Q: I am installing a new set of headers, and I’m trying decide on which hardware to use. Would you typically recommend header bolts, or should I step up to a set of header studs? Are there advantages to using one type of fastener over the other? What other tips do you have for choosing header bolts or studs?
A: First, we’d recommend you check out this earlier post on bolts versus studs. While the video post covers cylinder head studs and bolts, many of the same principles apply to your header fasteners. However, with header fasteners, you must pay close attention to the particular design of your headers. Specifically, access to studs can be a problem when header tubes are bent closely to the flange. In these cases, the protrusion of the stud may actually hit the tubing, making it impossible to access the fastener altogether. For this reason, header bolts are often the most optimal solution.
When using header bolts, it’s important to select a fastener with the proper under-head length (the distance from the friction surface of the bolt head to the tip of the bolt) also known as UHL. For secure fitment, header bolts need maximum thread engagement, without bottoming out in the cylinder head. If the bolts are too long, you’ll probably gall or strip the threads in the cylinder head (especially if it’s aluminum), or even break the bolts before the header flange is properly sealed against the gasket and cylinder head.
Most headers with 1/4-inch or 5/16-inch thick flanges will use bolts with a 3/4-inch UHL, while 3/8-inch thick flanges normally require bolts with a 1-inch UHL. It’s a good idea to measure the depth of the fastener holes in the cylinder head, add the header flange thickness, and then select bolts with sufficient clearance.
Having said all that, you may want to step up to header studs for some applications if clearance allows. Because studs don’t exert torsional stress like bolts do (refer to the video), they’re a good choice for the softer threads on aluminum heads. Studs also provide more even clamping force across the entire fastener, so many engine builders prefer them for high-pressure applications like forced induction engines.