Does the Go Gage Pitch Size Change After Plating External Metric Threads?

Yes, for external metric threads, the GO gage size is often the only gage requirement that changes after plating because the plating process adds material to the thread surface, increasing the thread’s effective size.

Before Plating

Unless otherwise specified, external thread tolerance is typically manufactured to the specified tolerance class, for example class 6g prior to coating. Thus, a 6g thread after plating is subject to acceptance using a basic size 6h GO thread gage. Minimum material would be inspected using a 6g NO- GO gage. The GO ring gage verifies the maximum material condition, ensuring that the thread is not oversized and will properly assemble with a mating internal thread.

Effect of Plating

The 6h GO ring gage has a larger pitch diameter than the 6g GO ring gage, effectively accounting for the plating buildup while confirming functional assembly requirements.

Why the NO-GO Gage Does Not Change

The NO-GO ring gage checks the minimum material condition, ensuring that the thread is not undersized.

Since plating adds material:

The thread moves away from the undersize condition.
The minimum size requirement is generally unaffected.
Therefore, the NO-GO gage requirement remains unchanged.

Summary

Plating increases the size of external threads.
The GO gage controls the assembly-related maximum material condition and must account for plating thickness.
A plated class 6g external metric thread, unless otherwise specified, is inspected with a 6h GO ring gage to verify proper functional fit after plating.
The NO-GO gage remains unchanged after plating. Once the determination is made that the parts do not accept more than 3 turns in the NO-GO gage before plating, providing you have a minimum of 6 threads.

This is why only the GO gage requirement is modified for the increased size of plated external metric threads, while the NO-GO gage remains the same.

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