Thoughts on screwdrivers and fasteners and sizes

Okay, there are apparently about 29 different slotted Imperial fastener sizes (or more) — I’d like to categorize the sizes used in watches/jewelry as well:

but typically only 9 different screwdriver sizes are in a set:

While I can’t recall ever having needed a larger screwdriver size than the 1/2" Stanley I have of my father, there have been lots of times when I’ve been dis-satisfied w/ screwdriver fit for more reasonable size screws — time was I’d just grind a screwdriver to fit, and I’d like to just have one long session of that and be done w/ it, but would like to source commercial bits for as many sizes as make sense.

I’m currently working on enumerating the Brownells Magna-Tips sizes:

and I guess I’ll do Wheeler Engineering next, and and I do have a Grace USA Gunsmith 24 Bit Magnetic Tip Screwdriver Set on order.

Anyone got any resources / links / sources I’ve missed? In particular, a standard listing of metric sizes? (having a hard time wrapping my mind around )

Did add the PB Swiss sizes:

Slotted heads are a bane to humanity…just saying. :wink: Thx for the resource though Will. I for one appreciate your dedication to providing resources and troubleshooting.

There are tables in Machinery’s Handbook (I am currently looking at “British Standard Slotted Countersunk Head Machine Screws”) with the standard data for many types.

For example, from the table: An M6 of the above type has slot width [1.66, 1.91]mm and depth [1.20, 1.80]mm, with minimum head diameter of 10.5mm, so the screwdriver blade should be 10 to 10.5mm wide and have a thickness at the tip no greater than 1.66mm to insure fit, but the tolerance is great enough that I have two tools for these, as I see them fairly often.

This is but one example (and a modern, tight tolerance by the trade) of why slotted screws are so hated. The slot sizes are not consistent between screw head types, and have such wide tolerance that getting a good fit can be problematic. Surprisingly, the tolerance is tighter for a number of the Imperial sized (US standard) than for metric, though I see way too many fasteners where that tolerance is not met.

I have way too many slot screwdrivers, but never manage to have the one I need.

For small (miniature, instrument, firearms, and watch), there are standards, as well, but again, the tolerances are a bit large, and there are so many standards, and so many that follow no standard at all, that you could fill a shop with nothing but screwdrivers for them and still not have the right one.

Then we get things like “coin slot” drive (slot with a rounded bottom… “Hi-Torque” is one trade name) and the other common-but-specialty flavours, where standards are made to be broken (which is why the oil strainer cap for the lifter chamber on every Harley Evo engine ever built looks like it was beaten with a chipping hammer)

If you don’t have it handy, I can take a shot of the table in Machinery’s for you.


Slotted screws for which one doesn’t have an appropriately sized screwdriver are awful, I’ll agree.

Added Felo bits (and will need to measure one I have at home, and buy one more), so the metric stuff at least is falling into place, though I’m surprised that there isn’t more correspondence between the Felo and PB Swiss stuff.

Thanks! But I actually have a (reprint) copy of the first edition on order — it should arrive sometime this week (really looking forward to it), though I guess I should buy a more recent one as well — any thoughts on which edition would be the optimal mix of up-to-dateness and not having removed cool information?

It’s also available on as a PDF if one wants a copy electronically — though it really wants to be retypeset — though I’ve been considering doing that with the competing American Machinists’ Handbook — another project.

any thoughts on which edition would be the optimal mix of up-to-dateness and not having removed cool information?

Short answer: I use the 25th the most, as it is the most recent I own, but I will soon be investing in a current (about $US110 for the 30th) or one-out edition ($US35 to $50 for the 29th if you look).

Long and rambling, because there is no easy answer:

Due to the nature of my work, and the work I have done over the years, I own paper copies of several editions of Machinery’s (5th, 12th, 19th, 21st, 25th, and one or two others), each with benefits and drawbacks. Then again, I have maybe 6 metres of shelf that is nothing but outdated, but useful, technical books. I have been passing a lot that I will never use again on to others lately (manuals for lathes I once worked on or repaired but will never see again, for example, freed up almost 1 metre. Maybe 5 times that still ready to go, most so specialized it may sit a while) I also have a couple editions of American_Machinist’s_Handook, Mark’s, about half a set of Colvin and (Stanley|Haas)-- the whole shebang elecronically, though-- many ICS books, Audel’s manuals, and so on… I also have way too many ASME and ASTM publications.

No, I swear I don’t have a problem…

That said, a Machinery’s from, say the 12th or 13th (13th is about as good as it gets), or maybe up to the 15th, covers late war to postwar giving the key info relevant to the era, and a recent (26th or later) fills in the rest.

