Acceptable thread tolerance help plz

[E T]

My book says for external 12.994 to 12.854

That's for a 6g thread tolerance.
I might be talking nonsense here, but I believe 6g could be for threads that are galvanised or plated afterwards.
Somebody of the collective that knows more about it than me might chip in.

 

[E T]

Yes, that is the 6g (external) standard. @E T has also given you the standard for your nut: the 6h internal standard.

Either way, your external thread is a little under-sized. The nut might also be over-sized.

No internal standard is 6H. Capital letter. 6h is external thread.

 

[Piero]

Yes

I am guessing the nut is oversized also

How would one measure the nut

 

[Bouledogue]

No internal standard is 6H. Capital letter. 6h is external thread.

I don't think that is correct. I think you have given him the internal pitch diameter. Forgive me if I am being dim: it happens a lot!

 

[E T]

I don't think that is correct. I think you have given him the internal pitch diameter. Forgive me if I am being dim: it happens a lot!

Sorry, but I'm pretty sure it's correct.

 

[E T]

Yes

I am guessing the nut is oversized also

How would one measure the nut

Internal thread gauge, but those are usually not cheap.
Otherwise go-no go gauges. If you've got a lathe you can make a few yourself with slightly increasing pitch diameter.

 

[Bouledogue]

Sorry, but I'm pretty sure it's correct.

Take a look at ISO 965. @Piero 's tolerances are correct for external. Yours are for internal.

 

[Bouledogue]

So you don't have to go digging for the ISO965 data, the attached PDF might be of help, @E T . Forgive the French. "Diamètre sur flancs" is the French for "Pitch diameter".

https://composants.emile-maurin.fr/custom/docs/modele/16-26_e/tolerance-filetage-metrique-iso-filetage-ldoc1.pdf

I believe @Piero 's figures are correct.

 

[Piero]

For now,
Assuming my machinery handbook is correct (as it does state, external threads at the top off the page)

What is the corrected pitch diameter

Ie,
With the measurements I've given using the measuring pins,

What is the correct results off the pitch diameter???

 

[E T]

 

[RichardM]

What engine is the crank for?

 

[Kram]

I suggest using @Dr.Al 's excellent calculator, which means you can use any set of three identical wires, or, if you know the exact OD, a single wire.

CGTK - Three-Wire Screw Measurement

 

[E T]

So you don't have to go digging for the ISO965 data, the attached PDF might be of help, @E T . Forgive the French. "Diamètre sur flancs" is the French for "Pitch diameter".

https://composants.emile-maurin.fr/custom/docs/modele/16-26_e/tolerance-filetage-metrique-iso-filetage-ldoc1.pdf

I believe @Piero 's figures are correct.

@Bouledogue

Internal thread 6H

External thread. 6h

 

[Piero]

The crank is for a tunned Vespa
60mm stroke, 110mm rod length

Cost a whopping £360 odd quid

Would you guys accept that poor thread tolerance on a top end crank ???

 

[Bouledogue]

@Bouledogue

Internal thread 6H
View attachment 531351

External thread. 6h
View attachment 531352

Thank you, @E T . Should @Piero use 6g or 6h?

@Piero . Do you have a lathe? Can you make a nut? Or do you just want to send it back?

PS measuring the nut is a faff.

 

[Bouledogue]

For now,
Assuming my machinery handbook is correct (as it does state, external threads at the top off the page)

What is the corrected pitch diameter

Ie,
With the measurements I've given using the measuring pins,

What is the correct results off the pitch diameter???

I don't understand your question, @Piero . You got your answer to this in posts 11 and 12, I think, unless you are asking a different question from the one I think you are asking?

 

[Pete.]

All that matters is will it hold the specified torque surely? Put the nut on, crank it down to spec and see what happens. If it fails send it back.

Personally, I would make a nut to fit the thread that's there.

 

[addjunkie]

All that matters is will it hold the specified torque surely? Put the nut on, crank it down to spec and see what happens. If it fails send it back.

Personally, I would make a nut to fit the thread that's there.

This is what Id do too…new nut that fits..

 

[RichardM]

Is it a new crank or a modified one? I would certainly call the manufacturers (and back it up with an email) expressing my concerns.

 

[Piero]

New crank

Never fitted

Just inspected and disappointed

Yeah, I could make a nut to suit,

Should I be doing so !! Not really

Is my payment full??? Yes, so the crank should meet standards ??? Yes

I get it's easier to just make a nut,

But the major diameter is under size.

So the thread engagement will be less no ????