Bolt sizes

gardenshed

gardenshed

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I've got a bolt that appears to be M10 approx 85mm long. the shank is slightly less than 10mm. What is the closest imperial size to M10?
The bolt has come out from the bellhousing of a perkins 4108 and was used to connect a hurth gear box to it.
 
Closest imperial size is 3/8". If it is actually metric it will probably have an 'M' on the head. If you use a pair of vernier calipers to measure it up accurately, along with a thread gauge or some careful counting to measure the tpi, you should be able to identify it precisely.
Some bolts used on engines have a slightly waisted shank, ie smaller diameter than the nominal thread size. Often this is the case with high tensile (eg cylinder head) bolts.
 
3/8" is smaller than M10. Whitworth is 16 tpi, BSF is 20 tpi, UNC is 16 tpi , UNF is 24 tpi

7/16" is larger than M10. Whitworth is 14 tpi, BSF is 18 tpi, UNC is 14 tpi, UNF is 20 tpi

Whitworth and BSF threads are 55°. UNC and UNF are 60° so although Whitworth and UNC have the same tpi the angle of the thread form is different. There are other differences in the thread form as well.

M10 has a pitch of 1.5mm which is approx 17 tpi. M10 fine has a pitch of 1.25mm which is just over 20 tpi. Metric threads are also 60°

The "reduced" shank may indicate that the thread has been rolled rather than cut. Common with metric threads.
 
UNC = Unified National Coarse
UNF = Unified National Fine
Whitworth = British Standard Whitworth
BSF = British Standard Fine

HT = High tensile

tpi = threads per inch.


My money would be on it being either 3/8" or 7/16" UNF
 
A bolt of approx 10mm diameter sounds like 3/8ths (=9.525 mm). They were making 4108 engines when I visited Perkins back in the late 60s, though I don't know when they started this range. Many engineering products had Unified National Coarse or Unified National Fine threads at that time, whilst British Standard Whitworth and British Standard Fine threads had become progressively less common in designs produced after the war. Many engineering designs adopted metric threads in the early 70s when UK "went metric"

You can tell which (Unified or BS or metric) by finding a spanner which fits properly on the head - the spanner will be marked with the thread size (eg 3/8 BS) if it is BS or marked with the cross-flats size of the head (eg 5/8 A/F) if it is Unified (or 14mm) if it is metric.

As to whether you have a coarse or fine thread, an engineer could tell by just looking. Otherwise, you can count the threads per inch, but you would still need to refer to a table of threads to identify coarse or fine.

I think your bolts are most likely 3/8ths UNC if they are screwed into threaded holes in the engine. If they just have nuts on, they may be UNC or UNF.
 
as posted a bolt should be easy enough to obtain, just note
if it is to screw in to a pre-tapped hole you will have to ensure
you get the right thread, i would try a local hardware store as
most stock a selection of metric bolts.
 
VicS was all this info right off the top of your head, its 30 years since i did bolts at collage and have forgot most of
lessons learnt.
 
thanks everyone for the tutorial on threads and thread sizes. I've got a BSc Hons in Mechanical Engineering, but don't know my UNF from my whitworth......... shame on me!
This is the forum at its best. Honest to goodness know how respectfully offered and very gratefully received.
"an engineer could tell by just looking" how true
Just glad that I didn't try and muscle in a M10 and then start a thread (no pun intended) about the merits of helicoils, oversize drilling and tapping or other repair bodges.
Best regards
GS
 
If you have problems with supplies locally, then "The Nut and Bolt Shop" - Google for "Namrick" do a mail order service.

The told me recently than UNC or mebe UNF bolts were becoming difficult to source.
 
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