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Crankshaft Counterweight Design

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Crankshaft Counterweight Design

Postby SchmidtMotorWorks » Thu Jan 10, 2008 3:27 pm

Here are two images to show different ways of designing counterweights to balance for the same engine and bob weights.

The main differences are:

In design 1, the CWs are positioned more closely to opposite of the nearest rod pin. This way is heavier, more expensive to balance because it usually needs heavy metal but it has lower bearing loads on all journals.

In design 2, the CWs are biased to being opposite of the nearest end rod pin this way can be lighter, rarely requires heavy metal but has higher bearing loads.

These differences are actually understated compared to the crank designs that were popular in the 70's. I think the most extreme of type 2 were Moldex. There was a cast crank made for 392s in the 60s made by Reath? that had a CW design like #1.

Forged cranks are rarely like #1 because it is a very difficult / expensive shape to forge.


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Postby C Stevens » Thu Jan 10, 2008 11:26 pm

Henry Velasco, the crank guy at Reath's back then, had the center counterweights welded to factory 392 cranks.
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Postby SchmidtMotorWorks » Thu Jan 10, 2008 11:47 pm

Henry Velasco, the crank guy at Reath's back then, had the center counterweights welded to factory 392 cranks.


I saw some of those too. The Reath crank I saw was a work of art. It was a casting and the rod pins were were even cored out to conform to the oil holes. It is funny to note how far CW designs went away from that style and now they have returned to it (at least the real hi-end stuff has).

I met a guy that had worked at a bunch of the old crank shops way back from the 50's. He told me that the origin of the end pin biased CW layouts originated from a half-finished weld-on rush job that happened to go in an engine that won some races for Garlits and the design became the hot tip of the day.
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Postby Stef » Fri Jan 11, 2008 5:54 am

Can you give any info on where we can learn more on counterweight design?
Any good papers, books, etc.
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Postby SchmidtMotorWorks » Fri Jan 11, 2008 4:54 pm

Can you give any info on where we can learn more on counterweight design?


I wish there were. The most enlightening thing you can do if you work with cranks is to cut one apart on the mains and rejoin it with springs or belvil washers. Then spin it up with bob weights (even if it is flat). Compare what you see with where the journals wear and you get a lot of info about your engine.
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Postby C Stevens » Fri Jan 11, 2008 11:06 pm

'Er, "back in the day" Chrysler hemi cranks were FORGED, not cast. :roll:
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Postby SchmidtMotorWorks » Fri Jan 11, 2008 11:43 pm

'Er, "back in the day" Chrysler hemi cranks were FORGED, not cast.


Having personally made a few hundred forged cranks and made foundry tooling and castings I know very well what the difference is between a forging and a casting.

This crank had a single fine parting line that went around the entire part on a single plane. Not a forging.

The rod pins were cored out, not a forging.

The CW design can't be forged unless it is a twisted forging but since the parting lines were on a single plane, it wasn't twisted. Not a forging.

I machined the od of the CWs when I balanced it; the chips were powder not strings, casting, not forging.

Other than that I'll take your word for it, it must have been a forging.
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Postby C Stevens » Sat Jan 12, 2008 10:55 pm

You don't have to take my word for it, look at any Chrysler literature from the time. Me thinks you might have run into one of the Mickey Thompson CAST strokers from the 60's.
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Postby SchmidtMotorWorks » Sun Jan 13, 2008 5:44 pm

It said Reath or Reeth on it, that was the only time I have ever seen a crank with that name on it. It could have been a MT casting that Reath worked on.
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