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Old 05-20-2007, 01:55 PM
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connecting rod positions?

i started assembling my engine last night and noticed that two of the pistons were backwards (directional KB pistons). Would it be OK to switch LH and RH connecting rod positions as long as they are on the same crank throw and the caps match the rods they came off of or bring it back to the machine shop and have them press them off and switch them that way?

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Old 05-20-2007, 05:44 PM
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I`m kinda confused by your question but will say this. The rods don`t have to be run in the exact same place they were prior, but many number them so the caps don`t get mixed up and so they go back on the same journal. However, if the bearings locks are facing to the outside, it don`t matter what hole the rods are in. So if you can switch sides and have the pistons correct, check the locks, if they`re facing to the outside of the block, then it`s okay. If it`s facing to the inside it won`t work and the pistons will have to be pressed off and corrected, however, it`s risky to have them pressed off and back on, as cracks can develop you can`t see the with naked eye.
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Old 05-20-2007, 08:46 PM
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There is a big chamfer on one side of the big end of the rod. This chamfer must face the outside of the rod journal on the crank pin.
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Old 05-20-2007, 09:35 PM
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to clarify, i am talking about the RH and LH AFT (Rear) pistons. they way it is installed right now the RH piston has the large valve relief in the upper AFT corner, it needs to be in the upper FWD corner. if i put the RH piston and connecting rod assembly in the LH side and vice versa, the valve reliefs will be in the correct positions.
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Old 05-21-2007, 11:59 AM
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Quote:
Originally Posted by deathbypontiac
to clarify, i am talking about the RH and LH AFT (Rear) pistons. they way it is installed right now the RH piston has the large valve relief in the upper AFT corner, it needs to be in the upper FWD corner. if i put the RH piston and connecting rod assembly in the LH side and vice versa, the valve reliefs will be in the correct positions.
Read the blog and am not sure how the total problem goes together. Here's what you need to satisfy:

1. There is a an arrow or a groove on one side of the piston crown, this must point forward or the pin to piston thrust skirt will be backwards this will cause the piston to slap the cylinder wall and will eventually break the piston skirt.

PLUS

2. The big end bore of the connecting rod has a chamfer on one side and not the other. The chamfered side must face the outside of the journal where the journal is radiused into the crank throw's cheek. Backwards installation will cause the rod to jam against the radius with really messy results.

MAYBE

3. This may not be a problem for you, but the rod cap and shank are a matched set. They are usually numbered and when assembled the numbers on the shank and cap need to be on the same side.

Bogie
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Old 05-21-2007, 03:48 PM
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Still not sure I follow what you mean, forgive me. But if your using flat top 2 valve relief pistons they don`t have a arrow, and the reliefs always go to the lifter valley side or top side.
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Old 05-21-2007, 04:48 PM
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pistons

Not sure what engine you are building..i don't think you said it anywhere, unless i missed it...

I assume it's a pontiac???

You are correct if you have left and right pistons that they can only go in the bore with the same valve location...When the pistons are installed the chamfer on the rod bearing needs to be on the correct side. You can't go by the rod it self because i have seen rods with bigger chamfers on the wrong side then the right side. You need to install a bearing shell and no rings, slide the piston into the bore and seat it on the rod throw and look at the radius on the rod throw and the bearing chamfer and see if there are on the correct side. If not get the shop to swap them around.. If they are correct your good to go.

Keith
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Old 05-21-2007, 05:38 PM
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Quote:
Originally Posted by DoubleVision
Still not sure I follow what you mean, forgive me. But if your using flat top 2 valve relief pistons they don`t have a arrow, and the reliefs always go to the lifter valley side or top side.
I don't know which part number you're using so don't know if these are cast or forged pistons.

Usually pistons are directional in that there is a .060 inch pin offset to the thrust side of the piston, there is usually a mark or some other way to indicate which way aligns the piston in proper orientation. The rod must be affixed properly to left side and then right side pistons, the fact that the rods are usually numbered is helpful. If they are not then you (whomever) as the assembling mechanic need to engage your brain. If ofset pin pistons are installed backwards the piston will snap thru the clearance dimension on every power stroke. Eventually the skirt fails from such loads.

Some pistons, usually forged, do not have a pin offset and can be used left to right as long as the valve relief's match the valve positioning, if the piston features such a difference.

K&B 2 relief pistons use the intake and exhaust relief for orientation. They should have an orientation with a large relief for the intake and the smaller for the exhaust. The orientation of these is in opposites for left and right sides of the engine. The orientation must be correct for the valve position above the relief ---AND--- the rod's big end MUST have its chamfer facing to the outside of the crank's bearing journal upon assembly.

The fact that the pistons when installed have the relief on the valley side as depicted in the K&B assembly cartoon, doesn't mean the rod is positioned correctly on the crank's journal. You could switch a left and right side piston and rod to fix the big end orientation ---IF--- it produces the valve relief in the proper intake and exhaust pattern for the valves in that head with the relief is sitting toward the valley side --- AND--- the rod's big end chamfer is facing out toward the cheek(s) of the rod journal.

If you can't get to this configuration, the pistons will have to be separated from the rods. This usually results in damage to the pin or piston or both. If the shop that assembled these screwed up, then it's reasonable for them to belly up to the bar and fix it with new pistons & pins. A situation that I'm sure will make them happy.

Bogie
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