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.
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"
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"
pitch dia 0.6703 ~ 0.6778"
major diameter 0.7087"
Thread height 0.0320"
Flats on top of thread 0.0148"
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)
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
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.
your book is newer them mine but the 25th addition has no data of any kind for a 4H fit on an 18x1.5mm intrenal thread. It's on page 1694. Now on 1697 it has 4g6g info for external.
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