MJ vs. M Thread Designation - All About Fastener Threads

[bobhall]

MJ vs. M Thread designation

Does anyone know what the MJ designation means in referece to metric thread gagging?

The tolerence is normally 6h4h, and one of our engineers wants a 6g tolerence, is this possible.

Thanks for your help in advance.

 

[Kevin Mader]

Hello Bob,

If nobody has welcomed you to the Cove, WELCOME!!

I do not know what the MJ designation means but I recommend that you call your supplier of thread gages/inspection gages. They should have the answer for you.

As for part two of your question, you can get a 6h in a 6g class. It has been a little while since using these metric classifications, but the higher the letter, the finer the thread. A 6g might be used as a pre-plated fastner check where as the 6h would be post-plating. The designations you state here are quite common, so lead times should be short should you need it.

I hope this helps.

Regards,

Kevin

 

[SMcD]

Calibration - thread

A good reference if you have it would be JIS Fasteners & Screw Threads 2002 (B 0209).

From what I can tell "g" typically refers to external threads and "H" represents a internal thread.

Be carefull > there are exceptions, "h" that I think has to do with plated or not plated.

 

[Kevin Mader]

As I recall, the difference with a 6g and a 6G is as is being mentioned here: the difference from internal and external threads. Case sensitivity needs to be observed. The change in letter, as we ascend with the alphabet designates a tighter thread class.

Another thought has entered my tired mind in that if you plan to match a 6h internal with a 6H external thread, you will ultimately find yourself in an interference condition where parts will only mate when they are in their least material state. As you reach maximum material conditions, you'll find bolts and nuts not going together.

Becareful, I may have confused the cases with the wrong internal/external designations.

Regards,

Kevin

 

[SMcD]

Your Engineer is requesting a 6g tolerance.

I believe we are talking about an external Fastener. Am I correct?

Is it plated or not plated?

If you answer the above I think I can give you the min. max. thread tolerances.

Is this for any particular OEM?
This could make a big differnece?
They all like to have their own ya know!

 

[bobhall]

Maybe I was unclear on my question, but what I really need to know is: what is the difference between a M and a MJ thread.

 

[Atul Khandekar]

Here's something I found:

Metric fastener thread profile M is the normal, commercially-available thread profile. Thread profile MJ designates the external thread has an increased root radius (shallower root relative to external M thread profile), thereby having higher fatigue strength (due to reduced stress concentrations), but requires the truncated crest height of the MJ internal thread to prevent interference at the external MJ thread root (just as the UNJ external thread profile requires the UNJ internal thread). However, M external threads are compatible with M and MJ internal threads (just as UN and UNR external threads are compatible with UN and UNJ internal threads).

http://euler9.tripod.com/bolt-database/22.html