What length exhaust bolts ?

Long story short - the top exhaust bolt (the one holding the exhaust into the head) on the back pot, has stripped the threads (in the head) at the start.  I'm guessing that whoever fitted the Highway Hawks replaced the original bolts but they look a bit too short.  I've replaced the one that came out (M8 x 25mm) with a 30 mm stud & it seems to torque up fine.  Does anyone know what the length of the bolt should be - I've been through the manual & can't find a length.   Is it worth going to the expense of having the head Helicoiled if it tightens up OK?  I imagine it'd be expensive & if I can use a longer bolt with no detrimental effect, I'll stick to it.

Hi Dave, 30mm thread length on the stock bolts, V&H are slightly longer 10/15mm perhaps? due to flange being slightly thicker, could be worth screwing a bolt in as far as it will go to see how much thread you've actually got left? I'd have thought anything over the thickness of a nut (10mm say) should give enough purchase as long as you don't over do it and use plenty of silicone to form a good seal.

Thanks Sim.  I'd had a similar thought, so I whipped the bolt out again & pushed a bit of welding rod down there - 40 mm to the end.  Used a 35 mm & torqued it up - I'm happy with that - I'll replace all the others as well.  Reckon the previous owner just used whatever allen heads he'd got lying around - 25 mm was well short on stock, never mind this having after markets fitted.

 Apparently, the general rule of thumb is that you should have at least the width of the bolt in thread depth, so if you're using an M8, you need at least 8 mm of thread