Manky Monkey Motors

can any body tell me what is the purpose off a starter solenoid and where is the cheapest place to buy one and are there different solenoids for different vehicles

The solenoid is a switch, one thats capable of handling large amperage.
One side is connected to the starter and the other to the battery via large round threaded posts, there will be another small spade connecter called the exciter and depending on fitment, there may be a fourth, which is an ignition bypass.
The exciter teminal connects to the starter switch (or button), and when power is applied to it, a copper plate inside the solenoid moves and connects the two threaded post terminals together, therefore supplying current directly from the battery to the starter motor.
Yes there are different types, you have the old fashioned seperate type for use with inertia starters, or the newer piggyback type for use with pre-engaged starters, and you`ll also have subtle differences between the two types.
your looking at between £10-£15 for an inertia starter one and double that for a pre-engaged type, depends what starter you have!

thanks for that TB what size cable should be running from  battery to starter solenoid and from solenoid to starter motor

25mm squared, 170 amp rating will be fine.