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75% thread engagemant would be a 0.6511" hole. Use a 21/64 drill (0.6562")

To get wizzy the spec for a class 6H is:
minor diameter should be 0.6447" ~ 0.6565" <- 0.6506" is what the hole really should be
pitch dia 0.6703 ~ 0.6778"
major diameter 0.7087"
Thread height 0.0320"
Flats on top of thread 0.0148"
 

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steelcomp was here
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75% thread engagemant would be a 0.6511" hole. Use a 21/64 drill (0.6562")

To get wizzy the spec for a class 6H is:
minor diameter should be 0.6447" ~ 0.6565" <- 0.6506" is what the hole really should be
pitch dia 0.6703 ~ 0.6778"
major diameter 0.7087"
Thread height 0.0320"
Flats on top of thread 0.0148"
21/64 drill, eh?:)devil
 

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steelcomp was here
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Easy enough formula is to subtract the thread pitch from the thread dia. and that's a good drill size. 18mm wiith a thread pitch of 1.5mm...18 -1.5 would be 16.5mm drill. (16.5mm=.6496...closer to the 75% than 16.8mm which = .6614):p
 

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http://www.korit.com/tapndrill.htm

Says 16.8mm drill...next smaller fractional would be 21/32, at about .050" smaller. That would work though.
I think you mean .005" smaller. (.050 would be a lot to take out with a tap.);)
No, the 21/32 is .050 smaller than the 16.8mm prescribed to drill with. It's the next smallest fractional size listed, and I'd imagine, easier to find than 16.8mm.
if that's the case, 11/16 would work better because it's .03125 larger than 21/32 therefore making it only .01875 smaller
hil:) Guess this is one of those cases where zero really doesn't mean zero.
.50, .050, .005, are different, who da thunk it:)sphss



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Easy enough formula is to subtract the thread pitch from the thread dia. and that's a good drill size. 18mm wiith a thread pitch of 1.5mm...18 -1.5 would be 16.5mm drill. (16.5mm=.6496...closer to the 75% than 16.8mm which = .6614):p
what about a class 3b fit? You know, like all the hose fittings in your boat should be. :)hand
 

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"The" masheenist
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You crazy kids all argueing about a tap drill size.
 

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steelcomp was here
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what about a class 3b fit? You know, like all the hose fittings in your boat should be. :)hand
AFAIK, metric threads aren't classified the same as American threads, so there wouldn't be a "class3b" here. It would be referred to as a 4H6H thread class.
 

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steelcomp was here
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frickin machinist, 4h6h? what the fck, I bail. I still trying to figure out why a zero here .xx5 changes things but one here .5xx doesn't. :)sphss
4H6H is the metric equivalent of a close tolerance American 3B (internal) thread class. Lower case 4h6h would designate external threads. (My mistake on my original post, but I edited it)
I'm guessing you're not going to hear about the .050-.005 number.:p
 

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Steel, do you like your metric thread info in metric units or in Imperial (inches)? lol

More then you ever want to know. Note the there is no 4H for this size. only 6 and 5H. kinda interesting I thought. Machinery's handbook doesn't have the 5H data listed either. Hummm


note there is no 4H6H spec for intrenal threads. But you are correct on the 4g6g for external except you got yer g and h mixed up. hehe




 

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Steel, do you like your metric thread info in metric units or in Imperial (inches)? lol

More then you ever want to know. Note the there is no 4H for this size. only 6 and 5H. kinda interesting I thought. Machinery's handbook doesn't have the 5H data listed either. Hummm


note there is no 4H6H spec for internal threads. But you are correct on the 4g6g for external except you got yer g and h mixed up. hehe




I got this right out of the Machinery's Handbook. 27th edition, p. 1783:
"Application comparison with inch threads...Tubular data are also provided for a tighter tolerance fit external thread of class 4g6g which is approximately equivalent to the inch class 3A but with allowances applied. It may be noted that a 4H5H/4h6h fit is approximately equivalent to class 3A/3B fit in the inch system"
H tolerance position = no deviation allowances.
Interestingly enough, machinery's Handbook lists the 18X1.5 as a "sparkplug" thread.
 
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