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 Post subject: Machining a crankshaft
PostPosted: Sat Nov 19, 2016 3:29 am 
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Joined: Fri Mar 07, 2014 5:38 am
Posts: 417
Location: South Lake Tahoe, CA, USA
Boat Name: Wayward Belle

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 Post subject: Re: Machining a crankshaft
PostPosted: Sat Nov 19, 2016 2:29 pm 
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Joined: Mon Dec 14, 2015 2:12 pm
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Location: Very eastern England
Boat Name: Platypus, Shelduck
Very clever, but I think I'll stick to SG Iron castings and finish grinding.


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 Post subject: Re: Machining a crankshaft
PostPosted: Sat Nov 19, 2016 4:27 pm 
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Joined: Wed Dec 07, 2011 6:41 am
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Location: Lopez Island, Washington State, USA
Boat Name: S.L. Folly
I don't get why this is being done. Maybe it is for a one time job. Techie performance art? No production operation would tolerate the waste of material inherent in starting with a solid bar. Mass produced cranks are forged and then machined.

I am for sure done with machining cast cranks. I think built up ones are the way to go. Crankshafts get almost all of their stiffness from the crank case and in spite of all sorts of low cunning with support schemes I still find it too much like matching a noodle.

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 Post subject: Re: Machining a crankshaft
PostPosted: Sat Nov 19, 2016 5:09 pm 
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Joined: Fri Mar 07, 2014 5:38 am
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Location: South Lake Tahoe, CA, USA
Boat Name: Wayward Belle
Most cranks for high output racing engines (1000hp+) are billet nowadays, especially one-off oddball applications. If you have your piston and rod assembly weights handy they can turn them with the balance in mind with no need for drilling the counterweights or adding expensive mallory metal.

My turbocharged Buick 300" v8 in my '65 Buick Wagon will be getting a billet crank... It is getting stroked from 3.40" to 3.90" and bored out from 3.75" to 3.90"... the only available cranks are either the stock cast cranks from the Buick 350 which have to have the mains turned down and can't handle where I'm going with the power, OR a one-off billet crank for around $2000.

Grandma faded fresh and staying that way...

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Last edited by TahoeSteam on Sun Nov 20, 2016 2:17 am, edited 1 time in total.

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 Post subject: Re: Machining a crankshaft
PostPosted: Sat Nov 19, 2016 11:36 pm 
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Joined: Fri Jan 29, 2016 3:56 am
Posts: 55
Boat Name: victoria
I had one in our old Jaguar.... sm block 412 FI engine. That way it could be easily INTERNALLY balanced, it's now in Finland, I wonder if it's still alive. But who here is old enough to remember Hirth crankshafts, WHAT just me!?, Dammit, they also did the cranks for MotoGuzzi V8 racing bikes, in fact some Brit loonies have built new ones if you REALLY need one!! Bring PILES O Pounds! rp


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 Post subject: Re: Machining a crankshaft
PostPosted: Sun Nov 20, 2016 6:01 am 
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Joined: Wed Dec 07, 2011 6:41 am
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Location: Lopez Island, Washington State, USA
Boat Name: S.L. Folly
As I said, if you want a one off crank then a billet job makes some sense especially when you consider the loads on an I.C. engine at high r.p.m. But for one of our little steamers it is a gold plated bitch to machine one without getting all sorts of chattering and flexing. I have tried both roller and solid steady rests and clamped on all sorts of lead and leather to dampen vibrations. You can grind the mains and throws but not on my lathe!

Now I just buy drill rod the right size for both main and rod throws with a wonderful finish, crank out the webs in a stack on the mill and press it all together. With a .1% interference fit it will not move. If I'm paranoid I might pin the main journal to web connection.

Don't be threatened by the job of truing the crank. With a heavy lead hammer and a dial indicator it goes quickly. I've done many, many motorcycle cranks with no problems.

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