As Antony says the three bolts that go through to bolt the cap on to the tank are somwhere around 40mm Long, I'll try and measure one of mine tomorrow, I do know they are M5 size ie a 5mm thread
measuring them is not difficult hoever they also want the pitch of the bolts. Blob of glue on the stem for the shorter ones. It is a really nice part but cannot see why they do not supply the bolts with the cap, as the OEM bolts just don't fit or sit flush.
M5 X 0.8 mm standard ISO 5mm thread with a 0.8mm pitch, remember Countersunk bolts are measured over their full length where as Cap heads etc are measured from the underside of the head, can be important when ordering