Crankshaft for Twin Launch

GaryK · Jun 10, 2013

I recently purchased a Stuart Twin launch on ebay and figured that I would start with the crankshaft.

After examining and measuring it I determined that it was so far out of tolerances that it would be impossible to turn it according to print.

Therefore I will be making one from a solid bar stock.

My question is what are the best materials to make it from?

1018, 4140, 303 SS...

Any thoughts would be appreciated.

Something relatively easy to machine. This engine won't be used except for demonstration purposes.

Gary

canadianhorsepower · Jun 10, 2013

4130 is probably your best bet

gbritnell · Jun 10, 2013

1144 stressproof steel.
gbritnell

idahoan · Jun 10, 2013

One more vote for 1144 stressproof; 1018 and 303 will walk all over the place. I would use plane old hot rolled steel (A36) before the others. I can't say on the 4140.

Dave

rcfreak177 · Jun 10, 2013

4130 and 4140 are my favourite, they do move around a bit during machining though if not stress relieved.
You do end up with a nice shiny and durable part.

1018 is crap to machine (in my opinion)

You say this will be used for demonstration and you also want something relatively easy to machine so I would have to go with the 2 above.
1144 stressproof.

Baz

Edit

Turns out 4130 is stress relieved, My bad.

GaryK · Jun 10, 2013

Thanks for the replies! Looks like 1144 is the way to go.

If I were going to be building this for real work and not just demonstration, what material would you recommend?

Any idea what the original Stuart crank was made of?

Thanks,
Gary

canadianhorsepower · Jun 10, 2013

Hierarchy Of Metals
Since the American Society for Metals allows for some leeway within each grade of metal, the tensile strength ratings listed in this chart and elsewhere in the story are approximate, not exact figures. Nevertheless, they do adequately allow comparison among the strengths of different metal grades. While these represent just a small portion of all the steel alloys established by the ASM, they are the ones most common in automotive applications. Here's a quick rundown:

MATERIAL:TENSILE STRENGTH:RATING:
Cast iron70,000-80,000 psi OE engines
Nodular iron95,000 psi OE engines
Cast steel105,000 psi strongest of the cast cranks
1010/1045/1053100,000-110,000 psi high-carbon factory-grade forging
5140 steel115,000 psi sportsman-grade forging
4130 steel120,000-125,000 psi premium alloy
4340 steel140,000-145,000 psi strongest alloy for cranks and rods

Read more: http://www.popularhotrodding.com/tech/0710phr_crankshaft_tech/viewall.html#ixzz2VpTFzhwV

GWRdriver · Jun 10, 2013

Gary,
So far as strength goes, all of the aforementioned materials are WELL beyond whatever internal stresses this engine will generate (in working mode or otherwise) so any one of the ones mentioned will do on strength alone. But finish, and ease of finish is important (to me), so I would choose what I could get the best finish on with the least trouble, and it was a plus if I had in the metals bin. The original Stuart cranks were forged low carbon steel which turned and finished very easily but also became pretzelized very easily. I now build-up all my cranks and drill rod (silver steel), or a free-machining stainless, are my usual materials for the shafts and scraps of whatever steel is handy for throws. That could be gauge plate or CRS, and is usually CRS.

Here's the beginnings of a D-10 with finished crank - drill rod and CRS.

Crankshaft for Twin Launch

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