Hi

I’m sure someone more knowledgable will reply soon, but as far as I know, drill shanks are left soft and they probably aren’t very tough. The HT socket head screw shank sounds a far better idea, as long as it’s dimensionally accurate enough.

Rob

The shanks of HSS drills are not hard that is why it bent and why they get chewed up if they spin in a drill chuck.

If you want to try HSS the buy a 4mm “drill blank” or round tool blank. Hss will be less likely to bend but it is brittle.

Dowel pins are also a possibility being ground and unles excessively long not easy to bend. They have a good finish and should be on size.

The shank of a 10.9 or preferably 12.9 could work, it’s somewhat of a gunsmithing secret for making firing pins.

Spring steel should also be a good choice if you can find it.

I would go for the 4 mm shank of an Unbrako socket head shoulder screw, Unbrako stuff is chrome moly steel and extremely strong.

Back in the day I turned up a gudgeon pin from a large Unbrako cap screw in the lunch hour for an  air-cooled outboard I bought for pennies because it rattled. I hammered that old outboard mercilessly for years until I sold it with no rattle.

Bear in mind that Ebay/Amazon capscrews are probably not Unbrako.

+1 for Dowel pins – hardened and ground – why go fa*****g around with bits of allen screw?

Was the OEM spec really 4mm – ? surprising for a sixties Norton?

This is a very reliable seller – 50 for a fiver..

https://www.ebay.co.uk/itm/303391264979?_skw=din6325+4mm&itmmeta=01JYVQ1XKPES92EM0W8GQCAG0J&hash=item46a38750d3:g:7RAAAOSwmXtd6Mo-

On 28 June 2025 at 17:12 Robert Atkinson 2 Said:

Rollers may also be soft internally. To get HT at exactly the right diameter I’d go for a socket head shoulder screw:

From a reptuatable supplier.

Why would you use those even from a reputable supplier. They are usually to a minus tollerance typically h8 so likely to be under nominal Dowel pins are ground to a plus tolerance usually m6 so will be ideal for good press fit

On 28 June 2025 at 18:25 JasonB Said:

On 28 June 2025 at 17:12 Robert Atkinson 2 Said:

Rollers may also be soft internally. To get HT at exactly the right diameter I’d go for a socket head shoulder screw:

From a reptuatable supplier.

Why would you use those even from a reputable supplier. They are usually to a minus tollerance typically h8 so likely to be under nominal Dowel pins are ground to a plus tolerance usually m6 so will be ideal for good press fit

The OP wanted something stronger. The original may have been a dowel. Dowels are about 600 MPa Socket head screws are around 1300 MPa. The MAX undersize is 2 thou. A drop of Loctite stud and bearing fit will take care if that. The shoulder screw is much closer to nominal than the cap head suggested by others.

EDIT: Your suggested drill blanks are also h8 tolerance.

Piano wire is exceedingly strong if you can get it in the right size

As already suggested, use a genuine Unbrako cap screw, but turn it up from a 5mm item to get the exact diameter. They turn easily with carbide to an excellent finish.