For an 8mm hole you will probably want an M5.5 according to this table
It's just one of those in-between sizes which doesn't quite work.
Easy enough to drill out to 9mm and put an M6 into it I would have thought.
Not sure where you got your figures from as they don't seem to agree with the chart. A couple of other online charts seem to be giving the same results which is why I'm questioning your figures - I'm no expert, I just remember having had a similar issue a couple of years ago and googled the figures again, so I'm quite happy to be proved wrong!
Thanks, M5.5 is probably right (I've tried M5 and M6). I measured it myself with a caliper, so it's a bit strange the table is different.
I'd rather avoid drilling, the extra effort and clean up is a bit annoying! I'm going to try and find a hardware store today with different sizes in stock (harder than it sounds) and see what I can find.
apologies, I was attempting to be humorous with my M5.5 comment
To the best of my knowledge no such thing exists.
I guess you could possibly go the other route and go for a smaller size rivnut and put washers with the correct diameter hole in front and behind the existing van holes (assuming you can access both sides)
If you do decide to drill, then a good tip is to place a magnet near the hole to catch the swarf as you're drilling. Saves a lot of time in clean up. And don't forget to paint the edges afterwards so they don't rust.
@robthehungrymonkey I'm going to be using rivnuts too. Drilling out to 9mm, paint and then an M6 rivnut - stainless steel so they don't corrode. Using them for all ceiling battens, wall panels and overhead cabinets so they're removable.
There are a couple places that sell for reasonable prices in small quantities. I ordered from Liberty. 1/4-20 plusnuts were $0.25 each. I don't know what the minimum is but I ordered 140 and was $58 with tax and shipping.
There are lots of rivnut sizes, I use a lot on my builds. One variation uses external splines which works well with slightly over size holes. It is a shame that the big supermarkets control the fastener market in the states. Fasteners made in Europe are higher quality and cheaper than supermarket junk. Eric.