Hey Matt,
You probably don't need to know what the standard head angle is. If you are building a bike for dirt track then you need to mock the whole thing up to the height that you think will work (including the tyres that you will use, as the Michelin trials are quite high profile) and then measure the head angle. Make sure that the bike is perfectly upright when you do this. Bear in mind that as you decrease the head angle you are swinging through an arc and will therefore raise the front of the bike, so you may want to take this into account with regards to the positioning of your forks within the triple clamps. You can then strip the bike and do the modification, subtracting 26 degrees from your previously measured head angle. This way it doesn't matter what the head angle is when you secure your frame to the workbench, as you are working on a difference, not an actual value.
Regards,
Pete.
