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Extremely short rod BBC?

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Extremely short rod BBC?

Postby rskrause » Mon Apr 09, 2012 12:19 pm

Anyone built a short deck BBC with a 4.75" stroke and a 6.135" rod? That rod length would allow a shelf 1.290" CH piston to be used. I suppose another possibility is a custom piston with a 1.040" CH and a 6.385" rod. Yeah, I know the rod ratio is "bad" at 1.29/1, that's why I am asking the question. The 6.385 would be better at 1.34 but that's not a of room for the ring package. Hopefully, someone has tried it and can report on results. I know the theories about rod angularity and side loads, but I'd like to find out about about real world experience with that or a similar combo.

Besides practical experience, if anyone has the ability to calculate or at least guesstimate the magnitude of the difference in side loading I'd be very interested to know that.

As you might be able to guess, I have a short deck Big M and that what everything else is set up for. Call me cheap, but I just paid my taxes and a new block/manifold, etc. is just not possible at this point.
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Re: Extremely short rod BBC?

Postby rskrause » Mon Apr 09, 2012 12:44 pm

Someone brought up the issue of skirt to crank clearance. DOH - wasn't thinking of that. So, playing with some numbers how does a 4.625" arm with a 6.385" rod strike you? The RR is a lot better at 1.38. A custom CH of 1.100" would be needed.
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Re: Extremely short rod BBC?

Postby Alan Roehrich » Mon Apr 09, 2012 12:50 pm

It'll hit the counterweights. It would probably cost $1K in heavy metal/labor to even try to balance it. Consider that you'd have 3/4" more stroke than a 454 crank, which is already externally balanced in OEM form. You'd have to have counterweights 3/8" smaller in diameter than those on a stock 454 crank, approximately. Not sure you could even do it with center counterweights.

A quick example is the Eagle 4.75 stroke crank requires a 6.660" rod to clear the counterweights, even with center counterweights.

Why not run a 4.375" stroke and a 6.385" rod? A 4.5" bore or so will get you up around 555 cubic inches.
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Re: Extremely short rod BBC?

Postby rskrause » Mon Apr 09, 2012 12:56 pm

Sounds non-feasible. I currently have a 4.385x4.560 572 and am looking to stretch it a bit.
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Re: Extremely short rod BBC?

Postby Alan Roehrich » Mon Apr 09, 2012 1:18 pm

You're not going to do it cheap, I don't think, and it probably won't work out well anyway.
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Re: Extremely short rod BBC?

Postby srv601 » Mon Apr 09, 2012 1:59 pm

The crank would need so much mallory it would cost a fortune and then it may fly out because of the material around the pieces is not thick enough to withstand the forces created and then you will have a pile of #4#$ even worse if oil gets on the tires. Not worth it
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Re: Extremely short rod BBC?

Postby rskrause » Mon Apr 09, 2012 3:19 pm

Thanks, I think I will forget the idea and move on.
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Re: Extremely short rod BBC?

Postby srv601 » Mon Apr 09, 2012 3:36 pm

rskrause wrote:Thanks, I think I will forget the idea and move on.

do you mind my asking why you need such a long stroke? what kind of racing are you doing?
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Re: Extremely short rod BBC?

Postby rskrause » Mon Apr 09, 2012 8:34 pm

srv601 wrote:
rskrause wrote:Thanks, I think I will forget the idea and move on.

do you mind my asking why you need such a long stroke? what kind of racing are you doing?


I bracket race but I also like to fool around and see how fast I can go. I don't have the $$$ to race heads up and bracket racing is for just an excuse (for me) to fool around with the car. Since I keep changing things it makes consistency problematic but I like what I am doing even though I don't have a lot of wins.
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Re: Extremely short rod BBC?

Postby Adger Smith » Tue Apr 10, 2012 1:16 am

If you want stroke and cuin on the "cheap" use a tall deck truck block and a set of 496 .060 pistons with the 4.75 stroke...
Take a little off the deck and bore it .060
That makes a poor mans 555
At +.100 over bore it makes 565
Use a .060 piston from a 496: 9.780 Deck 6.385 R/L 4.250 Stroke 0.020Deck CL1.250CompHT
Put it in a
Tall deck truck block with: 10.180 Deck 6.535 R/L 4.750 Stroke 0.020Deck CL1.250CompHT
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Re: Extremely short rod BBC?

Postby VMC » Tue Apr 10, 2012 9:51 am

I built a 4.500"X4.500" 9.8" deck 572 in the mid-`90's and I'd say 4.500" stroke is pretty much max in a short deck BB Chevy. Any more than that and the pistons will be hanging out of the bottom of the bores too far @ BDC. The skirts and oil rings on mine were awfully far down in a Mark IV Bowtie block with 6.385" rods and a 1.145" pin height.
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Re: Extremely short rod BBC?

Postby 18gullwingdragboat » Wed Apr 11, 2012 11:14 pm

That rod angle and ratio would be bad for sure! I broke an Ohio Crankshaft 4.75 steelie last July with a 6.7 rod in a tall deck application. Just ordered a Sonny Bryant billet to replace it and at 2900 before balancing with heavy metal (appx another 450) to my bob-weight --- it ain't cheap! If you do use a 4.75 stroke crank - put the best you can afford in there!
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Re: Extremely short rod BBC?

Postby Warp Speed » Thu Apr 12, 2012 8:25 am

18gullwingdragboat wrote:That rod angle and ratio would be bad for sure! I broke an Ohio Crankshaft 4.75 steelie last July with a 6.7 rod in a tall deck application. Just ordered a Sonny Bryant billet to replace it and at 2900 before balancing with heavy metal (appx another 450) to my bob-weight --- it ain't cheap! If you do use a 4.75 stroke crank - put the best you can afford in there!


Going off your member name.........hope you didn't get wet when it failed! :shock:
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Re: Extremely short rod BBC?

Postby Strange Magic » Thu Apr 12, 2012 12:21 pm

It's a disaster in every which way.
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