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Old 09-08-2009, 09:02 PM   #1
TruckNutzDude
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400 sbc rod length

I'm in the process of building a 2 bolt main 400 sbc with a +.030 bore. It's got a fresh grind on the crank (still has the grind sizes in marker on it) and I'm sending the block to get checked in the next few weeks. The engine was running when I got it, with a stock intake and cam, but smoked, I found a burnt piston skirt but no damage to the cylinder wall.

I do not know how to measure the rods for length. I've heard there are 5.7" and 6.0" rods for the 400's. I took a quick measurement from eyeballed center of the wrist pin to eyeballed center of the crank bearing and found about 5.7. Worst case, if I'm too dumb to figure it out, I'll just get new rods and pistons when I buy my rebuild kit and cam. Are 6.0's better than 5.7's?
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Old 09-08-2009, 09:05 PM
Posted in reply to TruckNutzDude's post "400 sbc rod length"
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4X4HIGH
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SBC 400 rods are 5.565" ALL others are 5.7" length.

The longer the rod the less side loading on the cylinder walls BUT a 6" rod in a 400 will put the wrist pin into the oil ring and for a street driven engine that's to see lots of mileage that isn't recommended. Also, if you wish to run a longer rod you must also change the pistons to accomodate the longer rods otherwise the piston will be out of the top of the deck.
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Old 09-09-2009, 08:10 AM
Posted in reply to 4X4HIGH's post starting "SBC 400 rods are 5.565" ALL others are..."
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I need new pistons anyways, that's the point of my question so I get the right ones. I figured I'd upgrade the rods "while I was in there" though there's nothing wrong with the ones I have.
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Old 09-09-2009, 09:30 AM   #4
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Keep in mind that if you go changing parts such as rods you NEED to rebalance the rotating assembly. If you don't you're just asking for trouble.
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Old 09-09-2009, 10:40 AM
Posted in reply to 4X4HIGH's post starting "Keep in mind that if you go changing..."
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About any piston company will pistons for a 400 with long rods. When I built mine 10 years ago, they were the same price as stock rod 400s. Mine pulled like a freight train off idle to ran out of road.....

While your at it, be money well spent, get a new flexplate and balancer, and like 4x4 said, get it all balanced.
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Old 09-09-2009, 11:27 AM
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Yes! Nail on the head there. A balanced engine is critical for getting good longevity. And you cannot just say that the weights on the balancer and flexplate/flywheel make it "Balanced."
Take everything down to a good machine shop and have every final part to weigh, down to the spiral locks and have it balanced properly.
In my findings, I have never seen a longevity issue from having the pin up in the oil ring groove. As long as the piston is designed well, and the rings and oil support rail is installed properly, I have never seen an issue. All of the trick LS series stroker hits have it that way.
I am not saying that it is not a problem in certain applications, or others engine builders findings, just what I have personally seen.
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Old 09-10-2009, 02:26 AM
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i have built 400's with both 5.7 and 6 inch rods and longer as well. it really depends on what im building the engine for as to what rods i use. if you use quality parts the pin being in the oil ring land wont affect reliability. make sure you balance everything.
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Old 09-10-2009, 04:01 AM
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The crank was balanced when it was ground about a year before I got it, but one of the pistons had burnt up due to running lean. I noticed the rods had been stamped on the caps to mark location, however none of the locations matched their markings. If I understand you guys right, "balancing" is making sure the crank/balancer/flywheel are zeroed so there are no heavy spots (like balancing a tire) and the rods/pistons need to weigh the same amount when assmebled. So my crank is balanced with these components and the only thing changing will be rods/pistons. I will make sure they weigh the same before I install them. I will either re-use the stock rods and get new pistons or get go with 5.7" rods/pistons.

This is going in my '39 chevy pickup 2wd backed by an m-21 4 speed with an undetermined gear ratio (haven't located a decent rear end yet)
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Old 09-10-2009, 11:07 AM
Posted in reply to TruckNutzDude's post starting "The crank was balanced when it was..."
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The pistons weighing the same as the others is important, but that is not it. It is balancing the crank, (balancer and flexplate) to the weight of the new pistons. If the new pistons/rings/locks/wrist pins/rods weigh exactly the same as the ones that came out of it, then the old balance job may be O.K. But that is a fat chance. A balance job is getting the counterweights of the crank balanced to what is hanging off of the other side of the crank. (The piston and assembly.)
So just because something was balanced once, does not help when you put new parts in it. Especially if you are changing rod length.
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Old 09-10-2009, 11:21 PM
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kevin400ex
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what wannabe said is correct.
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