All ( most good quality ) taps have a GH after the size . Each H is = to .0005 oversize . Check your tap .
I thought about that but the H number would not account for .3mm oversize would it? That is .012" over. Even an H7 is only .0035" over?
BTW People might like this:
Class of Threads, H Limits
Classes Of Threads
There are (3) established Classes of Thread, designated in the unified series by adding: "A" for Screws and "B" for Nuts (or other intenal threads) to show definite limits and tolerances.
Class 1B Thread
Is wherea 1A screw can run in readily for quick and easy assembly. The hole is classified as 1B. The fit is a 1B thread, (very seldom used in modern metal working)
Class 2B Thread
Consists of a 2A screw in a 2B hole. 2B thread has wide applications. It is used to accomodate plating, finishing and coating to a limited extent and threfore, has fair tolerance allowances.
Class 3B Thread
3A screw in a 3B nut or internal threaded hole, used where tolerance limits are close.
GH Numbers
GH Numbers are listed below. "G" designates Ground Thread. "H" designates the pitch diameter is on high side of basic. These two letters (GH) are followed by a numeral indicating the Tolerance of Pitch diameter oversize.
H1 | = | Basic to Plus .0005 | | |
H2 | = | Basic Plus .0005 to Plus .0010 | | |
H3 | = | Basic Plus .0010 to Plus .0015 | | |
H4 | = | Basic Plus .0015 to Plus .0020 | | |
H5 | = | Basic Plus .0020 to Plus .0025 | | |
H6 | = | Basic Plus .0025 to Plus .0030 | | H=Above Basic |
H7 | = | Basic Plus .0030 to Plus .0035 | | L=Below Basic |
Relation Of Tap Pitch Diameter to Basic Pitch Diameter
American Tap Manufacturers use a series of tap pitch diameter limits. These limits feature a .0005 tolerance in tap sizes #0 Thru 1" and a .001" or greater tolerance in tap sizes above 1" thru 1 1/2" diameter.
Example: 1/4-20. Relationship between Tap Pitch diameter limits and basic nominal pitch diameter.
Basic
Pitch Diameter: .2175 | | | | | | | ----.2200 |
| | | | | ----.2190 | | |
| | | | ----.2185 | | | |
| | | ----.2180 | | | | |
| | ----.2175 | | | | | |
| ----.2170 | | | | | | |