There is no reason to go pre-war unless you are specifically dealing with things manufactured pre-depression, in my opinion. Then you may need to hit it close, as there were a of changes as automobiles came into play, diesel engines were developed, aircraft (and aluminum) matured, and so on. The first edition is interesting, but very much an embryonic form, and I tend to prefer Colvin and Stanley _American_Macinists’Handbook, or the McGraw-Hill series (Tulley, Lowe, Collins, and so on being the authors/editors) from that era when I need specific information. By the 5th edition, it was much better (better index errors corrected, additional info, and so on).

I was looking at the screw standards in the 25th earlier, but I think that it will be pretty complete in any recent edition. I miss my 13th, as it was my grandfathers, but only hold a small grudge against the one that never returned it (a former editor for Machinery’s, of all people).


seconded!. …mmmm…post must be 20 characters long…challenge accepted

Additional (not in Machinery’s, and a manufacturer’s spec):

Miniature (imperial machine) screws:

see: page 3 of PDF (table 10) for slots and heads of some small machine screws. On later pages, there are specs for assorted other small screws, such as self tappers, but page 12 and 13 of PDF have all of the specs for the standard miniature series (used on most modern applications, such as eyeglasses), which is formally metric, but has a weird history, and is often treated as imperial in the US, which leads to some difficult fits on occasion.

Had to get onto the right computer to find this.

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Thanks! I’ll get that entered when I get a chance.

The mention of American Machinist’s Handbook is interesting since I just picked up a copy of Jigs and fixtures : a reference book showing many types of jigs by F.H. Colvin and L.L. Haas (4th edition from 1943, printed just down the road from me at Maple Press in York, Pa.)

I’m thinking the next book I ought to get should be Gages and their Use in Inspection? Or should I go for something more contemporary such as:

My favorites (The Machinist Handbook is great, but has SO MUCH information I find it really hard to find what I want):

Carroll Smiths Nuts, Bolts, Fasteners and Plumbing Handbook (~$30)

“Engineers Black Book” compiled by Pat Rapp . ( (~$35)

“Fasteners Black Book” ~$40 - If I could have only one reference, this might be it.

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Precision Measuring, presuming I was looking at the same one you are (the one from your link is rejected by acrobat reader), isn’t something I would bother with a paper copy of. Basic how to use a caliper and micrometer info is good, but not on paper for me.

MikeP’s suggestions are good, but I tend not to use the black book (I only own Engineer’s) because I am familiar with Machinery’s, and can generally find what I want quickly, if the book doesn’t already know where to open.

Other books I use a lot (work related and machine/making related) are the riggers handbook (some covered in black book) I got when I was certified as a crane signalman for a prior job, Blodgett’s Design_of_Welded_Structures, Schodek’s Strucures, Den Hartogs’ Strength_of_Materials, a bearing handbook (I actually have several, a set of older New Departure books, and a fairly current SKF), a Boston Gear catalog, a Stock Drive (SDP/SI) catalog set, Edmund Opticals catalog, a Helical Tools catalog (tooling) and, again, I could go on… Just looking at those that are on the desk rather than the shelf right now.

A lot of the catalogs are free (gotta get a new McMaster. Mine is over ten years old. MSC is only two, but I don’t use them much) and have really good information, especially the Stock Drive and the Boston Gear. Funny thing is I still have my old CRC Handbook. It was a HUGE investment and absolutely necessary back in my early career, but I don’t think I have opened it in 20 years, and it is so far out of date now that I doubt I ever will.

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A coworker had the CRC Handbook, and I’ve always wanted a copy (the husband of one of my English teachers had one — he published an engineering journal which I’d help out on from time-to-time, though arguably he spent more time teaching than I spent working).

There was some major reference work which was recently retypeset using LaTeX — was that it?

The other book I’ve been wanting is Foundations of Mechanical Accuracy: — some light reading for when I retire.

Foundations is an absolutely fantastic text. Not a general interest text, though.

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Okay, transcribed the sizes from Machinery’s Handbook into

and the even gradation of sizes makes it obvious why the chisel-shaped screwdriver blade became a standard — it facilitated tables such as:

which allowed one to use 9 sizes of screwdriver to drive 24 sizes of screws.

Got a little further along on the Brownell’s bits, and will hopefully finish those up and some other sizes presently, and before I get bored with this, work up one unified, sortable table which will allow me to purchase a set of bits which are nicer than the ones I bought in a Wheeler Engineering gunsmith set, and more generally useful.

As much as I’d like to just buy a set of matching Brownell’s Magna-Bits, I’m probably going to draw the line at a full set of metric sized screwdrivers to carry around — eventually I’ll get the Chapman’s Master Set, and when I retire, the Brownell’s — until then, I did indulge in:

which I’ve added to my traditional woodworking tool chest.

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