Carburettor Gasket Advice

[His Dudeness]

Metric bolts aren't too bad. Basically if you're buying metric bolts you need to know three things. What diameter the bolt is at the threaded end, the pitch of the bolt and the length of it. For metric they assign the letter M in front of the diameter at the threaded end so M8 is 8mm diameter at the threaded end, M6 is 6mm diameter at the threaded end etc. The next thing is the pitch of the bolt which is the distance between threads.  You can get a thread gauge that slots into the threads that will tell you the pitch but in general there's two types of pitch for each bolt fine and coarse. If you don't have a gauge you should be able to tell the difference between fine and coarse by looking at the bolt. A coarse pitch is the standard looking thread that you see most of the time, a fine pitch has more threads. There's a picture.



As an example if you look at the table in the middle of this page http://en.wikipedia.org/wiki/ISO_metric_screw_thread called "ISO 261 standard preferred threads". Say you measure your bolt diameter at the threaded end and it's 6mm. So it's M6. If you look in the table for 6mm diameter you get two standard pitch options. One is coarse and it's 1.0 and the other it fine and it's 0.75. So you decide it's got fine threads so it's a M6x0.75 bolt and you measure the length excluding the head and it's 40mm and has a hex head. So you now know you need a M6x0.75 40mm hex head bolt.