Engine studs: Understanding the advantages of using studs vs. bolts, and tips on achieving proper clamping loads
Text and photos by the author.
In far too many cases, the engine builder’s attention focuses only on selection of the proper components for a specific engine build, determining proper clearances, intake and exhaust volume and flow, surface finishes and precision assembly. Threaded fasteners, the vital link that secures everything together, are often taken for granted. Here, we’ll discuss a variety of issues and concerns directly relating to the most important components of any engine — studs and bolts and their handling.
For a performance or heavy-duty application, the use of studs is preferred whenever possible instead of main cap bolts, in those instances where a choice is available. Studs provide the ability to obtain much more accurate torque values because the studs don’t twist during tightening as do bolts. Because the studs remain stationary during nut tightening, the studs stretch in one axis alone, providing much more even and accurate clamping forces. Also, because the use of studs results in less wear applied to the block’s threads, this extends the life of the threaded holes in the block over periods of servicing/rebuilding. This is especially important when dealing 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.
Main cap stud installation tips
Before installation begins, clean the block, the caps and the studs thoroughly. Any debris on the threads can easily affect thread engagement quality and can cause incorrect torque wrench readings.
In order 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 that 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 even 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 pre-set value. Quick-banging to reach the “click” on a ratcheting click-type torque wrench can result in uneven and inaccurate values.
In the majority of cases, screw the studs into the block FINGER-TIGHT ONLY, or with a very slight amount of pre-load (specific manufacturers may recommend as much as about 8 to 10 ft.-lbs. of pre-load). 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 “fixed” installation (locking the studs in place for future servicing ease), the studs may be chemically adhered to the block threads. If stud removal is required in the future, the application of heat will break the compound down. If a locking compound is used, be sure to immediately install the main cap before the compound sets, to avoid any possible misalignment of the studs in relation to the cap. This means installing the cap and tightening the nuts to specs, before the thread compound hardens. This allows the necessary preload to be placed on the stud-to-block thread engagement before the compound hardens.
Remember: The use of a locking chemical really isn’t necessary, unless you want to ensure stud position during repeated and hurried main cap changes.
When installing the studs, simply make sure that full thread engagement has been achieved. In other words, make sure the studs “bottom-out,” with all threads engaged.
Once the studs are installed, install the main caps and check for stud-to-cap alignment (check for binding — the caps should slip over the studs smoothly).
Lubricate the exposed (fine) threads of the studs, and be sure to also lube the washers and nuts. NOTE: Torque specifications will vary, depending on the lubricant used. Moly lube offers a more consistent torque reading than various oils. If Moly is used, be sure to follow the torque specs for Moly. If oil is used, follow the specs for oil. The specs will be different! If you lube with oil but tighten to specs listed for moly, you’ll under tighten. If you use moly but follow specs for oil, you’ll overtighten.
With caps in place, the nuts should be tightened to the specified torque value three times. As noted earlier, if a locking compound is applied to the stud-to-block threads, the nuts must be torqued before the compound cures.
If the block is to be align honed, to achieve best overall (and optimal long-term) results, the block should be align-honed after stud and main cap installation. In other words, if you’re switching to studs, don’t use bolts for the align honing process. The main caps must be secured with the same fasteners that will be installed during final assembly.
Cylinder head studs
Again, as in the case of main studs, the use of studs is always preferred as opposed to bolts. However, this may vary depending on application. On a street engine application, or in a situation where the heads would be difficult or impossible to remove with the engine in place, studs may not be the best choice from a cylinder head service standpoint. For example, if a master cylinder or other component prevents a cylinder head from being removed or installed with the engine mounted in the vehicle, bolts may be a better choice simply in terms of practicality. However, if the situation allows, the use of studs will theoretically provide a superior assembly.
The use of head studs will aid in cylinder head installation, simply from a standpoint of gasket and head alignment. This is especially helpful in an application where frequent head removal will occur.
In terms of function, the use of studs provides much more accurate and consistent torque loading. When a bolt is installed, the act of tightening results in both twisting (torsional load) and stretching (vertical, or axial load). This results in the bolt being exposed to two forces at the same time, as well as experiencing frictional loads at the thread engagement. When the nut is tightened on a stud, the stud stretches on its vertical axis only. The exposed end (top) of the stud features “fine” threads, which allow more precise and therefore accurate, torque readings when the nut is torqued (or torque/angle tightened) to spec.
Head stud installation tips
Make sure the threads in the block and on the studs are clean to provide accurate torque readings. This is critical!
Since many applications feature cylinder threaded holes that are open to water jackets, it is critical to coat the lower (block end) of the stud threads with a quality thread sealer. If you are certain that the holes are not open to water, you may skip this step, but if in doubt, apply the sealer. It can’t hurt. As with main cap studs, if you prefer that the studs are installed more “permanently,” you may apply a locking compound to the threads. CAUTION: If you plan to chemically “lock” head studs in place, do not use an anaerobic locking compound, because this type of locking compound may expand as it cures. Depending on cylinder wall thickness, this can result in a cracked cylinder wall (as the compound expands and places pressure against the back of the wall). If you plan to lock the studs in place, a better choice would be a non-expanding material such as JB Weld or equivalent. If you need to remove the studs in the future, apply concentrated heat to the stud, which will break the compound down enough to allow stud removal.
Whenever a locking compound is applied, the cylinder head must be installed immediately following stud installation so that the nuts can be torqued to value before the compound sets (the studs must be preloaded to allow set-up to occur in the proper tensioned state, to assure stud alignment). Install the studs into the block finger-tight only (or with a light preload if recommended). DO NOT double-nut the studs and tighten severely. As long as full thread engagement occurs (the “course” end of the stud is screwed into the hole fully), clamping load will be achieved by tightening the nut. If the studs are too tight in the block, they may splay and cause a misalignment with regard to the head gasket and the head, which will, at the very least, make it difficult to remove or reinstall the head at a later time.
Prior to installing the nuts, lubricate the upper “fine” threads on the studs, the washers and the nuts and torque the nuts in the proper sequence and at the values specified. As stated earlier, torque value will vary depending on the lubricant used (oil or Moly lube), so always pay close attention to the torquing instructions supplied by the stud manufacturer. The nuts should be torqued three times.
Always read the stud maker’s installation instructions, and follow them to the letter. Even if a specific type of head stud is designed to be installed with a slight preload, NEVER double-nut and severely tighten any stud, regardless of its design. In most cases, studs should be installed finger-tight, but as already noted, some specialty studs and/or certain applications require a slight preload. Quality aftermarket studs will feature a female hex at the upper tip to allow the use of a hex wrench for installation/preloading/removal. Read the instructions!
Studs, as opposed to bolts, also offer component installation advantages. Studs act as guide pins when aligning items such as gaskets, engine covers, etc. Studs are available (ARP for example) 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 carburetor studs, distributor studs, exhaust header studs and oil pan studs. Instead of trying to hold a gasket or part in place while fumbling with bolts, trying to align the bolts to their holes, the gasket or part can be dropped over the studs (so alignment poses no further problems). With the part aligned in place, the washers and nuts can be installed without concern for the part and/or gasket being shifted out of alignment.
Another advantage of using studs is that you don’t need to worry about insufficient clamping load because a too-long bolt bottomed 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, due to component design. 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 in those cases, 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.