Hi to all,re building a 42" second hand cat,-in short found motor ,shaft etc to far faward of 1/3 mark(CG),this my first cat build ,so my question is the CG mark,for the centre of motor or exit hole for shaft,just want to get it rite,no instructions to guide so have to fly blind,any help would be muchly appreciated.

In terms of CG with gas powered cats at that length, most have had to get CG at between 31 & 34% to prevent blow off's.
Then prop choice is pretty important to find the correct lift and push balance.