Helicoil issues

Hello all, I'm having problems with placing a helicoil into my aluminium engine honda varadero 125v8 it's one of the cylinder head bolts I've drilled and tapped the hole cleaned the hole placed the helicoil in but when trying to put the bolt in it get a few mm in and gets very tight what could I be doing wrong ? The bolt is a 8mm wide 1.25mm thread the drill bit I'm using is 8.3mm. Any help would be much appreciated. Also I've drilled into the engine the filings from drilling, Some have fallen into the engine itself. Can anyone know a good way to clean them ?

Thank you.