View Full Version : Metric Threads

10-20-2009, 11:43 PM
I need some help with what I think are metric threads. I'm working on a pressure washer pump that had a plastic plug in the end of the passage for the bypass. It's broken and the best I can figure the threads are metric. They are NOT pipe as there is an o-ring under the head of the plug to seal it. I can't just find a metric plug that fits as there is a recess on the internal side of this plug that the bypass relief spring seats into.
Measureing the thread with an inch thread gauge 16 tpi fits real nice. Has anyone ever checked metric threads with an inch gauge and came up with any correlation of the two.
Is the pitch of a metric thread so many threads per millimeter, or???


People say I'm getting crankier as I get older. That's not it. I just find I enjoy annoying people a lot more now. Especially younger people!!!

10-21-2009, 12:30 AM

Metric threads are so many mm per thread, as in .4 mm, .5 mm, .75 mm 1 mm, 1.25 mm. If your 16 pitch thread guage is close, likely it is 1.5 mm pitch. About 59 thou crest to crest, where 16 TPI is .0625. Roughly 3 thou per thread coarser.

'Member, that's cumulative. 3 thou per turn, couple 3 turns you start seizing. Mike, or better yet, if you have a digital caliper, set it for metric and caliper the OD. It should be 5 to 10 thou, .1 to .2 mm under nominal size. 10 mm would be about 9.9 mm, so 10 X 1.5 mm fitting.



10-21-2009, 12:33 AM
George beat me to it.
So I'll throw a chart at ya fur readin' later.


doctor demo
10-21-2009, 01:08 AM
Off the top of My head that sounds like a standard AN plug .
I'll have to go check.


10-21-2009, 02:27 AM
The one thing I failed to say is that this mikes .862 as best I can tell with what is left of the plug. The closest size I figure is 7/8" but the 3 threads I can find are 9, 14 and 20 tpi.
As George posted and using Willy's chart I see .866 is 22 mm and there is a 1.5 and 2.5 pitch listed. In the upper part of the chart is a 17 tpi listing that converts to 1.5 pitch. My gage dosen't have a 17 tpi and the 16 is close but not exactly right.
Now to see if my lathe will cut 17 tpi.

Thanks for the quick replies I'll let ya know how this turns out.


People say I'm getting crankier as I get older. That's not it. I just find I enjoy annoying people a lot more now. Especially younger people!!!

doctor demo
10-21-2009, 02:34 AM
SAE J514 Straight Thread O-Ring Boss
Dash Size 8. Inch Size 1/2'' Thread 3/4''-16 Male Thread O.D. .75'' Female Thread I.D. .67''

SAE J1453 O-Ring Face Seal
Dash 8 . Inch Size 1/2'' Thread 13/16''-16 Male Thread O.D. .81'' Female Thread I.D. .75''


dr pepper
10-21-2009, 02:41 AM
Could be a parallel pipe thread, in the uk bspp british standard pipe parallel, if its a chink or jap job, then it could well be their equivalent which is jic, bsp and jic are near to whitworth from memory, if your not a perfectionist then a cut down whit bolt with a fibre washer or dowty washer might do the trick.

John Stevenson
10-21-2009, 05:08 AM
22mm x 1.5 is a very common thread over here for fine parallel pipe fittings like bungs and unions on truck air brakes.

Things like load sensing valves and charging valves.

We originally bought 22 x 1.5 and 24 x 1.5 taps and dies in the 1980's, as they were coming in them as a lot of air brake manufactures were making similar fittings.