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Discussion Starter · #1 ·
I am in limbo about having my rod replaced w a hotrod or a carillo rod. My rod has too much up and down play. I'm gonna say around .008-.010. I'm just wondering if this will throw off my balance? Is the 400ex crank balanced by the counter balance and flywheel or will it need rebalanced w the piston and rod I'm gonna use? Are the aftermarket rods and pistons stock weight? I know the crank balancer timing gear will have to come off ,I suppose, in order to get the new rod/bearing/pin off and on. What if this doesn't go back on aligned up perfect. I planned on letting sloans do it or someone that is recommended. Any advice would be appreciated! :p
 

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Any up and down play on the big end of the rod is too much. It is designed as a zero clearance fit.

The counter balance has nothing to do with crank balance. It's job is to counter the vibration inherent in single cylinder engines.

If you are racing and need specific mods to the rod and crank, then buy the rod of your choice, have it installed by an experienced crank shop.

For my money, I just buy a complete crank assembly. It is usually less than buying a rod kit and having it installed. (on the single cylinder engines - multi cylinder engines are a different story)
 

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Discussion Starter · #3 ·
I set it up with the piston on it and it wouldn't just stop at any spot. It wanted to swing with rod in the lowest point. To me that indicated it wasn't balanced? I can save about 100 on having a new hotrod installed vs a stock new assembly. Is the hotrod better being that it is brass bushed at the wrist pin where the stock one isn't or does it even matter? Are the aftermarket rods heavier/lighter or the same? I know some people just put aftermarket pistons in and don't balance anything. Should they be rebalanced when something like that is done? Or is the crank suppose to be 0 balance? Thanks!
 

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A crankshaft is typically balanced without the piston or rod on it. From 1/2 the length of the rod downward to the crankshaft is considered rotating mass while everything upward from the middle of the rod is considered reciprocating mass. This is the simple part. Here's a you tube video of a crankshaft being balanced. https://www.youtube.com/watch?v=LvgVI2PTKzI

It is a multi-cylinder crankshaft while you will be working with a single cylinder unit.

Piston manufacturers try to make even their "BIG BORE" pistons weigh the same as the stock piston, but a large variation in weight is OK because it is the reciprocating part of the equation which does not have as much effect on balance as the rotating part of the mass.

Here's a good article that explains the physics of crankshaft balance How To Balance An Engine - Engine-Balancing Basics - Car Craft Magazine

The bronze bushing is not important. As a matter of fact, I worked on an old Honda XR that had a seized wrist pin. After extracting the pin, the small end of the rod was damage beyond use. Neither a rod or crankshaft was available. I sent the crank to a machine shop along with a new piston pin. The machinist bored the top of the rod, pressed in a bushing, bored and polished the bushing to fit the pin thus saving the rod and the engine.
 
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