The chance of new crankshafts and rod etc

[Hedd_Jones]

If I was to make a crank at home (as you do) I'd probably use 4340 (EN24T)
I know of one A7 crank made "at home" from a slab of this and it's fine 25 years later

c

Thats what the crankshaft we had made for my traction engine is made from.

[Bob Culver]

Having got this far some further may interest a few.

The fatigue limit is the stress below which steel "never" fails. Well designed car engines only fleetingly operate above the fatigue limit, and as several million cycles just above are required for failure, no problems occur in normal lifetime. 

More disturbing, with components more occasionally operated just above the fatigue limit, as original Seven cranks obviously do, a very small increase in stress dramatically shortens the life of millions of cycles at that stress. So Sevens with raised CR and/ or operated at high rpm have reduced crank life. It is no coincidence that the few owners who claim very long life of original cranks have unmodified engines. By the same token, raising the fatigue limit slightly, can very dramatically increase life for the same usage. 

Increasing the tensile strength of the steel say 10% may typically increase the fatigue limit  by about 4%. So limited scope for improveement over the original, or one ht steel compared another. But nitriding a suitable steel may increase the fatigue limit  20% and considerably more. Hence the success of nitrided replacement  cranks.
(Editted to fix obvious error)

[Charles P]

In the absence of any controlled testing this narrative doesn't help explain whether using a high spec material, going up to 1.5" or nitriding makes the most difference, and allows a user to know if an EN40B 1.5" crank is perhaps overkill in many circumstances, which was Tony's problem.

Charles
BSc Metallurgy and Microstructural Engineering

[Stuart Giles]

interesting stuart.

we had taken into account modern cars having more jernals, also a smaller throw. and staggered jernal cranks.

here are the materials we were offered.

ASTM 4340 used for, engineering structual. used as require good toughness. high strength. as well as the important conditioning of large size. heavy machinery such as high load of axial, turbineshaft. larger than 250mm helicoptor rotor shaft. turbojet engineshaft, blade. high load of transmission parts. fasteners of the crankshaft, gear etc.

As above ASTM 4340 is equivalent to EN24T or 817M40T as the modern British Standard calls it.

ASTM 4140 used for, many applications as forgings for the aerospace and oil and gas industries. a myriad of uses in automotive, agricultural and other defence industries, typical uses are forged gears and shafts, spindles. fixtures, jigs, and collars.

thanks tony

ASTM 4140 is equivalent to EN19 or 708M40 on the latest BS standard. It's used a lot by US manufacturers of forged high performance aftermarket conrods etc.

In general terms there's a lot of confusion over material specifications, not least because the UK, Europe and the US all use different nomenclature for engineering steels, but also because lots of people (me included) use the (obsolete since the late 1970s)  EN naming system because it's easier to remember EN16T than 605M36.

Just to add some extra confusion, there is a newish BS EN system in place now (where the EN stands for Euro Norm) which was fun when I needed some bright round engineering steel from a stockholder who was initially convinced that I wanted a hot rolled steel beam.

[Bob Culver]

I seem to recall that the original cranks were EN19 (the Companion? or was it axles), somewhat higher uts than the EN16 or EN12 used for other makes. Dunno what tempered to, but  the scope for a ht steel of further increased uts imay be quite  limited.

[Tony Betts]

at the same time as myself,

one of the mini race companies were also after new cranks.

the mini was originally EN16, they went forward with the 4140 nitrided.

tony.

[Stuart Giles]

at the same time as myself,

one of the mini race companies were also after new cranks.

the mini was originally EN16, they went forward with the 4140 nitrided.

tony.

All the Run of the mill BMC 'A' Series Cranks were forged EN16 AFAIK. But the Cooper "S" cranks were always EN40 Nitrided. Probably the main reason that the classic mini guys would be keen on that spec. The EN16 cranks were/are OK for a Sunday afternoon drive but don't seem to live very long in a tuned motor. This crank Magnafluxed good when the motor was built -admittedly a few years before this occurrence:

[Tony Betts]

length of "string theory" then stuart.

at least you now know how long yours is?

tony

[Steve Hainsworth]

Hi
I have only just read the latest entries to this topic.
I am a toolmaker by trade and have worked in the automotive press tool industry and Plastic mold making until teaching engineering in the last 10 years.
my knowledge of metallurgy and heat treatment tells me that although 4140 is a nitridable high tensile steel ,it is not however regarded as a nitriding special steel. Those materials invariably contain aluminium in small concentrations.
One trade name for this was Nitralloy and I think it was developed during WW2. It exhibits superior characteristics to 4140 and is fine grained
So I believe there are better alternatives to 4140 (EN40b).  In New Zealand 4140 is only available in rounds so it cant be used for flat billet cranks here.
P20 is the modern plastic die steel  of this aluminium alloyed nature, used in industry for plastic molds and manufacture of stressed parts including crankshafts and gears among others, it is supplied in a heat treated stress relieved condition which means it is able to be sawn and milled/turned in a toughened state and ready for nitriding once machined and ground.
it is usual to polish the journals after the nitriding operation. it is around 60 tons tensile in this state and the hardness of the surface 63 Rockwell C plus.
A piece of this material of the dimensions required is around $160 NZ or 80 pounds UK.
I am certain of its availability in UK (West Yorkshire Steel Has it in stock in plate form - 45mm thick heat treated and stress relieved).
Heat treatment does not mean hardened ,but heated until it is tough and machinable and stable after machining.
I hope this is useful and would be interested if other engineers agree with my opinions on this.
Cheers Steve Hainsworth, Wellington NZ

[Tony Betts]

hi steve,

thanks for adding to this.

it proves my fears of useing another material, right or wrong there will always be a chance to say you used the wrong material.

we did look at other materials available in the uk from the start, and prices.

the price of E40b in the round over here to machine billet cranks was coming in at £500 per crank. we couldnt get it big enough in bar.

BUT we could get EN30b big enough in bar, at a price of £50 per crank. then large sections could be laser cut out to reduce machining costs.

thanks tony