SteelTownMadDog

iTrader: (2)

As the subject says...playing on adding 6" rod in a 350 rebuild. Just wondering if any block clearancing is required at the bottom of the cylinder bores for the longer rods.

SpitotRs305

why do you want a longer rod?

92droptopws6

iTrader: (10)

You should be fine w/6" rod just buy a good quality H-beam rod

sofakingdom

iTrader: (1)

Depends on the rod.

There is nothing about rod length, as such, that determines clearances. All of the part of the rod that's different is up inside the piston. The part near the crank is not affected by the length.

But to answer the question, no, as long as the stroke is only 3.48", then there should be no problems. And, little to no benefit from the extra expense. In fact, it's possible that it could COST you power; in particular, if the long rod + piston weighs more than the short rod + piston.

jonmark1985

Actually rod length plays a big difference in clearance issues. Like you sofakingdom said, not much around the crank, more so around the piston, but clearance around the camshaft is gained .

I have some diagrams to finish up to prove it. Hopefully i don't have too.

Personally if you have already bought and balanced the rotating assembly, then go for it. If not I wouldn't think clearance is such as an issue and i would go for the 5.7" rod, unless your building wild enough of a motor that you need a huge strong rod that just wont clear the cam, kinda doubt that.

SpitotRs305

i would like to see these drawings... they are in a cad or solid works right with actual measurements from real rods and an actual SBC blueprint?

jonmark1985

CAD, with a true deck height, compression distance, and so and so, ill post em in the morning

SpitotRs305

oh an which rods scat? eagle? oem?

but that isnt the point that little bit of extra clearance that MIGHT be gained around the cam isnt necesary as there is already enough to begin with

EvilCartman

iTrader: (1)

Mine had to be clearanced a little bit. Running 6" Eagle H-beams with Wiseco pistons.

SteelTownMadDog

iTrader: (2)

I'm just looking at plain Scat rods, and there's no price difference from the 5.7s to the 6s and that's why I was considering it.

I thought I remembered hearing a while back that the longer rod actually get closer to the bottom of the cylinder bore (shaft section of rod) because it goes in higher into the bore.

Engine I'm building should be in the neighborhood of 400-425 HP, roller cam setup, with trick flow aluminum heads. Nitrous will not be used.

Apeiron

Clearance anywhere depends far more on the shape of the big end of the rod than it does on the length.

jonmark1985

the longer rod changes the angle however which has an effect also, basically the long rod allows a slightly wider rod to be used in its place.

I'm having trouble posting the autocad .dwg and problems post a bitmap, so pm if you want the diagram.

sofakingdom

iTrader: (1)

Not sure what that's supposed to mean....

The 2 things that most affect the need for clearancing are the STROKE, and the ROD DESIGN. If you use stock 350 rods in a stock 350 block with the stock 350 stroke, there is no clearance problem. If you take those same rods and put them in the same block but instead use a crank with a 3.75" stroke (i.e., build a 383), you DEFINITELY WILL have a clearance problem; because now, the volume that the rods sweep through, is ¼" bigger in diameter (1/8" radius), and the rod WILL try to occupy the same volume as block material. Widely considered an adverse outcome.

The big end of the rod, which is the part that goes around the crank and therefore affects clearance, is not affected by the rod length. All that rod length does, is move the location of the wrist pin up and and down in the piston. That is, if you use a different length rod OF IDENTICAL DESIGN but the same stroke, the volume at the big end that the rod occupies will be essentially unchanged. Therefore no clearance issues will occur.

The most famous clearance issue is when people build a "budget" 383; the kind we all used to build back in the 70s and 80s before all these aftermarket cranks became available. We would take a stock 400 crank, turn the mains down to 350 size, and use stock 400 (5.565") rods and 350 pistons; bolt up and go, down the road. The next thing we would try was to use stock 350 rods and custom pistons, or any piston we could find that had a 4.0x0" bore and a 1.435" "compression height", more or less. That combo DEFINITELY required grinding, because a stock 350 (5.7") rod IS NOT the same at the big end as a stock 400 rod. The LENGTH wasn't the problem, the DESIGN was. The 400 rod uses a shorter bolt with a different shape shoulder. The 350 rod, even if you cut it off and re-welded it to the same length as a 400 rod (not that anyone would do that, but if you did) would still not fit, because the whole bolt area is simply too big.

Understand this clearly: the reason the 5.7" rods didn't fit was NOT the extra length; it was THE SHAPE of their big end.

Same deal applies with aftermarket rods. If you look at some mfr's rods that are available in different length, like Eagle or Scat or the like, you'll see that the big end of the rod is EXACTLY THE SAME regardess of length; and therefore, for any given stroke, will require THE EXACT SAME clearance work no matter what length is used.

If you run a 350 stroke (3.48"), then virtually ANY rod, of ANY length, will fit witout clearance issues, or at most, with very minor ones; because the LENGTH isn't what creates clearance problems. The combination of INCREASED STROKE along with BIG END DESIGN is where those issues come from.

Cap screw rods will almost always fit better than bolt-and-nut (stock style) ones; the bolt head is where the biggest clearance problem is.

Apeiron

I don't need the diagram, I know what you're talking about. The rod angle is changed by a whole 0.9 of a degree. Not even worth mentioning.

sofakingdom

iTrader: (1)

"Slightly" would certainly be the concept...

Typically one would look for .050" or so of clearance.

I'd be REAL surprised if the difference between 2 OTHERWISE IDENTICAL rods of different lengths, would amount to 1/10 of that.

"Measure with micrometer, mark with chalk, cut with axe".

In other words, concentrate your efforts and attentions on the BIG differences, and let the insignificant ones disappear into the noise and general uncertainty. If you're going to be cutting with an axe, then a .001" difference in your measurement isn't going to make the length of your firewood different enough to matter.

Same kind of a deal here. The effect of rod length on clearance is one of those insignificant things. Basically, if you've got enough of clearance with a 5.7" rod, you'll have enough with a 6" rod too; even if there's a tiny "theoretical" difference. It's hair-splitting and a waste of time.

jonmark1985

Other people claimed to want to see a diagram, it wasn't aimed at anyone particularly, hope you didn't take it wrong.

Yeah the difference is pretty minimal, like i said, i personally wouldn't worry about it, unless i had all the parts aquired already, also the shorter compression distance on the pistance will pose more of a threat of wreaking havoc, than if a person would just use the 5.7 rods.