Part of the fun of building an engine is ensuring that all the threaded holes in the block are in good shape and ready to handle the torque loads for fasteners like head and main cap bolts. While you might not give much thought to the quality of these threads, it's a royal pain when the threads pull out when you're torquing that last head bolt.
Standard taps are actually designed to cut new threads rather than chase existing ones. Every time you use a standard tap to chase a set of head bolt threads, for example, the tap will also remove more of the existing threads. This reduces the overlap of the female threads in the block to the male threads on the bolt. This reduced overlap can often lead to stripped threads on these larger bolts.
In the old days, savvy engine-builders only ran a tap through these bolt holes once, and employed a very old or used tap that did not cut as deeply into the female threads. While that's still a trick that you can use today, ARP has come up with a set of new taps that will accomplish the same goal. These taps are specifically designed to be used as thread-chasers to clean dirt and junk from the threads, yet remove as little parent thread material as possible. If you enjoy building engines, keep this little trick in mind the next time you get ready to clean those threads. It might save you grief down the road.
 ARP sells thread-chasing taps...  ARP sells thread-chasing taps that will clean the female threads in your block without removing excess material that could reduce the torque load of the bolt hole. The idea is to retain as much of the threads in the block as possible. |
 Employing a used tap is a...  Employing a used tap is a good idea if you don't have access to one of ARP's thread-chasing tools. A used tap will remove less thread material than a brand-new tap. |
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Part Numbers
We've listed the combination packs of thread-cleaning taps, but the National Coarse taps are also available individually.
Thread cleaning taps, 1⁄4- through 1⁄2-inch PN 911-0006
Metric combination 1.25 pitch PN 912-0009
Metric combination 1.50 and 1.75 pitch PN 912-0010