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Thread: Steel for a Crank shaft.
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23rd Jul 2013, 04:54 PM #1Senior Member
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Steel for a Crank shaft.
Gentlemen, a question for the ''brains trust''.In the not to distant future I have to make a crank shaft for a 1922 Diatto. Its own is broken.The original is machined from a solid billet. Blank billet will need to be 8-10 inches in diameter and 3 feet long.Be a lot of swarf in the bin for the end result.Am seeking a heads up for advice on material and possible suppliers?As there will be heat treatment and normalising processes to go through am throwing my hat in the ring for advice. Thanks in advance John.
Diatto an Italian car was a low volume car there are 4 here in Australia 2 locally. They were to become Maserati in 1925 .F Y I. John.ps Squizzy Taylor the gangster drove 1 in Melbourne in the 1920's.
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23rd Jul 2013, 06:51 PM #2Most Valued Member
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If there is going to be any heat treatment involved wont you need a specific alloy?
Stuart
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23rd Jul 2013, 07:10 PM #3
Hi John,
Sounds like a great project!
I'd try asking around the after-market racing fraternity and see what they are using.
I googled it and came up with 4340 Holden V8 Billet 4340 steel crankshafts | COME RACING
When it comes time to heat treat it, go talk to Hills Heat Treatment in Bayswater, Hills Heat Treatment - Bayswater, VIC - whitepages.com.au
Regards
Ray
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23rd Jul 2013, 10:25 PM #4Senior Member
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23rd Jul 2013, 10:30 PM #5Senior Member
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24th Jul 2013, 08:27 PM #6Senior Member
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Crankshaft Alternative
John,
Another alternative to a billet crank is to find a donor crankshaft with the same bore spacing and a similar stroke and see if you can adapt it to fit.
When I was rallying MKII Ford Escort's I developed a stocker crankshaft using a donor crank. The crank nose was machined down to the Escort dimensions, the big end journals where offset ground to increase the stroke and a custom flywheel machined. I then used off the shelf rods and custom pistons that took the 2.0lt out to 2.4lt. All this was done for half the cost of a billet crank from the UK.
What is the stroke and bore spacing on the Diatto?
Cheers
Piers
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24th Jul 2013, 09:38 PM #7Senior Member
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I was interested in the heat treatment involved and found this link which may be useful... seems very involved.
Machining operations for a crankshaft from a 4340 billet
Saw this on PF too... good to find someone who shares the same interests
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24th Jul 2013, 10:07 PM #8Intermediate Member
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En25
I can recommend good old EN25 (826m31, Werkstoff 1.6743, 32NiCrMo10 4) Can be obtained in the annealed state speak to the supplier and ask for a hardness through out, homogeneity, mico cracks and an ultrasonic. Rough Machine all over allowing two mm and then stress relieve.
Straighten as required then Final machine / grind then finally nitride to a depth of ten thou gives a hard surface and improves the resistance to fatigue, ( very special M/C engine stuff with sprocket teeth cut into the centre as well that spin up to 18,000rpm)
Not caused a problem with distortion the final nitride operation if it has they can still be pressed straight again
Good luck with it mate those pineapple roughing cutters you will love if you take it on yourself
Bruce
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24th Jul 2013, 10:34 PM #9Senior Member
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Mate has done a few in EN26 - Bohler was the supplier, not that it helps given your location, heat treat and final grind was performed in Wollongong. Interrupted cut for what seemed hours on end, not my idea of fun.
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24th Jul 2013, 11:44 PM #10Most Valued Member
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Knowing little about these things I'm wondering. Are we talking about biulding a 300hp crank for a 40hp motor?
Stuart
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25th Jul 2013, 08:51 AM #11Senior Member
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Thanks for the current replies Gents one and all.The crank on these is marginal at the outset.If you can think of a ww1 fighter plane with a 4cyl in line water cooled engine this is what we got here.It was an aircraft engine fitted later to an automobile.
The engine is 2 litre OHC.The cam is driven from the front of the engine with a small 2 to1 spiral bevel gear set via a vertical shaft.All these cars have an inherent harmonic vibration phase at about 2500 -2800 rpm.Mind you this vehicle comes in a 1.5 tons and develops about 60 hp.It is a tractable vehicle and will run with city traffic quite easily,its only scary point is its 2 wheel brakes.Modern motorists don't realise or understand the limitations of braking deficiencies of old cars.Leaving your safety braking margin is grabbed usually by some ones daughter driving a nippy runabout of Japanese/Korean origin sporting P plates.Not fun in metro traffic especially on a wet day.brakes work fine is lack of friction co efficient road surface to tire.They run a 440x23 beaded edge tire 60psi [thats the pressure] that gives you a braking area of about 10 square inches of tire road contact.The last price per tire is about 700plus dollars each.[Yes my girlfriends wear expensive shoes]
Back to the crank,the harmonics described earliier is a problem with vertical shaft driven cams.MG had it Wolsley and Bentley.The first 2 had a generator supplanted in between on the vertical shaft.This load on the shaft dissipated the back lash.On a Bentley W O [Walter Owen] designed his from the original aircraft engine he did his better and drove the generator[dynamo] directly from the rear of the camshaft protruding through the fire wall [clever] then generator was out of the sh ite and avoided the oil bathing and potential fire hazard the Nuffields products had.
Diattos resolve was to place 4 dummy rockers in the cam box with super heavy duty springs[some of the oldies here will remember the check coil springs on holden car doors]they are about 1/4 dia coil.Loading the cam this way was its design.
Most of the breakages of these cranks happen at No 1 big end journal.Break contained by the connecting rod.
Over the years i have fitted thousands of cranks to engines have made 2[model steam traction engine] modified many but never embarked on a crank of this proportion.Is worth the risk and I feel would only ruin the first attempt at the price of the raw material.
Hoping this diatribe has not sent you to sleep reading and why I threw my hat into the ring of forum members for advice. Cheers John.
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25th Jul 2013, 10:41 AM #12Diamond Member
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If the Diatto engine is a WWI design it presumably it has no counterbalances, if they could be included with the new crank that could well get rid of the harmonics of which you speak. (Hudson were the first auto manufacturers to use counterbalanced cranks in 1916. Even Packard were still using non-balanced cranks 10 years later).
Given the price of the raw material, $1400 for a 250dia x 900 or $2000 for a 300dia x 900, I think I would be having serious discussions with the guys who do this sort of thing for a living before I tackled it myself. Yes they will be making a dollar out of you, but you get an item that is absolutely spot on and all the risk is on them.
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25th Jul 2013, 11:06 AM #13Most Valued Member
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An estimate on the hours would be interesting.
Rob
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25th Jul 2013, 12:27 PM #14Senior Member
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Hello John,
I was going to ask if you had considered welding repair as an option, but the location of the break might rule that out. You may be able to eliminate a considerable amount of material and machining if you adopt the crank design used by Peugeot in the 1950-60's where the counterbalance weights are bolted on, not integral with the shaft. Have you been able to determine the cause of the fracture; is it fatigue due to harmonic stresses, or weevils in the steel? Perhaps the journal radius is too sharp, causing stress concentration? I think your final decisions will depend on the type of service expected from the rebuilt engine. Any chance of a photo of the shaft and fracture?
Chas.
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25th Jul 2013, 12:40 PM #15Intermediate Member
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Your local blacksmith Flame cutter
Ha
Reinventing the wheel here its a flat plane crank get a profile of it blasted out of some 350 grade plate 75 mm should do it
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