Any question answered...

Vello, you need to measure the length of the bolt you need to replace, not including the head, and get one that is about 10mm longer. If (a) is 10mm , it is probably an M10 x 1.25 thread size, but that is a guess. I wouldn't use studding as it is made to hold ceilings and ducting up, not huge engines.
Think about the clearance you have around the head, and whether you will be able to get a socket or spanner in there, or whether an 'Allan' bolt is a better bet.
I don't think Clerkenwell Screws will have what you need, but these people may have something suitable,
http://www.pro-bolt.com/

There are three options for M10 bolts - standard pitch (1.5mm pitch), fine (1.25mm pitch) and the other really fine fine (1mm pitch). You'll need to know which one it is before buying the bolt. The easiest way to work it out is to take the bolt out, and measure 10 threads. In my experience most Japanese engine bolts (Honda Blackbird, ZX9R, R1) are M10x1.25.
Namrick is a useful place for buying bolts (www.namrick.co.uk) is a good place for buying small quantities of unusual bolts, although I don't think they do metric fine cap heads. You'll probably want a 12.9 spec bolt, which most cap head bolts will be.
What kind of bike is it? Might help narrow the options, as would some more information about what bolt head it has.

Thanks guys.

Actually reading your answers I realize I should have explained that I need a threaded rod, not just a bolt, as it goes all the way thru the engine, lateraly: I've made a little drawing to clarify.
With regards to the thread, my plan was to buy the nut and rod at the same time to avoid compatibility problem with existing: nothing is threaded on the frame or engine, I can use whatever I want providing the diameters is correct for the hole, and the existing rod is 10mm diameter.

The bike is an old Suzuki tourer, GSX1100G (shaft driven)

Loic

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