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Barking Mat
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Scale would help.
Pigeon_Droppings2
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There is a phone app you can download which will help you identify the thread....I've used it and it works OK....you hold the bolt next to the gauge on the screen (can't just take a picture).
I tend to replace with stainless only in non critical applications due to the difference in strength ratings....I guess I'm asking what the bolts are for?
The one I used is called "thread pitch" but I'm sure there are lots out there. I use mostly metric and they come in diameter eg M6/M7/M8 and thread pitch...eg 1mm/1.25mm etc etc...you need both bits of info to match the bolt.
I tend to replace with stainless only in non critical applications due to the difference in strength ratings....I guess I'm asking what the bolts are for?
The one I used is called "thread pitch" but I'm sure there are lots out there. I use mostly metric and they come in diameter eg M6/M7/M8 and thread pitch...eg 1mm/1.25mm etc etc...you need both bits of info to match the bolt.
Robert Weaver
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Assuming they're metric, measure the diameter across the thread crests (major diameter) and distance btwn threads (pitch) and compare to standards for coarse and fine. M8 coarse is 8x1.25 mm and M10 is 10x1.5 mm. They're probably not imperial if off a vw.
Pigeon_Droppings2
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vw1
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Its not a vw roof rack not sure on brand but its from the 70s and my dad had it on his vw beetle . Its been in my garage for 30+ years . I decided to restore it and bolts have to much rust to use again .
Munkul
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No-one can identify a thread with their eyes, at least not unless you already know what threads are used on a particular machine 
The reliable way of measuring threads, is a thread gauge, a caliper, and a Zeuss book. I highly reccomend these three tools to anyone at all who doesn't have them - they don't break the bank, and they'll bail you out on enough occasions to be well worthwhile!
See? not expensive. You should have a caliper anyways, you can buy a cheapie for a tenner, and you can use the internet to look up Metric/BSW/UNC thread sizes etc if you're too cheap to buy a Zeuss book.
The reliable way of measuring threads, is a thread gauge, a caliper, and a Zeuss book. I highly reccomend these three tools to anyone at all who doesn't have them - they don't break the bank, and they'll bail you out on enough occasions to be well worthwhile!
See? not expensive. You should have a caliper anyways, you can buy a cheapie for a tenner, and you can use the internet to look up Metric/BSW/UNC thread sizes etc if you're too cheap to buy a Zeuss book.
A good clue would be the spanner size that fits the head. Then look at the internet as there's loads of sizing info on it. Still need to identify thread pitch though.
As an aside, these are called set screws as they are threaded for all of their length. Bolts only have thread for part of their length.
As an aside, these are called set screws as they are threaded for all of their length. Bolts only have thread for part of their length.
vw1
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Yeah I do have a caliper but its dead and have not got a battery for itNo-one can identify a thread with their eyes, at least not unless you already know what threads are used on a particular machine
The reliable way of measuring threads, is a thread gauge, a caliper, and a Zeuss book. I highly reccomend these three tools to anyone at all who doesn't have them - they don't break the bank, and they'll bail you out on enough occasions to be well worthwhile!
See? not expensive. You should have a caliper anyways, you can buy a cheapie for a tenner, and you can use the internet to look up Metric/BSW/UNC thread sizes etc if you're too cheap to buy a Zeuss book.
but I shall invest in the above. 
Bladevane
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From a quick scale of the thread diameter and pitch relative to the battery and taking into account they are from the 1970's I would estimate these are 1/4" BSF. The real test though is accurate diameter and pitch measurement then relate those to Zeus (or similar) tables.
zx9
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^^^^^^^^^^^^ WHS ^^^^^^^^^^^^No-one can identify a thread with their eyes, at least not unless you already know what threads are used on a particular machine
But Bladevane's guess could be close.
Metric tend to have grades marked with numbers on the head 8.8, 5.8 or similar newer inch sizes have radial lines on the head to denote grade this may help a little however very cheap bolts tend to be made of toffee and are un graded.
FWIW Dad had loads of old bolts from dead lawn mowers all BSW or BSF and I don't recall seeing any with obvious grade markings beneath the green paint.
Barking Mat
Cuddler of hedgehogs.
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M8's ?
123hotchef
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you need this
Agroshield
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And if none of your metric spanners fit, it is a good indication that the setscrew is imperial.
An option for identifying the thread is to buy some nuts from Toolstation and see which fit: I think they do from M4 to M12.
gordon stephenson
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They look like bolts with full size Whitworth heads.
Munkul
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I get the same result, scaling on screen, counting threads and using vernier calipers. BSW, UNC and UNF are too far out with the TPI, which comes out at 25.5tpi for me, thus 26 BSF.
It could also be M6x1mm....
