Main Bolts or Studs

When it comes to using main bolts or studs in an engine block, there seems to be a lot of confusion regarding the subject of both strength and whether the conversion process requires that the line bore be honed. To gain a full understanding of this subject, we must first understand the difference between available fasteners, their strength and how their different clamping styles impact main housing bores.

Difference Between Main Cap Bolts and Studs

Main Cap BoltMain cap bolts are traditionally used on most OEM engine blocks as they are affordable for manufacturers to produce and provide adequate clamping force for daily driver vehicles. The diameter of the bolt and the threaded receiving hole material (cast iron or aluminum) often dictates how many foot pounds of torque each bolt is tightened to. The head of each main bolt also contains a fixed underhead washer that has a tendency to dig into the block, when tightened, and can cause erroneous torque readings. When a main cap bolt is torqued to specification, the bolt actually twists before achieving its final torque. This twisting force places additional stress on the fastener and threads inside the block’s main registers, but is acceptable for most stock applications.

ARP Main StudMain studs are the holy grail of the performance automotive aftermarket industry. Unlike bolts, studs are threaded on both ends and use nuts and washers to create a superior clamping force. Main studs are typically installed finger tight, with some engine builders preferring to use Loctite red on the threaded section that enters each main register. When the nut on a main stud is torqued to specification, the clamping force is better distributed throughout the entire length of the stud while avoiding the stress on the block’s threads that occur when bolts twist during the torque process.

Converting Main Bolts to Studs

Most aftermarket main studs are available in a variety of materials that have been hardened using specialized metallurgy processes. 8740 chrome moly material, for example, is most commonly used in performance engines with tensile strengths of 200,000 PSI and greater. Though the material used to produce studs may be identical across the many economy and premium main stud kits that are available, the grade of steel used in the fasteners and heat treating process used for hardening are generally what distinguishes the quality of each kit. When selecting a main stud kit to replace bolts, be sure to identify the appropriate PSI rating that best meets the needs for your application.

Since the use of main studs is confined within the oil pan, black oxide is the most common finish available unlike studs for heads which are also available in stainless steel. Standard hex and 12-point nut kits can also be purchased from leading manufacturers including ARP, Milodon and others. Please keep in mind that if your performance engine will be using splayed caps or a windage tray, in the oil pan, special fasteners will be needed.

Just like main bolts, studs are designed with a course thread that inserts into the engine block and a fine thread for the nut. Since studs are normally installed finger tight, the actual torque value assigned to main studs is for the fine threaded nuts that are to be fastened after the main caps have been installed. The appropriate lubricant (10W30 motor oil or moly lube) should be used when torquing any main stud or bolt.

When converting main studs to bolts it is important to torque all the main caps into position with the lubricant and check the line bore with a dial bore gage. In most cases, the conversion to studs will distort the line bore and require that the block be line honed. Because of this, it is always a good idea to convert to studs when new billet main caps are being installed or a girdle will be used.

Now that you understand the difference between main bolts and studs, which will help you determine what fastener is most appropriate for your engine block, we invite you to learn more about engine blocks here. Areas discussed in detail include the deck surface, freeze plugs and other important features of an engine block.


Engine Block Tips

In this section of our website you will find important tips that will help you to identify and purchase the best engine block for your application.
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Main Cap Bolts -vs- Studs