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Nut & Bolt Sizes: Explanation Needed

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Mark Jones

Mark Jones

Jedi Warrior
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I'm about to rebuild the rearend on my Spitfire and I know that I'm going to have to make a trip to the local fastener supplier to pickup some replacement nuts and bolts. In the past I have just taken the bolt with me and said I need a nut and bolt like this because I didn't know how nut and bolt sizes are determined. Can someone explain to me how to properly ask for the nut and bolt I need without taking in a bunch of rusted metal with me? Thanks.
 
First, you have to determine what you have. You can use a size/thread gage purchased from an auto supply place. Common way of specifying size is "Diameter - Threads per inch X Length" (ex. 1/4-20 X 3) for inch sizes and "Dia. X Pitch - Length" (ex. M8x1-30). And then there is the type of head -- socket or (hex) Cap. Unfortunately, there can be a lot of other factors that need to be taken into account when "cap screws" are used in machinery. Other things that may be important are:

1. Strength -- such as Gr. 8 of 8.8 for metric bolts
2. Cap size -- special machined bolts may have a different head size -- hex or socket head
3. Thread length -- that part of the body diameter that is threaded.
4. Drilled -- for safety wire
5. Finish -- plated or not

I try to determine the basics when I do it but still end up taking the cleaned up part with me and a visualization of where/how it fits to find the replacement.
 
Mark, this is straight from the book: (The nomenclature is theirs, not mine. All notes in parentheses are mine.)

Bearing cap to housing, 3/8"x24 bolt. (No length given.)
Crown wheel to differential case, 3/8"x24x.63 length (5/8")
Drain and fill plug, 3/8"x18 dryseal plug.
Front mounting plate to axle, 3/8"x24 bolt. (No length given.)
Hypoid housing, 3/8"x24x1.84" length.
Hypoid housing to axle housing, 5/16"x24x1" Socket head capscrew.
Inner axle flange to drive shaft,(halfshaft) 3/8"x24, (No length given.)
Pinion flange to pinion, 5/8"x18 Pinion nut. (Castle nut)
Propeller shaft to flange, 3/8"x24x1" length.
Hope this makes your life easier.
Jeff /ubbthreads/images/graemlins/cheers.gif /ubbthreads/images/graemlins/hammer.gif
 
Hello Mark,
for your Spitfire the majority of the fasteners are U.N.F. , some will be U.N.C., these are normally used in castings (e.g. studs or bolts in an aluminium casting like a gearbox). The 'F' being fine thread, that is many threads per inch run of the fastener and 'C' being coarse, fewer threads per inch. These fasteners relate to spanner size. 1\4" UNF or UNC = 7\16" AF, 5\16" = 1\2"AF, 3\8" = 9\16"AF, 7\16" = 5\8"AF and so on. Incidentally a Metric 11 mm spanner will fit a 1\4"UNF(C) fastener and a 14mm will fit a 3\8"UNF(C) fastener.
Both AF and Metric spanners sizes relate to the size across the flat of the fastener. Whitworth spanner sizes, used on much older British cars, related to the fastener diameter.

Good luck,
Alec
 
Re: Nut & Bolt Sizes: Explanation Needed

Mark;

Nice list. I'm going to print that out and put it in my service manual.

Mark
 
Re: Nut & Bolt Sizes: Explanation Needed

A handy tip for determining the size of a bolt is to use an open end wrench as a measuring tool on the bolt shaft. If the bolt shaft (not the bolt head) fits into a 5/16 wrench, it's a 5/16 bolt. This works great for older british and American cars that use non-metric boltx. After a while, I've realized that "the majority" of bolts on my Triumphs are either 1/4", 5/16", or 3/8" Fine Thread in various lengths.

Newer cars have a mixture of metric and standard bolts. I have yet to be able to figure out an easy way to tell the difference.
 
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