I've been asked to fix a friends boat where some self tappers have pulled out over time.  I have access from 1 side only so no bigheads I'm afraid.  Its 10mm thick GRP.  Can I use a helicoil in it?  Or should I thread it out to M12 and bond in an M12 piece with an M5 tapped hole down the centre?  Is there anything else I can do?

How much of a load is being applied? There's a few options available but would have thought a bonded insert would work

There's a fair load on them.  They hold a foot bar on the side of a skiff.  I agree a bonded fastener would be best but there's no access to the back.  It was this reason why I thought an M12 slug would be good.  The thread is fairly course so should be ok in the laminate.  I thought the helicoil might also work but the thread is obviously much finer (M5 thread required).  It also means drilling a small hole so might be a first try before the slug.

Have a look on the Memfast website. Is there a cavity behind the panel? I have used hollow wall anchors and rivnuts in the past. The wall anchors may be more suitable for a thick panel.

Carbon Copies Ltd

I've seen the memfast slugs before.  It was these that inspired my ideas!  With 1cm to play with I'm sure this will hold it. 

I've got the Memfast kit at home, its fantastic ( I bought all the thread sizes I could afford at the time!) I used it to build the kit car but have used it loads of times since on other projects so it was well worth the purchase. It works really well in GRP but make sure you select the correct rivnut for the job.