I notice that crankshafts specs in addition to stroke normally specify rod length, why? What difference does it make to the crank - "HOW DO IT KNOW??.

Can you use a different length rod than the what the crank specs specifies? If you do, what are the consequences.

Thanks.

 

I have not assembled a stroker yet but my best guess is that the crank is roughly balance with a particular rod/piston cobo in mind and will likely require a small amount of tweaking, if you use a different length rod you will change the weight of the ro/piston combo and make a whole lot more work for the guy balancing it, so costs will add up fast.

 

Sounds logical to me. Thanks.

 

here's another example. on my forged eagle 3.75 stroke crank with 5.7 rods, the piston skirts act. touch the top edges of the crank counterweights even though the crank is supposed to be ok for use with 5.7 rods. so the tops of the crank will have to be milled down some to clear the piston skirts lower edges. so if you use a crank designed for a 6.0 rod w/a 5.7 rod, expect there to be clearance issues also. if you had a choice, just go with the 6.0 rod. Im now wishing I did. I thought I was goin to save $50 buying the 5.7 rods from a guy that never used them, turns out now I have to pay for the extra machining to turn down the crank in a lathe so the pistons dont hit the bottom edge of the skirt.
chris

 

I have a crank designed for 5.7" rods, but really wanted to go with 6.0 inch rods. I'm probably going to go with 5.7" rods now. Everything I've read about rod length differences seems to say that the advantage of one length over another is very slight, so I don't think that hassle of having to pay for more balancing is worth the advantages of a 6.0" rod over a 5.7" rod.

 

most any quality rods and pistons come with in 2 grams. so balancing is done on the crank. get the 6" rods. however piston pin height is different so make sure your rods match the pistons. good luck.
matt

 

It's got nothing directly to do with balancing, as piston weights and rods vary all over the map. It's the diameter of the counterweights. If they are relatively large (eg for a 6" rod) there may be interference with the piston skirt when a short rod is used. The weights can be cut down if you still want to use a crank designed for a longer rod with a shorter rod, but this will mean more weight has to be added, depending on the piston and rod chosen.

Rich Krause

 

Rich. Am I to understand from this post then that there probably wouldn't be a problem using a longer, (6.00"), rod on a crank designed for a shorter rod, (5.70").

Thanks.

 

That is right.

Rich

 

Thanks man.