metric pitch question FINAL ANSWER

[gearman]

In measuring an 8mm 1.25mm pitch I do not get that number when measuring crest to crest length.Am I doing something wrong? Thanks  OK,The long story is my friend who knows I was a machinist for 40 years calls me and asks me how to measure a metric thread pitch. I tell him  ,from crest to crest in mm. He says no that is not correct.So I tell him to measure his 8mmx1.25mm bolt. He does and it comes in at a wrong number than I had thought.I told him that I always just used a gauge  so maybe I am wrong. Hence my question here.Well I get up this morning and go out to my shop and measure an 8mm bolt.I'll b durnned if it was right on 1.25mm apart. WTH?????An hour later my friend comes over and tells me that he found out what was wrong.He said that he had measured 4 crests to get a better resolution but counted one too many and therefore got the wrong result.So sorry for the bad info but maybe some here learned something.

[LowRyter]

don't know.

I just take by bolts and screws to Ace hardware and screw them into their test station.  Then I know.

[normzone]

I should know this, but I let that lobe fall into disuse years ago.

I think the pitch distance is from some arcane point on the side of the crest (before you get to the peak) to that same point on the next crest. So peak to peak would give you a different measurement.

[Chuck in Indiana]

Just put a thread gage on it. It *should* be 1.25 from root to root, though..

[oldbike54]

 Yu Darvish ?

 Seriously , just use a pitch gauge like John and Chuck mentioned .

 Dusty

[dxhall]

The pitch of metric threads is not a linear measurement.  It’s a ratio of some kind.  The only way to determine pitch which I know of is to use a pitch gauge.

[pyoungbl]

I know that the Internet is full of bad info so I'll throw this out and you can make your own determination:
https://www.boltdepot.com/fastener-information/measuring/tpiandpitch.aspx

Thus it looks like the thread pitch for metric bolts is measured from one thread tip to the next.  I suspect you get the same number if you measure from the bottom of one groove to the next.   Bottom line is that a pitch gauge is still the easiest way to be correct.

Peter Y.

[wirespokes]

It's confusing since it's different than imperial threads. As already mentioned, easiest is a thread pitch gauge or take it to the hardware store and use theirs. I looked it up just now and it says it's the distance between threads - crest to crest or valley to valley.

[pehayes]

In measuring an 8mm 1.25mm pitch I do not get that number when measuring crest to crest length.Am I doing something wrong? Thanks

How about telling us what you DO get when you measure crest to crest?  Yes, you SHOULD measure crest-crest or valley-valley and you should get the nominal dimension of that thread.

If this is a used bolt, could it have been over-tightened and stretched?

8 by 1.25 is a very standard number.  But there are other pitches related to 8mm diameter.

Might someone have substituted a nearly matching non-metric bolt?

Otherwise, you are doing a simple crest to crest measurement.  If you match against a thread gauge, you might see where some threads were stretched under load while the others all match up perfectly.


Note here that there are five potential thread pitches for 8mm diameter:

http://mdmetric.com/tech/M-thead%20600.htm

Patrick Hayes
Fremont CA

[Idontwantapickle]

It's not complicated or mysterious.
Measure over multiple thread peaks for an accurate result.

If 5 threads are 4mm then it's 0.80
If 5 threads are 5mm then 1.0
If 4 threads are 5mm then 1.25
If 4 threads are 6mm then 1.5
Etc.

It's some simple math.
A pitch gauge is nice but if you already know it is metric then the possibilities are narrow.

Hunter

[canuck750]

From the interweb;

If you normally work with inch fasteners, pitch can be a little confusing—pitch is the distance between adjacent threads. In the example above, a pitch of 1.0 means that from one thread to the next thread, the distance is 1.0 millimeter. Inch fasteners are described as threads per inch, which is sometimes mistakenly called pitch. If you need to know the threads per inch of a metric fastener, first convert pitch from millimeters to inches (multiply by 0.03937), then divide 1 by that number.

[Chuck in Indiana]

The reason I said root to root is the root doesn't get beat up /filed down, etc. Just the same, a M1.25 thread is exactly that. If it isn't, it either isn't that thread, or you aren't measuring it correctly. <shrug >

[Huzo]

I should know this, but I let that lobe fall into disuse years ago.

I think the pitch distance is from some arcane point on the side of the crest (before you get to the peak) to that same point on the next crest. So peak to peak would give you a different measurement.

Would it..?
I'd wind the nut up to the head and drop a vernier over the nut and head and get a measurement.
Wind the nut 10 turns and take the new reading.
I'd expect it to be 10 x 1.25 = 12.5 mm larger than the first reading.
As mentioned, taking the reading over 10 pitches will reduce the error in reading one pitch to 10%

[pete roper]

I know that the Internet is full of bad info so I'll throw this out and you can make your own determination:
https://www.boltdepot.com/fastener-information/measuring/tpiandpitch.aspx

Thus it looks like the thread pitch for metric bolts is measured from one thread tip to the next.  I suspect you get the same number if you measure from the bottom of one groove to the next.   Bottom line is that a pitch gauge is still the easiest way to be correct.

Peter Y.

This.

[molly]

It could of course be a non standard pitch thread or a 5/16"(7.94mm) ) thread of some description.
 Pitch gauges are the best method, failing that count multiple threads as already mentioned.

[Mike Tashjian]

I use the Machinery Handbook that is full of good information on any thread.  I had a oddball recently while repairing an old tool.  The thread is 8-40 which I had never seen before but was in the book.  Thread gauges and a vernier caliper are the first things I grab as they easily get you in the ball park on thread count and size.  I have a nice reference chart on the wall that lists many common thread sizes.   Then you can narrow your search quickly.  Mike

[kirb]

when to drop collision insurance

[tris]

......8 by 1.25 is a very standard number.  But there are other pitches related to 8mm diameter.......

Correct, but we should remember that there are preferred metric thread sizes - 1.25 in the case of the M8

The non preferred sizes are generally very rare.

 I think that the"sacred screw" on the CARC bikes being an example

[gearman]

I posted the answer now on my op. Thanks for your input

[pehayes]

Now you have to go back and correct one sentence in the modified original post.  You now wrote "Well I get up this morning and go out to my shop and measure an 8mm bolt.I'll b durnned if it was right on 8mm apart. WTH?????"   If you measured and 8mm diameter bolt then the crest-crest distance should be 1.25mm, not 8mm.  Hate to be pedantic but I realize you were dug deep into the trees and this would be an easy oversight.  The oversight error was easy to spot.  8mm crest to crest would be some seriously large bolt!

Patrick Hayes
Fremont CA

[jas67]

Correct, but we should remember that there are preferred metric thread sizes - 1.25 in the case of the M8

The non preferred sizes are generally very rare.

 I think that the"sacred screw" on the CARC bikes being an example

[gearman]

Thanks Patrick.

[molly]

Correct, but we should remember that there are preferred metric thread sizes - 1.25 in the case of the M8

The non preferred sizes are generally very rare.

 I think that the"sacred screw" on the CARC bikes being an example

Maybe not that rare Tris. The bolts that hold the brake calipers to my Griso 1100 forks are M10x1.25 (hard to get hold of) but the rear caliper is held on by standard M10 x1.5 bolts.

[Dukedesmo]

Just use a nut as a 'gauge', if it fits...