Head Studs vs Head Bolts | Reusability & Installation Differences

The question of head bolts vs. studs is not a new one. The use of both studs and bolts to clamp the cylinder head to the engine block or cylinders dates back to the beginning of the automobile. However, lately that question has become a bit more important simply because modern performance engines are capable of making a ridiculous amount of power. Your hot street engine in the 70’s that ran 10:1 compression and made 400 horsepower was likely going to be fine with the old stock head bolts that you cleaned up on the wire wheel in the corner of the shop. These days, it’s not uncommon for an engine that once lived in a soccer mom’s SUV to be built up to make three or four times the power that it was designed to withstand. The incredible amount of cylinder pressure that is generated by big boost and high compression requires a corresponding increase in clamping force to keep everything happy and in one piece.

Head Studs vs. Head Bolts | Why are Head Studs Better than Bolts

 
An increase in cylinder pressure, caused by either high compression or a power adder, will eventually require more force to hold the cylinder head to the block. There are a few different ways to get there; you can either increase the number of fasteners, increase their size, or increase their strength. It depends somewhat on the application, but most likely drilling and tapping to increase the number of fasteners per cylinder or make them bigger is going to be tricky. The easy answer then is to make the existing faster count and diameter stronger and more resistant to stretching. The way to do that is by increasing its tensile strength. How do you do that? The answer is to make them from a stronger alloy.
 
Let’s pause here. The answer to the question of why are head studs better than head bolts is only partially answered by an increase to tensile strength. After all, you could make a bolt out of superior chromoly alloy and partially achieve our goal here. Another important point to consider is the simple mechanics of what happens when you install and torque a head bolt vs. a stud. When you’re spinning in a head bolt and then torquing it to spec, you are not only applying that stretching, pulling force to the fastener, you are also applying twisting force. That matters because of the potential fatigue to the bolt as well as a slightly less accurate torque reading.

Perhaps more importantly, especially when working with an aluminum block, is the fact that you’re also applying that combination of stretching and twisting to the threads in the block as well. We’ve all felt that heart-sinking moment when the torque wrench is about to click, but instead suddenly starts to rotate easily. Cue sad music. Head studs can help you avoid that heartbreak.
 
Head studs thread into the block only until they’re snug. Once the head is positioned, washers and nuts are spun on and the torque wrench spins the nut only, not the whole fastener. This is clutch here because that saves the threads in the block from being subjected to twisting as well as pulling. Ditto the fastener; all the stud sees of that torque is the pulling and stretching force being applied in a straight line, without the addition of the twisting force. This is better for the block as well as the fastener.

Series 4135, Series 8740, and ARP Head Studs | Which is Best for my Engine

As discussed above, greater tensile strength (when the diameter of a stud is held constant) results from making the stud out of a stronger material. To simplify a complex discussion, for our purposes tensile strength can simply be defined as the resistance of a material to a pulling or stretching force. In our world here, that is the torque applied and then the force applied by combustion in the cylinder.
 
Head studs are typically made from chromoly. Steel alloyed with chromium and molybdenum is hard and strong, and different alloys have different tensile strengths. A stud with greater tensile strength will withstand greater pulling force before stretching and eventually breaking. To some extent, that makes the stud itself more resistant to the force being applied by combustion. More critically, greater torque can be applied to the stud that clamps the cylinder head more tightly to the deck. That’s where high quality studs really shine, allowing for much greater torque and more securely holding the heads in place. Combined with a quality head gasket, that reduces the potential for failure in extreme situations.
 
Speedway Motors offers head studs made from various grades of chromoly steel, allowing the engine builder to balance the engines intended use and power level against the budget. Series 4135 studs feature a tensile strength of 190K PSI and offer the benefits of studs without the cost that is often associated with them. Series 8740 studs are rated at an even more impressive 200K PSI tensile strength for even more strength. ARP head studs offer the ultimate in strength while also being the most impactful on your engine building budget.

With all of this said, are there any disadvantages to choosing head studs over bolts? In many mild performance engines, head bolts will work just fine. They have been commonly used on production engines for a century because the are inexpensive to produce and generally sufficient for factory power levels. High performance head bolts made from chromoly are available as well. Their inexpensive nature is one of the kickers in this argument. In a mild performance engine being built on a budget, studs are more expensive and will take money out of your wallet that may be better spent on better components elsewhere. Speedway Motors Series 4135 and Series 8740 Head Studs offer the benefits of studs at a more affordable price, but will still cost more than most head bolts.

Another thing to consider is the space consideration in the engine bay. Studs require that the heads be pulled straight off all the way over the end of the tall studs. While the studs acting like big dowels that guide the head onto the block can be a perk when assembling the engine on the stand, they can make it hard (or impossible) to remove the heads after the engine is installed in the car.

Are Head Studs Reusable

Are cylinder head bolts and studs reusable? The answer is yes and no. First of all, any factory torque-to-yield bolt needs to be routed to the trash can. GM Gen III and IV engines that used them are great candidates for upgraded aftermarket bolts or studs. Otherwise, bolts and studs can generally be re-used once you've looked very closely at the condition. Any bolt or stud that was rusted due to a leaky head gasket should also head to the scrap bin. Same for damaged threads that need to be chased. Removing material with the thread die can weaken the faster and should be avoided. Otherwise, bolts and especially studs in good condition can generally be re-used.

In the end, head bolts, especially upgraded aftermarket bolts, are a sufficient budget-friendly choice for mild engines. Once you start adding serious power, especially with boost, then head studs start to become necessary. They offer the best combination of strength, re-suability, and protection for your engine.