Shrinking threaded holes is Greenfield Tap tech tip from Cutting Tools Chicago aka General Cutting Tool. At times, threaded holes will “close in” or shrink due to either heat or build-up. In both cases, the threaded hole becomes tighter in the process. To correct this condition, use a slightly larger tap to compensate for shrinkage. This practice is common for parts to be plated.
If build-up occurs, try this simple solution: Multiply the amount of build-up by four, and then increase the tap’s H limit size accordingly. For example, if the build-up equals .00025″ build-up, then .00025 x 4 = .0010. .001 ” is equivalent to two H limits; therefore, if you were using H6, you should use H8.
If the shrinkage occurs due to heat, the solution is to use a larger tap. While there is no formula that will give you an exact number for choosing a larger tap, our recommendation is to use your GO gage as a guide. If the GO fits snugly, but still enters all the way, one or two H limits should be sufficient. If the gage locks up tight from the start, then three, four, or more H limits are probably necessary. (Each H limit is .0005″ larger than the previous one.)
One important note regarding heat shrinkage: When parts shrink, they shrink in ALL directions. A threaded hole affected by heat shrinkage will get smaller in diameter and also shrink along the axis of the thread, producing less than the desired pitch thread. (Think of this as if you were compressing a coiled spring, making the distance between each coil smaller.) The effect would be similar to attempting to screw a 1.5mm pitch thread gage into a 1.25mm threaded hole. While this kind of error isn’t overly significant, it does happen. Refrain from choosing a tap with a long lead, anticipating that it will shrink to normal after heating. Generally, the configuration of the part determines the amount of shrinkage. We recommend starting with a tap that is a few H limits larger to see how it gages.