Helicoils in block.

[jonblob33]

Hi all, i’m Looking for a bit of advice please. Just found water in no 3 cylinder. Took head off to find stud between no2&no3 cylinder pulling out of block. Re helicoiled with 2d 5/16 bsf helicoil fitted with thread sealant. No water coming up stud now but unable to torque to 20 ft/lbs. I think the helicoil is pulling out again. My question is I am thinking about re helicoiling to 3/8 bsf but not sure if there is enough area round the stud for the head gasket to seal ok. Has anyone done this mod? Any suggestions or advice much appreciated. 
Thanks. Kevin.