You need to talk to your Ariel in the same way as you are talking to the plants in your windows.

Many years ago, when I restored my cast iron Sq4, I decided to reline the cylinders back to original size. Since then I have had on my shelf a set of usedd pistons in a big oversize; about 67,4 mm. I took measures of them in 2 directions. They are coming here: Nr1: 67,29 - 67,39. Nr2: 67,30 - 67,40....

And at that time multigrade oils were not invented, so don't believe what you read.

It is like the oilcap on my 37 Sq4.

Yes, and the surplus ones were used on export bikes. This one is on a partly restored bike, which arrived in Denmark in 1931. The rear light is not original, but has been on that bike for more than 6 decades, so it has earned its place.

could there be indentations in the fork bearings?

Dish washing soap is not good. It contains a lot of salt, causing rust. You must neither use it to mount tyres for the same reason.

I'm rather sure, it is cycle thread. 1/4 x 26TPI.

My main concern now is, that I need some 3 mm rivets for a project. But if the need is strong enough, I can turn them on my lathe.

It seems to have a thick compressionplate under the cylinder.