THE SHOP

Performance & Hotrod Business - July '15

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4 n PRECISION ENGINE n July 2015 PRECISION ENGINE nature, these tools cut the parent metal in order to create the threads. When you need to "restore" or clean up "boogered" existing threads, use a dedicated chaser tap, if available. By using a regular cutting tap in a block's cylinder head bolt holes, you could easily remove too much thread material, weakening the threads and possibly result- ing in cylinder head gasket failure down the road, (along with a resulting warped head, oil/coolant cross-contamination, etc.). Tap Chamfer Styles Three basic styles of tap tips are avail- able, based on the tip's chamfer. These include: tapered chamfer, plug chamfer and bottoming chamfer. Tapered chamfer taps feature a notice- able taper, or chamfer at the tip, featuring about 7 to 10 threads in this tapered area. This eases tap entry and alignment. Plug chamfer taps feature only about 3 to 5 chamfered threads at the tip area. Both tapered and plug chamfer taps are okay to use in through holes. Bottoming chamfer taps allow you to "bottom" the threads in a blind hole, (creating threads as close to the bottom of the hole as possible). Bottoming chamfer taps feature 1 to 2 chamfered threads (the slight chamfer offering initial entry into the hole). Pay attention to the type of tap cham- fer. When tapping a through-hole, any of these types will do. However, when tap- ping in a blind hole, if it's critical to use up all of the hole, you'll need a bottoming tap. And if you need to tap all the way to the bottom, it's best to use all three types in progression—start with the taper chamfer tap, followed by the plug chamfer tap, and finishing with the bottoming tap. Drilling a Hole to be Threaded Don't blindly drill a hole that looks like the right size in preparation for tapping. Consider the bolt size you plan to use, then refer to the drill chart (usually included with any decent tap & die set). Or, refer to the charts listed here. The drilled hole must be large enough to allow tap use, but small enough to allow creation of properly-sized thread diameter. Basically, we want to drill a hole that equals the root diameter of the desired threads. This will provide enough meat in the hole to allow the tap to cut the major diameter. It's a really good idea to keep a precision caliper (dial caliper or digital caliper) with your tap and die set. In some cases, your tap and die set sizing chart may call for Types of CuTTing Taps Spiral point taps are also referred to as gun taps because of the angular point that shears and shoots chips out ahead of the tap. This minimizes the chance of the flutes being clogged with chips, which in turn reduces the chance of tap breakage. This type of tap can be run faster, with less power required. To be used only for through-holes or holes that are at least two diameters deeper than the required thread length. For instance, if you're using a 1/2-inch-diameter tap (cutting, say 1/2x20 threads), if the hole is blind, you'll need at least 1 inch of extra depth where threads are not required. Hand taps feature straight flutes. Hand taps can be used either by hand or by machine, (for example, on a lathe). Spiral flute taps feature spiral flutes (also called helical flutes), and are best used on soft materials such as aluminum, brass, copper or magnesium. The flute spiral offers a lifting action that removes stringy chips that form when tapping soft metals. Good for deep holes, blind holes and holes that feature a keyway or a gap. Pipe taps, as the name implies, are designed to create the specific diameter and pitch required for pipe threads. Note that not all pipe taps are tapered. If an NPT (tapered pipe) thread requires a cleanup, I'm not aware of any specialty NPT chaser taps, so a conventional NPT tap will suffice. Just don't get carried away by tightening the tap into the hole too deeply. You don't want to cut new and deeper threads. Only run the tap to the existing thread depth. Since the hole is tapered, by cutting deeper you run the risk of the NPT plug seating too deep relative to the outer component surface. Never guess at thread diameter. If in doubt, use a checking card to identify thread diameter. This exam- ple shows a 12mm bolt diameter. If in doubt, always use a thread gauge to accu- rately identify the thread pitch. Using the wrong thread pitch chaser (or cutter) tap will destroy the thread integrity, forcing you to repair the hole with a thread insert. A spindle thread chaser set (Snap-on RD8), appli- cable to most domestic and import front-wheel drive spindle threads, which can be used on-car or off-car. (Photo courtesy Snap-on)

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