How bad of an idea would it be to use a tap to add an internal thread (probably M20 based on the internal diameter of ~18.5mm) to a pair of aluminium mountain bike handlebars?

I'd keep the depth pretty minimal, maybe 7mm.

I want to knock up some handlebar storage units, and a threaded bar plug that can twist out by hand is very appealing to some strange part of my brain.

I'm assuming the strength won't be crucially compromised by 7mm of threading but figured some of the engineering folks here might have a better insight than me sitting in my living room imagining the stresses on some bars.

Shouldn't cause any issues and 18.5mm is bang on for M20 with a 2.5mm pitch.

Rad. Thanks. You reckon 1mm pitch will work? I wanna use hollowtech crank bolt if possible.

For a 1mm pitch your 18.5mm hole is a shade too big but it should still cut a safe, useable thread.

Pretty much what stu said, while 18.5 is a bit oversized for an M20 female thread you'd be absolutely fine for small length and load ...

Thanks for the advice, guys. I'll give it a bash and see how it goes.

Once I've found a supplier for the tap that doesn't look like an AliExpress scam, anyway.

A few on ebay for around £20ish

https://www.ebay.co.uk/sch/i.html?_nkw= ... iMQAvD_BwE