THE SHOP

Performance & Hotrod Business - July '15

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PRECISION ENGINE July 2015 n PRECISION ENGINE n 1 A ll too commonly, female thread cleanup is performed with any appropriately sized tap that's handy, which usually results in using a cutting tap. The reason: many folks are unaware of the risk of thread damage and/or are not aware of available chaser taps. Especially for high clamping load thread holes such as an engine block's cylinder head bolt holes or main cap bolt holes, using a cutting tap can easily compromise the integrity and strength of the threads. The correct choice is to use a chaser tap, which is specifically designed to re-form existing threads, as opposed to a cutting tap, which is designed to remove material while creating new threads. Whenever you need to clean up existing female threads in order to remove burrs or light corrosion, always use a chaser tap (sometimes referred to as a clean-out or follower tap) instead of a common cutting tap. A chaser tap is designed to re-form the threads, as opposed to a cutting tap, which will cut its way through, possibly removing too much thread material. Other dedicated restoration taps include those made specifically for restoring spark plug threads in cylinder heads. Restoring cylinder head bolt holes in block decks and spark plug holes in heads are likely the two most common applications. Are Chaser Taps Really Different? Yes. These taps are specifically designed to re-form and clean existing threads. The spirals on a chaser tap are designed to follow an existing helical thread spiral path without removing material. While you may be able to accomplish the task by using a standard cutting tap, you run the risk of weakening the existing threads. When you're torqueing cylinder head bolts to upwards of 70 or so foot-pounds, you don't want to compromise the threads. Weakened head bolt thread engagement can result in inadequate clamping force, which can easily translate into head gas- ket leakage. While removing thread mate- rial from cast iron or aluminum is to be avoided, the risk is greater with aluminum, (head bolt holes in aluminum blocks and spark plug holes in aluminum heads, as prime examples). Chaser taps (male taps for cleaning female threads) are available in virtually all fractional and metric sizes, but only the most common sizes seem to be readily available, (machine screw sizes 4 through 12; 1/4-20, 1/4-28, 5/16-18, 5/16-24, 3/8-16, 3/8-24, 7/16-14, 7/16-20, 1/2-13, 1/2-20, 9/16-12, 9/16-20, etc.; and metric sizes 4mm through 16mm in various pitch sizes). Precision chaser taps are available from sources such as ARP and leading tool suppliers in both inch and metric sizes. Also, for spark plug hole thread repair, a special thread chaser is available that will clean and re- form the existing spark plug hole threads with- out cutting, (so no metal chips will be dropped inside the cylinder). An example is the 14mm spark plug thread chaser offered by Goodson Shop Supplies as P/N SPB-14. Bringing back male or female fastener threads. By Mike Mavrigian An example of a pro- fessional-grade thread chaser from Goodson Tools, designed for engine builders. It fea- tures high-quality hard- ened steel and massive flutes designed to follow, straighten and restore female threads. This Goodson example is size 7/16- 14, a common size for cleaning/restor- ing cylinder head deck threaded holes on many V-8, earlier- generation domestic engine blocks. Quality- wise, it just doesn't get any better than this. RestoRAtion

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