Looking at:
How to drill drill out or hollow
bolts.
and
'Where to buy' hollow (drilled) bolts.
It looks like
people are happy to 'Get Technical' about it with talk of hollow bolts being 'Stronger' and maths on the subject. Hole vs shank diameters etc-etc!
Then there's talk on
'Where to Buy' Hollow Bolts and
Aluminum Bolts and even
Hollow Titanium Bolts!
So this is just a bunch of links to forums discussing the 'how to's and maths and 'where to buy's, for now:
https://www.gt40s.com/threads/source...t-bolts.38380/
https://www.practicalmachinist.com/f...uminum.365860/
https://www.accessnorton.com/NortonC...d-axles.14507/
The overlap with racing is noteworthy.
ie: If you want to make an Eco Car;
Talk to and copy the racing guys 1st; DO what they do, then just drive 'Eco' rather than 'race'.
Well... until a nice twisty, quiet bit of tarmac presents itself!
There is more (fun) to life than just 'Eco'!