MGB Bolt Sizes/Taps and Dies
by Les Bengtson

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The MGB uses a combination of US, British and Metric nuts, bolts and fasteners. While the chrome bumper cars are more likely to use US standard threads and the Rubber bumper cars are more likely to have metric, there tends to be a mixture that continues over the years. In some cases, the US and metric bolts are so similar as to be interchangable. In other cases, significant changes have been made and what works on one car will not work on a different year. Over the last several years, I have had access to a 68 MGBGT and a 77 B roadster. I have determined the size of all the bolts and studs that I have had to remove and made a list of them. I am adding to this list all the time, but hope this initial listing may be of use to some people right now. Hence, I am publishing an initial draft and will, as time goes by, update it as more information becomes available.

Taps. A tap is a hardened piece of tool steel which has been machined to the size of a standard bolt, hardened and fluted (grooves cut along the long axis) so that it is capable of cutting threads into steel, cast iron or aluminum. There are three types of taps--taper, plug and bottom. The taper tap is designed to cut threads in a hole that goes completely through a piece of material. It features a fairly long taper on the bottom end which helps to align the tap with the hole so the threads are cut square to the top of the material. It is the best tap for general use. The plug tap has a shorter tapered area for working in "blind" holes--holes which are not drilled all the way through the material being threaded. Because of the shorter taper, the tap will cut full sized threads somewhat lower into the hole than a taper tap will. The bottom (or bottoming) tap has no taper and is only used in holes which have already been tapped part way. It allow one to extend the partial threads cut by the plug tap for a few threads more. I would recommend that you purchase taper taps for general use and purchase plug or bottoming taps on an as needed basis. The taper tap is capable of cleaning up the majority of buggered threaded holes and, with its longer taper, tends to help prevent cross threading. Most hole damage tends to be near the top of the hole if it has been buggered. If it is rusted, the entire inner surface is contaminated by rust and the taper tap will remove it efficiently and leave it sized properly.

Taps are made of two materials--carbon steel and high speed steel. The carbon steel is somewhat more brittle and is less expensive. High speed steel (tool steel) is less brittle and more expensive. Both have their places in machine operations. For automotive use, the high speed steel (HSS) is the best one to use.

Dies. Dies are available in two different styles--a "re-threading die" and a standard machine die (simply called a die). The re-threading die has a hexagonal external shape and is used to clean up burred bolts and studs, being turned by a large wrench.
The standard die is circular in shape and has a hole in the middle which has been threaded. Three or four relief cuts are made so that chips of steel or aluminum will not foul the cutting action. They come in various exterior sizes and are held in a "die handle" which is a circle with two handles coming out the sides. The die is inserted into the handle, locked in place by one or two set screw and then used to cut or recut a thread.

Caroll Smith, in his book "Nuts, Bolts, Fasteners and Plumbing Handbook" states that bolts and studs should never be re-threaded and should always be replaced. This is completely true with safety related applications. It is less true with non-safety related applications. I would not re-thread a head stud. I would re-thread a bolt that holds the trunk hinge on.

Using Taps and Dies

Taps and dies should be lubricated to prevent sticking and promote clean cutting. With a tap, the tap itself is normally lubricated. With a die, the piece being threaded/re-threaded is normally lubricated. Since the majority if holes being re-tapped are being cleaned up of rust or paint, you do not need to use an exotic tapping fluid. Motor oil, ATF, light machine oil or a similar compound will work fine. If you are making a special tool or have drilled a hole larger and are tapping it for the first time, Tap-Magic or a similar compound is recommended. Exactly what to use will depend on the material being tapped and the recommendations of a machinery supply store should be used. I use the same oil can that I use to top up my SU carbs and the tranny, finding that Castrol 20W-50 works well for general clean up work.

Taps are round in shape with a square end at the top. A device known as a tap handle is used to hold the tap and turn it. Most of these have a collet arrangement and are designed to fit a range of taps ( 0-1/4" and "-1/2" being the most useful). There are also tap handles which are two pieces of metal with a "V" cut in them and having two screws to tighten them together. These are especially useful if the larger taps are used, but not required for most automotive use.


The size of bolts, etc. is determined by three dimensions. The first is the diameter of the bolt, known as the "major diameter". The second is the number of threads per inch or millimeter. The third is the length. The length is normally measured from the underside of the bolt head to the end of the threaded area. This same form of measurement is used with most items having a head, such a socket headed cap screws or button headed cap screws. Set screw and studs are usually measured as the overall length of the item. All of this is easily measured with a dial caliper or ruler.

The major diameter is also easy to measure with a dial caliper or micrometer. Normally, on partly threaded objects such as studs or longer bolts, the major diameter can be determined directly by measuring the unthreaded portion of the stud/bolt. The size is normally expressed in either millimeters or fractions of an inch. Machine screws, of the smaller sizes, are measured as a series of numbers going from 0 to 14, with 0 being the smallest and 14 being the largest. When you do not have any unthreaded portion of the bolt/screw to measure as a major diameter, you measure the threaded portion. The threaded portion is normally slightly smaller than the nominal major diameter, so a small amount of interpretation is required. For instance, a " (.250") threaded bolt may only measure .242-.245" from the top (crest) of the threads. By comparing the measurement to a table of standard threads, the size of the bolt may be determined.

The last measurement is the thread pitch or number of threads per inch. This can be measured in two ways. The best way is to use a thread pitch gauge. This is a small tool having a number of leaves similar to a feeler gauge. Each of these leaves has a series of notches cut into it representing the number of threads it represents. This number is stamped into the gauge leaf. You hold the bolt in one hand (or a vise) and try to fit the various leaves into the threads until you find one that fits exactly. I find that the use of an optivisor or a magnifying glass helps, especially with the finer threads. When you find the one that fits perfectly, you read the number stamped on the leaf and, therefore, the number of threads per inch on the bolt.

The second method of determining thread pitch is less exact, but will work when no thread pitch gauge is available. You measure " on the threaded portion of the bolt and then count the number of threads covered by that area. Then, you multiply by four to determine the number of threads per inch.


Dial calipers can be purchased from most stores selling tools, including hardware stores, auto parts stores and machine supply stores. A very usable set may be had for under $25, made from stainless steel in China. The more expensive, American made tools, are not necessary for most automotive applications. My $18 Chinese version is just as accurate as my $125 American tool.

Thread pitch gauges are available, normally for under $10, from machinery supply stores. You can find these in most major cities through the yellow pages. Rutland Tools and Enco Tools are major mail order discount tool sources.



This is an on-going work and will be updated as I have time and opportunity to measure and catalog more sizes.


Size-Fractional (decimal) Threads UNC Threads (UNF)
1/4" (.250") 20 28
5/16" (.3125") 18 24
3/8" (.375") 16


7/16" (.4375") 14 20
1/2" (.500") 13 20
9/16" (.5625") 12 18
5/8" (.625") 11 18
3/4" (.750") 10 16

Bolts smaller than 1/4" are measured in machine screw sizes, running from Zero (0) to 14, with 0 being the smallest and 14 the largest. They run O to 6, then 8, 10, 12 and 14. Each has a standard UNC/UNF thread specification. These are the most commonly encountered. There are, however, a number of specialty machine screws either dating from before there were standardized sizes, or to meet special requirements. These will seldom, if ever, be encountered in automotive work.

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

Copyright 2001 by Les Bengtson Home - Classic Sports Car Pages - Tooling for Classic MG Restoration - L. Bengtson Arms Co.