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bolt sizes, heads and threads


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I never really understood how can I identify my bolts as being a M8 or M10 or whatever...well I know that I'm dumb as a mule but I can learn.....the same goes to threads and head styles/shapes...ok I know that the narrower the thread the more pressure the bolt withstands so I was told......but this is not enough to fulfill my needs :eusa_whistle:

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I've got a digital vernier that was really quite cheap, about £5 from lidls in one of their "I know you come in to buy carrots and yogurt, but I'm going to tempt you with something completely different" sales. With that I can measure the inside and outside of holes and bolts, plus on the back it has the typical threads for any given metric bolts etc, plus the sizes of the hole to drill if you're to make your own threaded opening. Then with a set of thread guages, I can see if the bolt I want to replace or whatever is a standard thread, or if it's a fine version etc.

My vernier looks like this, although mine has the little table of sizes etc on the back and I'm not sure if this does. If not a chart of sizes etc off the net'd do you fine.

My thread guage is like this, although to be honest It's got loads of sizes I'm never going to be using; I only ever use 3 or 4 perhaps.

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As Lorenzo says buy a cheap Vernier from lidl or wherever. With this, place it across the threaded part of the bolt, and it will read the diameter. ( If it reads 5.7/5.8 it will be M6 etc etc)

ISO metric threads are M3 x 0.5, M4 x 0.7, M5 x 0.8, M6 x 1.0, M8 x1.25, M10 x 1.5, M12 x 1.75

Motorcycle threads are USUALLY the same upto M8. After this it could be any of these. M10 x 1.0, M10 x 1.25, M12 x 1.0, M12 x 1.25, M12 x 1.5

If you havent got any thread gauges, if you are careful, you can measure the pitch of the thread by using the vernier. Place the fixed blade of the vernier onto the crest (top edge of one ot the threads) , and move the "moving" blade until it corresponds with the next crest, read off the vernier, and it will give you the pitch of the thread. It is more accurate if you measure over more than 1 crest (pitch) ie 5 Pitches and then divide the answer by 5. Probably sounds complicated, but try it on a thread you know the size of, you'll soon get used to it.............(honest)

Tapping sizes for metric threads; if you know the pitch of the thread subtract this from the OD (outside diameter) and hey presto, the size of drill you require to drill your hole. IE, M8 x 1.25, subtract the pitch (1.25) from the od (8) = 6.75. The ISO drill size for M8 thread is 6.8mm (as in zeus books etc)

Heres a list for std ISO metric threads. M3 tapping size 2.5mm, M4=3.3, M5=4.2, M6=5.0, M8=6.8, M10-8.5, M12=10.2.

ISO head sizes of hex head bolts;

M4=7mm M5=8mm M6=10mm M8=13mm M10=17mm M12=19mm

Bike sizes

M4=7mm M5=8mm M6=10mm M8=12mm M10=14mm M12=cant remember and its wet and dark...........

If you want to bored anymore about threads etc, pm me and i'll do me best to help.


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M6 +4 = 10 ... or ... for a m6 bolt you add 4 to get a 10mm spanner.

Most people can judge the spanner size needed after a bit of wrenching and as said higher up the thread pitches are fairly standard. Most bikes GSX-R1100W's and the like use an m6 as the most common bolt size - so buy a few different length from a bolt seller on e-bay

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