We learned this little lesson through a friend who had just completed assembling the short block on his newly machined small-block Chevy. With the heads installed and new, ARP head bolts in place, he began the torque sequence when the threads pulled out of one — and then a second — head bolt as he approached the peak torque spec.
I called ARP to get their take on what had occurred. Older small-block Chevys are somewhat known for pulling head-bolt threads out of the block, especially if no-name Grade-8 head bolts are used. Often these bolts have undersized or loose-fitting threads that minimize thread overlap. If those bolts are then used on a cylinder block that has been subjected to multiple head bolt-hole thread cleanings with a normal tap, this decreases the thread overlap even further. To prevent that, thread-chasing taps like these should be used instead of cutting taps.
After trading emails with ARP’s engineer, Chris Brown, he suggested using a Go/No-Go thread gauge to check the head bolt holes in the block. Often what can happen is one or two head bolt holes can become significantly looser than the rest of the holes in the block. In our experience, this seems to occur more often along the bottom row of head bolt holes for a small-block Chevy.
A Go/No-Go gauge is simply a highly precise pair of male threads on a hand-held tool. The “Go” section is machined to thread easily into a standard 7/16-14 head bolt hole. The No-Go side is machined slightly larger, so if the female threads in our block are in good shape, this should present significant difficulty to thread into the block. However, if the No-Go side threads easily into the block, this indicates the head bolt holes are oversized and could easily fail even when using high-quality, rolled-thread fasteners like ARP’s head bolts.
We immediately searched online and found several companies such as Grainger and MSC Industrial that offer these Go/No-Go tools. These tools are somewhat pricey at around $100. Still, when you consider the extra work involved with discovering a number of bad bolt holes only after you’ve begun the process of final assembly, the price seems reasonable. It’s better to know at the beginning of the block machining process rather than at the end with the engine half-assembled.
The ideal solution to repair the offending bolt holes is not all that bad as long as you don’t have to repair all 34 holes in a small-block Chevy! You will drill out the existing threads, cut new larger threads, and then thread in a new threaded reducer into the new hole.
One way to improve the chances of drilling the repair hole straight is to use a homemade drill guide. These are generally 2 -inch or thicker steel plates with holes perpendicular to the deck surface in the appropriate size. This drill guide will ensure the drill is square to the block deck. Then a threaded insert can be used to install new threads in the block and then you’ve saved your block and can now finish the assembly.