# Thread Conversion Question



## PTownSubbie (Dec 14, 2011)

Alright, I have been working with Leroy to determine the thread diameter and pitch of the Jr Gent I RB front sections. It has been determined that the threads are M7.8x0.8.

I take the 7.8mm and convert that to 0.307" which is really close to 5/16" (0.3125").

The problem I am having is ensuring the pitch can be converted the same way.....

Does a pitch of 0.8 convert to a SAE thread count of 32?

Can someone confirm? I think the M7.8x0.8 converts to 5/16-32?


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## MarkD (Dec 14, 2011)

In metric threads the pitch refers to the distance between the threads. In this case each thread would be .8mm or .0.031496 inches apart. If you divide an inch by 32 you get 0.03125 which is quite close. I'm just not sure if it's close enough or not to work.


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## PTownSubbie (Dec 14, 2011)

MarkD said:


> In metric threads the pitch refers to the distance between the threads. In this case each thread would be .8mm or .0.031496 inches apart. If you divide an inch by 32 you get 0.03125 which is quite close. I'm just not sure if it's close enough or not to work.


 
I think a couple of 10,000th of an inch is plenty close enough....But then again, I might be wrong.

Thanks! You sort of confirmed my guess. I went about it differently but we both got the same results. My calculation showed 31.75 tpi.


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## IPD_Mr (Dec 14, 2011)

You will still have to chase the threads in the Jr Gent I.  It should be minor enough to not cause any problems.


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## MarkD (Dec 14, 2011)

I just recalled that when I made an adapter to attach a stylus on the opposite end of the barrel I used  8 x 0.75 threads which wasn't exact but seems to hold OK. If my memory is correct the threads on both ends of the lower section were the same so a 8x 0.75 tap might work for you also.


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## PTownSubbie (Dec 14, 2011)

Alright, here is the real question...

The threads according to the manufacturer are M7.8x0.8

Would I be better with the M8x0.75 or 5/16-32 tap and die? Both are off by a little but which does everyone think will work better?


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## MarkD (Dec 14, 2011)

I guess the best answer is to get a M7.8x0.8 tap and die. My second best choice would be to try which ever I have. If you have to buy something probably the 5/16-32 tap would be more readily available and based on the numbers I think it would be slightly closer to the M7.8x0.8.

Good Luck!


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## alphageek (Dec 14, 2011)

MarkD said:


> I guess the best answer is to get a M7.8x0.8 tap and die. My second best choice would be to try which ever I have. If you have to buy something probably the 5/16-32 tap would be more readily available and based on the numbers I think it would be slightly closer to the M7.8x0.8.
> 
> Good Luck!



+1 on this... If you can't get an exact then - I think the answer is pretty clear if those are your choices.   The 5/16 is closer both in diameter and pitch, unless I'm looking at the numbers wrong.   The only reason to attempt the M8x.75 would be if you had it, but in the US the 5/16-32 is pretty common, so I'd try that and see how badly it feels and how much it changes the existing threads.


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## mredburn (Dec 14, 2011)

Victor Machinery carries both the m8x.75 and the 5/16 x 32 in both tap and dies prices are pretty much the same as well.


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## PTownSubbie (Dec 14, 2011)

Both the 5/16-32 and M8x0.75 are available at Victor Machinery. That is why those are my choices. 

If there is enough interest, a group buy for the M7.8x0.8 is the right answer but I don't think there will be enough......Having a single one manufactured is not economical.....


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## soligen (Dec 14, 2011)

I have played with 3 "close enough" SAE taps.  I have has limited success if I run the tap in and out may times to make the threads a bit sloppy.  It also depends on how many turns of the threads you want to engage.   I tis preferable If you can chase _both_ parts with matching tap and die you shuold be ok and not have to do this.  Note that metric .8 is 31.75 TPI metric .75 is 33.87 TPI.   8mm is .0025 bigger than 5/16, so the 5/16 x 32 is closest on both counts.

The formula for converting metric to TPI (Threads per inch) is 25.4 / metric_pitch


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## PTownSubbie (Dec 18, 2011)

I have been told by the supplier that the Jr Gent I front section threads are M7.8x0.8.

7.8mm (0.307") ~ 5/16" (0.3125")
.8 pitch (31.75tpi) ~ 32 tpi

So, I ordered a 5/16-32 thinking that it would be plenty close enough. NOPE. Threads in a thread or two and stops. 

Luckily, I also ordered a M8x0.75. This one threads just fine....Go figure....


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## Smitty37 (Dec 18, 2011)

*Checking*

I asked Rizheng if they can provide a Tap & Die in the correct size or a source where they can be had off the shelf.  I'll let you know what they tell me.





PTownSubbie said:


> I have been told by the supplier that the Jr Gent I front section threads are M7.8x0.8.
> 
> 7.8mm (0.307") ~ 5/16" (0.3125")
> .8 pitch (31.75tpi) ~ 32 tpi
> ...


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## Smitty37 (Dec 20, 2011)

*Will provide but............*

Rizheng will provide them but they will charge $50.00 per set (prpbably with additional shipping) --- seems a little high to me.  



Smitty37 said:


> I asked Rizheng if they can provide a Tap & Die in the correct size or a source where they can be had off the shelf. I'll let you know what they tell me.
> 
> 
> 
> ...


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## mredburn (Dec 20, 2011)

$50.00 a set would be less than having sets made here. I have asked for quotes from tapco on something as simple as 10x.75 and a set would cost 67.00 + shipping in minimum orders of 12.  I continue to look for a manufacturer of taps and dies that fit our needs.


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## Smitty37 (Dec 20, 2011)

*I asked about 10*

I asked about 10 sets if it was a larger order they might go a little lower.  If enough people are interested I will do it for no profit for me.


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## PTownSubbie (Dec 20, 2011)

Leroy,

Thanks for checking into this. If it doesn't go, the M8.0x0.75 works fine. I threaded a piece of aluminum (used this because the threads are solid unlike in PR or UHMW) and the front section threaded in just fine.

However, if you can get them for around $50 for both a tap & die, I might be interested....


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