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Pitch size and diameter of bolt

This has got nothing to do with cross threading but want to gain abit of knowledge of bolts in general as this will help in the future .

If the female thread ( not nut ) is interrogated into the engine it self or any other place around the engine bay does it have to be the same diameter length wise and pitch size as the male thread in order for it to engage into the male threads. Like for example M8x1.25 does the female thread have to be that size .

Some people have been really helpful on here and really do appreciate the help

Here you go.
Bet you thought threads were simple, huh? :grin:

Yeah quite allot of information but does the thread size e.g major diameter have to be the same size as the female internal thread that is integrated into the engine and the same pitch size. Didnt seem to explain on that website you gave me

Because that is taken for granted. A 6-32 nut works with a 6-32 screw only.

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Thank you Bill for saving me the trouble of stating the obvious.

Im talking metric though not inches

Doesn’t matter. They have to be the same size.

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Absolutely. The length is a different issue. If there is a blind tapped (threaded but only to a certain depth) hole in a block you should use the same length bolt or less in that hole. If you don’t, you can screw it in far enough that it bottoms before the part you are trying to attach is tight or it breaks out the backside of that blind hole. And really bad things can occur if that happens.

I’m fairly confident 6mm is metric.

Yup. The same applies to metrics as to inches.

There’s a book called “The Machinists’ Handbook” that goes into great depth on threads. It isn’t inexpensive, but if you really want to know in-depth about threads, I recommend it. It’s the “bible” when it comes to threads.

YES, to answer your question.

For a little more insight…
Metric threaded fasteners, use a format like M8x1.25mm to indicate that the screw/bolt (or hole that it fits) is 8 millimeters in diameter and has a thread “pitch” of 1.25 millimeters, meaning that each thread is 1.25mm apart from the adjacent threads.

What is left out of the M8x1.25 screw/bolt dimension is the length of the fastener. An M8x1.25x 25mm length would have 20 threads (count them!), from head to end (25mm divided by 1.25mm).

An M10x1.5 x 30mm (length) fastener would also have 20 threads. Although it is longer, the thread pitch is coarser (distance between threads is greater).

Not to confuse the issue…
Inch size fasteners (non-metric), like 3/8-16 use a little different method for designating thread pitch. So, a 3/8" (diameter) bolt with a thread pitch of 16, has 16 threads per inch (rater than a distance between threads). A 3/8-16 x 2" length bolt would have 32 threads from head to end (16 x 2").

Have you used a metric ruler very much? Pick up a metric ruler or tape measure and get to know it a bit if you haven’t done so. Try it out! measure how long an 8Mx1.25 bolt is and divide that length by 1.25. Then count the threads. The number should match your math.

I’ve seen a number of people mate different thread fasteners together…

Breaking them off or getting them out later is a different issue altogether.

A neighbor of mine came over one evening and was having trouble stabbing a manual trans back into his truck after a clutch job.
Of course he did not know the clutch disc had to be aligned so he offered me 40 bucks just to stab it.

Once stabbed, I’m looking around in the debris on the floor for the two 10 MM nuts that fit onto the lower studs mating the trans to engine block.
He says he has them. He crawls back under the truck with what is obviously two 3/8 NC Grade 2 fence post nuts and starts pounding them onto the studs with a hammer.
I asked him what he was going to do if they had to come back off. “Probably won’t have to…” and at that point I bailed out.

That is funny but sad @ok4450 .

I once saw a farmer hanging a small shed door. He told me that he couldn’t keep the hinges on the frame.

At that point I witnessed him drive the screws as if they were nails.


One of the times I worked at a Volkswagen dealer, we had a strange assortment of “mechanics.” We had a hippy (with a large “PEACE” painted on it) Volkswagen type 2, guys who “moon-lighted” as “mechanics” (one guy’s OTR truck diver uncle), a fire-arms dealer, and one “mechanic,” Moose, who didn’t own any metric tools! When he’d get in a “tight spot,” he’d yell to see if he could borrow a metric wrench from a neighbor.

They all had nicknames… Happy Apple, Johnny O, Yugi, etcetera. The guy named “Suds,” no joke, would say, “A good cross-thread is better than a lock-washer!” Given this place and people, I never really knew whether he was joking or not.

When getting parts for customer cars, he’d almost always say (for parts under about 5 bucks), “You’d better give me 2! I always screw the first one up!” His customers were charged.
I actually think he was just supplying his toolbox for week-end work at home (I hope).

The “size” of a bolt is its diameter measured on the threaded portion. The hole diameter has to match up to that, but be a little bit larger obviously. See a tap size vs hole diameter table for info about that. Sometimes confusion develops about which socket (or wrench) size corresponds to the bolt size. The corresponding socket size is usually larger than the bolt size, b/c the socket fits on the head of the bolt which has a larger diameter. A one inch bolt requires a larger than one inch socket in other words.

The other things you have to worry about being fit-compatible is the thread pitch, metric M8-1.25 vs an inch size like 3/8-16, and you need a bolt of the proper length for the application of course.

Bolts and nuts are made in different materials, different coatings, and different strengths too.

I see no one has talked about - and I’m searching for the correct word and I think it is - “Class of Fit”.

There are several different classes of fit for thread fasteners - from loose (where the nut is quite a bit larger than the bolt) to interference (where the nut is actually smaller than the bolt) These are standardized and labeled and every bolt and nut should have a class identified - although I understand that “Normal” is usually not specifically identified.

Thankyou for all the information , now I know a little bit more , it does
makes sense that the pitch size and major diameter has to be the same size
in major diameter and pitch size for the female ((internal thread) just
wanted reassurance in the end and a little bit more Informatioun . Cheers
for the advice thankyou .

Another question is how come you get play in the bolt when you turn the threads like you can move slightly left to right . Surely that is bad because then it wont be aligned as it would come out and be misaligned.

To complicate things even more, some high strength precision bolts are purposely threaded so that the female threads have a slightly different pitch than the male threads. This is to compensate for bolt stretch when tightening the bolt so that the thread pitch matches exactly when the bolt is torqued to specs resulting in all the engaged threads sharing the load equally.

ah that does make sense but it baffles me how it could have play when turning the bolt up by hand only slight and you can feel it if you did it by hand like tilt left or right , how is tht caused .