it measures 138.86,
heres something i found out whilst doing a bit of digging which surprised me
A new std head should be 139.0mm according to Cosworth. However I have used several brand new heads over the years and some have been as low as 138.82mm from the factory BRAND NEW. Most NEW heads are 138.90mm. (I have three brand new heads in the workshop and they all measure 138.90mm.)
the above statement by karl norris