of studs as opposed to bolts provides a more accurate and uniform clamping load pattern throughout the cylinder head.

Studs also offer component installation advantages over bolts. Studs act as guide pins during alignment of such items as gaskets and engine covers. Studs are available with "bullet" noses, where a slightly diminished-diameter bare tip is featured. This greatly eases nut installation, allowing the nut to be dropped into position before thread engagement begins. Good examples of this include studs for the carburetor, distributor, exhaust header and oil pan. Instead of trying to hold a gasket or part in place while fumbling with bolts, then trying to align the bolts to their holes, the gasket or part can be dropped over the studs (so alignment poses no further problems). The washers and nuts can be installed without concern for the part shifting.

Another advantage of using studs is that you don't need to worry about insufficient clamping load resulting from a too-long bolt bottoming out in its hole. The stud is stationary, and clamping load is achieved by tightening the nut.

Granted, studs can't be applied in all situations, because component design might not allow it. For instance, a V-block engine's intake manifold (depending on design) may place the fastener locations on opposing angles (where the bolt holes on each bank angle inboard). Obviously, studs can't be employed because you'd never be able to drop the intake manifold over the angled studs. Studs, where applicable, offer both installation convenience and superior clamping uniformity.

Main Studs

For performance applications where a choice is available, studs should be used whenever possible instead of main cap bolts. Studs provide the ability to obtain much more accurate torque values because they don't twist during tightening as bolts do. Because studs remain stationary during nut tightening, they stretch in one axis alone, providing much more even and accurate clamping forces. Also, because the use of studs results in less force applied to the blocks threads, this extends the life of the threaded holes in the block. This is especially important with alloy blocks. The use of studs also eases main cap installation, and contributes to main cap alignment. There is less chance of main cap walking because the studs remain stationary during cap clamping.

Always check the edges of main fastener holes for raised edges that would prevent proper main cap flush mating. While the use of main cap bolts might draw the upper threads up, creating a raised edge, studs are not installed at the high torque values that bolts would experience, so creating raised edges around the holes is eliminated by using studs.

Main Cap Stud Installation Tips

Before you begin any installation, clean the block, caps and studs thoroughly. Any debris on the threads can easily affect the quality of thread engagement and cause incorrect torque readings. To ensure that the block's female threaded holes are clean and uniform, they should be chased. Never use a cutting tap to perform this task, as this will cut, shave and remove thread material, which can reduce thread strength. Instead, use only dedicated thread-chasing taps, which will restore threads by "forming" instead of cutting. Also, make sure your torque wrench is properly calibrated. All torque wrenches should be checked for calibration at least once each year (more frequently for heavy use). Even new torque wrenches should be checked, as it's not uncommon for some new wrenches to be out of calibration by as much as 10 ft.-lbs.

Use consistent tightening techniques. In other words, don't turn quickly on some nuts and slowly on others. The best method is to slowly "creep" toward the preset value. Quick-banging to reach the click on a ratcheting torque wrench can result in uneven and inaccurate values.

In the majority of cases, screw the studs into the block finger-tight only. Do not double-nut the stud and tighten severely. Remember, the torque value given for the installation refers to the tightening of the nut only, not the stud itself! If you desire a permanent installation, apply a small amount of anaerobic compound to the coarse threads that engage in the block. Loctite 242 or a similar grade is acceptable. If you use Loctite 271 or similar high-strength compound, the studs will be extremely difficult to remove at a later date without the application of high heat.