How To Repair Strip Thread In Engine Block

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How to Fix a Stripped Head Bolt (Ask Away with Jeff Smith)

Posted by on December 5, 2014 at 9:27 am

I finally collected all the parts to assemble my small-block Chevy 350 but when I torqued the head bolts, one of the holes stripped. I've heard there's a way to fix this with a thread insert kit, but I've never done it before. Do I have to take the engine to a machine shop to get this done, or can I do it myself? I'm kinda stuck until I get this repaired. Luckily, the engine is out of the car if I have to take it to the machine shop.

M.K.

Jeff Smith: The good news is that you can repair this problem yourself with just a few simple hand tools and a ½-inch drill motor. There are several kits on the market for thread repair, but I've used the Heli-Coil kits in the past with good results. In the case of the small-block Chevy, the head bolt is 7/16-inch with 14 threads per inch. So this will require the Heli-Coil kit PN5521-7. The kit comes with the unique tap, insert tool, and six thread inserts. You will also need a 29/64-inch drill bit to open the hole up for the required tap. The most important point of this operation is to make sure the hole you drill is perpendicular to the deck. To ensure that the bolt hole is in fact perpendicular to the deck, you will need a drill guide.

ASK-12-02 You can purchase dedicated drill guides designed for 3/8-inch drill motors, but these tools are mainly designed for woodworking and the reviews I've seen are critical of the quality. A better idea is to make your own. There are a bunch of different ideas for creating one. The simplest is an aluminum or steel block about two inches thick and roughly six or seven inches long. The block must be perfectly flat on the cylinder block face. Next, you could use transfer punches and a cylinder head as a guide to drill three holes. The first two are used to position head bolt holes that will locate the drill guide on the engine block. The third hole is the most important one. It needs to be larger – enough to accommodate a length of hardened steel drill guide material. This bushing will be the same inside diameter as the outside diameter of the drill bit–29/64 or 0.4531-inch. The guide will then be pressed into the block to securely locate it. That might be a job for a local machine shop. Once that's done, you can use this block to center the drill bit over the head bolt hole. You can double-check your work by threading a head bolt into the block from your fabricated drill guide. If the bolt won't screw in, then the position is off. You don't need a depth limiter for enlarging the stripped hole since all production small-block Chevy head bolt holes open into the water jacket. If you were doing this on an engine with blind head bolt holes, you would definitely want to indicate drill depth with a mark on the drill bit. With the block accurately in place, you can use it to guide your drill bit through the engine block. I know this sounds like a lot of work, but it's important that drilling the Heli-Coil hole is as perfect as possible. It's a bunch of work to ensure that the 20 seconds it takes to drill the hole that it's as accurate as possible. Our machinist friend Don Barrington said that you need to be careful with head bolt holes nearest the cylinder as they have a tendency to crack right into the cylinder wall while you are drilling. When that happens, you have a much larger problem! Just be careful and don't say we didn't warn you.

Next step is to tap your newly-drilled hole with the supplied Heli-Coil tap. This will also require the tap to be perpendicular to the block. I would suggest using the drill guide to help start the threads. Once the tap has started, you can back it out, remove the drill guide, and then finish cutting the threads. Finally, follow the instructions for installing the thread insert. It may require a second insert stacked on top of the first. You will also have to break off the tang at the bottom of the insert to allow the head bolt to fully engage. Once the offending head bolt hole has been repaired, you should clean the other holes. But don't use a normal tap. Normal thread taps remove block material that will only increase the chances of pulling threads in other holes. ARP makes a thread cleaning tap that is designed to sweep the threads but not remove metal. For your 7/16 x 14 head bolts, the cleaning tap is PN 911-0004. This is expensive, but considering the alternative of installing several more Heli-Coil inserts into your block, the cost doesn't seem so bad.

I would also warn against using stock head bolts and I certainly avoid no-name "high performance" Grade 8 head bolts. Many years ago, the guys at ARP showed me what's wrong with these generic high performance fasteners. I chose a hole in a SBC block and threaded a stock head bolt in about four or five turns into the block. The bolt wiggled a little bit, which meant there was quite a bit of clearance between the bolt and the female block threads. Then I tried a generic performance bolt and the "wobble" clearance was shocking. The bolt moved so much it seemed like the threads were undersized–which they are. The thread overlap in these bolts is marginal, which can and most often contributes to pulling threads out of the block. The final bolt I tried was an ARP bolt. It actually required slightly more effort to thread it into the block in the exact same hole. This revealed the increased thread engagement with the ARP bolt compared to the other two bolts. That's why now I only assemble an engine with ARP head bolts. In your case, assuming stock head bolt lengths, the ARP kit is 134-3601. I clean the block with the ARP thread-cleaning tap, coat the threads with thread sealer (for any bolt holes that extend into the water jacket) and use ARP's Ultra-Torque underneath the bolt head flange and on both sides of the washer. All ARP head bolt washers come with a relief on one side that is designed to match the radius on the underside of the ARP bolt head. Make sure the washer radius is installed correctly.

This may seem like a lot of effort for just bolting on a set of heads, but it all will lead to a properly assembled engine where you won't have any head gasket problems because the load is even across the entire face of the cylinder head. If you take care of the details, the details will take care of you.

Tags: ARP, cylinder head bolts, engine fasteners, head bolts, Heli-Coil, Jeff Smith, stripped head bolts

Author: Jeff Smith Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.