Are 3/4" 16 UNF and 3/4" 16 UNF-2B the same thread

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Both are the same diameter and thread pitch. The "2B" specifies how tight the threads will fit together.

From the link: https://en.wikipedia.org/wiki/Unified_Thread_Standard

https://upload.wikimedia.org/wikipedia/commons/4/4b/ISO_and_UTS_Thread_Dimensions.svg

Tolerance classes
"A classification system exists for ease of manufacture and interchangeability of fabricated threaded items. Most (but certainly not all) threaded items are made to a classification standard called the Unified Screw Thread Standard Series. This system is analogous to the fits used with assembled parts.

Class 1 threads are loosely fitting threads intended for ease of assembly or use in a dirty environment.

Class 2 threads are the most common. They are designed to maximize strength considering typical machine shop capability and machine practice.

Class 3 threads are used for closer tolerances.

Class 4 thread fit are even tighter than a Class 3.

Class 5 fit is not a loose thread. It is a interference thread, used on items like spring shackles on an automobile.

The letter suffix "A" or "B" denotes whether the threads are external or internal, respectively. Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.

Thread class refers to the acceptable range of pitch diameter for any given thread. The pitch diameter is indicated as Dp in the figure shown above."


I'm assuming the 3/4" 16 UNF is a class 1B thread, since there's no designation at the end. A class 1 or class 2 internal thread on the filter base plate should fit fine on the filter mounting center spud.
 
Most oil filters threads seem pretty "loose"...
but occasionally I run across some that are obviously a lot more snug.
I wonder if it's just poor machining/dull tooling more than anything, though.
 
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