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

[iansoadyParticipant @iansoady]

Now lost for choice. I think I’m wavering between the dowels – which are available in exactly the right size – and the piano wire.

There’s also a lot of play on the pivoting end of the pawls which won’t be helping any so I’ll probably replace their pins as well. This is what the pawls / pins look like (not my bike).

[Tony Pratt 1Participant @tonypratt1]

‘Proper’ dowels are mild steel case hardened, so basically a hard case and soft centre, fairly cheap so worth a go. Not sure why we see so many stainless steel dowels. Never used or saw the need for them in my working life, in fact I didn’t know they existed until a few years ago?

Tony

[MacolmParticipant @macolm]

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.

[Graham MeekParticipant @grahammeek88282]

From what I can see of the fracture, (a blow up of this would have been helpful). The pin is “case hardened” and has a soft core. Any commercial dowel is going to be of the same construction. A Needle roller in this size would probably be hardened all the way through, but may well be tempered for maximum hardness and thus could be too brittle for this application.

The shank of a HSS 4mm Drill will be soft as you have already found out. An Allen Cap screw shank will not give you a much better performance than Silver Steel which is soft. Which you have tried.

Round HSS tool or Drill blanks would I suspect be the best way to go. Drill blanks in particular will be on size if you purchase from the likes of Dormer, etc.

Without hardening and tempering facilities for Silver Steel I would be inclined to go for the Drill Blank option, but these may well be down on size and not a press fit. High strength Loctite would be my choice here, a grade that will tolerate oil.

Was the shank of the 4mm Drill a “press fit”?

I know it would have been down on 4mm by about 0.025mm, so I am really wondering if you do not want 5/32″ diameter material. This assumption is based on the bike being a Norton.

Regards

Gray,

PS I would replace all 3 with the new material, it will always fail at the weakest point.

[iansoadyParticipant @iansoady]

Many thanks Gray, excellent points. The original (I assume) pin broke rather than bending but both the drill shank and silver steel replacements bent. Both, at 4mm, were a light press fit. I agree re replacing all three so have now removed the pivot pins and pawls. There is a fair amount of visible wear on the pivot pins and the holes in the pawls are (intentionally I assume?) about .010″ larger than the pins and the retaining holes for the pins. This gives a lot of “float” on the pivots.

As you may be able to see all pawls are significantly worn on the face that engages the ratchet, but one (not the one that keeps failing) far more than the others indicating it has taken most of the load.

Advice from the owners’ club experts is to have the worn faces built up with weld which I will need to have done as my welding skills are minimal but I’m sure I can find someone to do it locally. Then I’ll be able to file/grind back to original profile. The pawl material is obviously quite soft.

The operation of the device is that the pins only serve to pull the pawls into engagement with the ratchet attached to the end of the crankshaft and once in contact these should be held in position by the torque from the starter motor until the engine starts when the ratchet spins faster than the starter, pushing the pawls back out so they clear the outer part of the ratchet.

[Tony Pratt 1]

On Tony Pratt 1 Said:

… Not sure why we see so many stainless steel dowels. Never used or saw the need for them in my working life, in fact I didn’t know they existed until a few years ago?

Tony

Two reasons I guess:

• corrosion resistance matters sometimes
• the best Stainless Steels are a bit stronger than most alternatives.

Problem buying dowels off the likes of ebay is finding out what the stainless alloy is.  A stainless dowel made to resist corrosion could be significantly weaker than one made for strength.  Definitely worth a try though.

Reading up reveals an unreliable machine put together by a company on the brink of bankruptcy.  More development was needed and they didn’t have the money.  They probably also recognised the bike had limited sales appeal.  Maybe the factory pins were never quite man enough for the job!  Therefore, I suggest making sure the starter has an easy life by keeping the engine in tip-top condition. Should spin without undue friction and fire a healthy spark into an ideal mix of fuel, with the bike kept warm and dry.   Back in the day it seems owners kicked the motor over a few times before trying an electric start…

Dave

 

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