High strength 4mm steel?

[iansoadyParticipant @iansoady]

I have a Norton Electra motorcycle which has a notoriously weak electric starter system. Part of this is a set of 3 pawls which are pivoted and rotated by 4mm steel pins which engage with arcuate slots in a “wave” spring. The pawls drive a rathcet arrangement on the end of the crankshaft.

One of these pins broke and I am struggling to find an appropriate material to replace it. One owner of a similar machine suggested using 4mm HSS drill shanks: I tried this and the pin bent on first use. I replaced it with a pin made from 4mm silver steel (BS1407) but again the pin bent after a few starts. I used the material as supplied as I have no hardening facilities.

Can anyone recommend a suitable material for this purpose? Another owner has suggested using the shank of a 4mm high tensile socket head screw but I’m dubious.

[Grindstone CowboyParticipant @grindstonecowboy]

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

[John HaineParticipant @johnhaine32865]

Roll pin?  Generally made of hardened springy steel.

[JasonBModerator @jasonb]

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.

[bernard towersParticipant @bernardtowers37738]

I’m with Jason definitely a job for a dowel pin,strength and on size.

[FulmenParticipant @fulmen]

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.

[Nick HughesParticipant @nickhughes97026]

Another option is Needle Rollers, such as here:-

https://www.ballandrollerstore.com/loose-steel-rollers/4mm-x-15-8mm/

[iansoadyParticipant @iansoady]

Many thanks all, much food for thought. I believe another Norton owner has used roll pins. Most of the dowel pins I can find are stainless steel (A2) so I’m not sure about the strength.

Thinking further about roll pins, I believe they’re made larger than the nominal diameter so they close up when fitted. Not sure this would suit me as the free end needs to fit in the arcuate slot of the wave spring.

Re socket head screws – the 4mm diameter is quite critical as this needs to be a press fit in the pawl. I suspect that a nominal 4mm screw will be undersized for this.

Needle rollers may be a good option.

This shows the general layout – the ratchet etc are shown at the bottom.

[Pete RimmerParticipant @peterimmer30576]

12.9 socket cap screws are very tough. I turned a large one down, milled a square in the end and used that to broach a wallered-out square hole in a 1960’s Triumph brake pedal. It cut a perfect square hole in the pedal and didn’t even mark the edge of the square.

[howardbParticipant @howardb]

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.

 

[Robert Atkinson 2Participant @robertatkinson2]

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.

[DiogenesParticipant @diogenes]

+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-

 

[iansoadyParticipant @iansoady]

Those dowels look good. Yes, 4mm is a bit odd and it may be that the original spec was 5/32″ which is a couple of thou under that. 4mm gives a nice interference fit however.

[Robert Atkinson 2]

On 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

[not done it yetParticipant @notdoneityet]

Questions:  Are you replacing all, or just the one?  If only one, might it be taking the full load before the others?

[JasonB]

On JasonB Said:

On 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.

[JasonBModerator @jasonb]

Press fit was only mentioned after I said drill blanks😉

Nor did I see a need for the replacement to be harder. Just suitable.

I took the suggestions of cap head screw shanks as raw material to be turned down, quite common practice for model engine crankshafts.

[duncan webster 1Participant @duncanwebster1]

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

[duncan webster 1]

On duncan webster 1 Said:

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

Macc models claim to sell it, likely from a known supplier

4mm dia piano wire 1 metre long

Spring tempered, phosphate coated, carbon steel wire ASTM A228
https://www.fushunspecialsteel.com/astm-a228-music-wire/

Bill

[Author]

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