On measuring my coach bolts, they were indeed too long @25mm long, (300mm counting the 5mm that recesses in the plastic end cap. The original coach bolts were rusted through, and had to be angle grinded off, so couldn't measure.
So I have cut the coach bolts down, and now they measure 11mm, plus 5mm which will go in the plastic end cap (see below photo)

Will 11mm be ok, or do I need to cut some more off?
Thanks in advance.
