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Old 08-21-2013, 10:13 AM
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wrong piston in bore?

as I am installing my pistons in my motor it reminded me of my friend who did not mark anything when he rebuilt his 302 years ago. it ran just fine. he is that lucky type. but it brings up a question. if the rods got mixed up, how would you figure out where they go? is it really that important?

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Old 08-21-2013, 12:40 PM
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Quote:
Originally Posted by snupytcb View Post
as I am installing my pistons in my motor it reminded me of my friend who did not mark anything when he rebuilt his 302 years ago. it ran just fine. he is that lucky type. but it brings up a question. if the rods got mixed up, how would you figure out where they go? is it really that important?
Rods are directional in the crank pin, there are two chamfers on the rod a large one that faces into the direction of the crank cheek and a smaller one that on pairs of rods will both face in to each other.

The other thing which does have a relation to the rod direction is pin offset of the piston. This is intended to quiet the piston by keeping the skirt loaded against the cylinder wall all the time so it doesn't snap over making a click with load and directional changes. current trend with hypereutectic pistons and of high silicon forgings is to eliminate this becasue these fit so tightly in the bore that the torque over doesn't make a sound anymore. But if these are offset pin pistons they are directional so that when the rods are properly oriented on the crank pins the piston will have a groove or arrow on the crown which must always face to the front of the block.

So what you need is 4 sets of rods with pistons that have the arrow on the front side of the bore with the big chamfer on the rod also facing forward.

Then another 4 sets of rods with pistons where when the arrow faces the front of the block the large chamfer of the rods big end will face to the back.

Bogie
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Old 08-21-2013, 01:14 PM
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Originally Posted by oldbogie View Post
Rods are directional in the crank pin, there are two chamfers on the rod a large one that faces into the direction of the crank cheek and a smaller one that on pairs of rods will both face in to each other.

The other thing which does have a relation to the rod direction is pin offset of the piston. This is intended to quiet the piston by keeping the skirt loaded against the cylinder wall all the time so it doesn't snap over making a click with load and directional changes. current trend with hypereutectic pistons and of high silicon forgings is to eliminate this becasue these fit so tightly in the bore that the torque over doesn't make a sound anymore. But if these are offset pin pistons they are directional so that when the rods are properly oriented on the crank pins the piston will have a groove or arrow on the crown which must always face to the front of the block.

So what you need is 4 sets of rods with pistons that have the arrow on the front side of the bore with the big chamfer on the rod also facing forward.

Then another 4 sets of rods with pistons where when the arrow faces the front of the block the large chamfer of the rods big end will face to the back.

Bogie
so it does not matter about the cylinder just has to be on the correct side? or am I mis understanding?
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Old 08-21-2013, 01:42 PM
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sorry but this will be a nitemare for you if you do not start over.
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Old 08-21-2013, 01:42 PM
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Originally Posted by snupytcb View Post
so it does not matter about the cylinder just has to be on the correct side? or am I mis understanding?
Yes it does matter.

All pistons with a directional arrow or groove mark on the crown (usually located above a pin bore) must have the arrow facing to the front of the engine.

Half of the pistons (4 on a V8) while having the arrow/groove facing the front of the engine must have the larger of the chamfers on the big end of the connecting rod also facing forward.

The other half of the pistons must still have the arrow/groove facing the front of the block, but the larger of the chamfers on the big end of the rod must be facing the rear of the engine.

The arrow or groove on the piston crown tells you there is a pin offset. That arrow/groove aligns the piston so the pin is offset in the thrust direction. Get this backward and the piston will slap the cylinder wall, it will at the least make noise and at the worst bust the skirt if installed backwards.

For the rods the large chamfer on each rod must face outboard for the crank pin thay are mounted on. This matches the larger relief machined on the ends of the crank pin as it merges into the cheek section that supports and ties the pin to the main journal. If the rod is installed backwards it's small chamfer will become binded by the large chamfer on the outboard sides of the crank pin. Rotation of the assembly will be stiff if at all were this to happen and a great deal of damage will be done to the offending rod(s) and the crankshaft.

So you need to look at these most carefully, machine shops have crossed this up more than once.

Bogie
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Old 08-21-2013, 02:07 PM
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No offense old boggie but I cant even understand some of your engine lingo!

The Rods Chamfered edge go towards the crankshaft counter weights, and the flat surface faces the other rod. all tangs face the outside of the block if these are press fit the machine shop will have set this up correctly but check them anyways if they are press fit pistons just line up all your tangs (for the bearings) to the outside of the block and you'll be good

usually rods are marked upon disassembly for the re-assembly process.
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Old 08-22-2013, 04:08 AM
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Quote:
Originally Posted by snupytcb View Post
as I am installing my pistons in my motor it reminded me of my friend who did not mark anything when he rebuilt his 302 years ago. it ran just fine. he is that lucky type. but it brings up a question. if the rods got mixed up, how would you figure out where they go? is it really that important?
To actually answer your question we need to know WHO took it apart, were the pistons and rods marked for bore location and how much actual refinishing took place.
If you tore the engine down yourself and didn't mark anything you WILL have issues. Not only from what Oldbogie was saying but, without a bore job, new rings and reconditioning of the rods, getting stuff out of place will effect ring sealing and rod bearing longevity. As the parts "wore in" in the engines previous life the pistons, rods and crank make their own wear patterns. These patterns are different to every piece just like finger prints. Mixing and matching makes for poor sealing and bearing clamp during assembly.
Mark
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