Substituting Pistons/rods

[TURBOTIMMER]

I'm in the process of rebuilding a stock 109 shortblock. The original rods were slightly out of round. Question is, can I use pistons and rods from another low mileage 109 without rebalancing the rotating assembly?

 

[TurboBuRick]

Won't know till you do a weight comparison. Ideally you would change the entire rotating assembly.

 

[TURBOTIMMER]

Won't know till you do a weight comparison. Ideally you would change the entire rotating assembly.

That's what I thought. Thanks Rick.

 

[earlbrown]

How far out of round? I hand lap every rod cap I touch if they aren't freshly resized.

 

[TURBOTIMMER]

How far out of round? I hand lap every rod cap I touch if they aren't freshly resized.

I don't have a good answer for that one. All I know is the bearings were missing material where the caps meet the rods. I never measured. Took them to a machine shop. I have the rotating assembly from a rebuilt 109 somewhere here with stock rods that were already done.

 

[earlbrown]

I don't have a good answer for that one. All I know is the bearings were missing material where the caps meet the rods. I never measured. Took them to a machine shop. I have the rotating assembly from a rebuilt 109 somewhere here with stock rods that were already done.

Oh OK. The only way to know is to torque up the rods with no bearings and put a bore gauge in there.

Odds are they're still usable. Rods really aren't a weak point in these engines.

 

[turbofabricator]

Agree with Earl. I have never broken a stock rod or bolt. Bent a few sets over 28 years, though.
With that said, we spin these things less than 6K. Unless you are upgrading crank, rods, or pistons, you will not notice a balance issue unless you screw something up. You can easily have the big end of your rods resized, though.

 

[TURBOTIMMER]

Ok, how critical is it that the pistons go back in the exact hole in which they came out? Just picked the originals back up this morning, and the machinist didn't stamp them.

 

[TURBOTIMMER]

Forgot to mention, all fresh. New rings, bearings, fresh hone, etc.

 

[Ttype6]

Forgot to mention, all fresh. New rings, bearings, fresh hone, etc.

Measure the pistons. They should all be the same.

 

[TurboBuRick]

Forgot to mention, all fresh. New rings, bearings, fresh hone, etc.

There are 3 left bank and 3 right bank. Should be something on the pistons that indicates front of engine. Other than that it should not make a difference what piston goes in what hole IMO

Rick

 

[Ttype6]

There are 3 left bank and 3 right bank. Should be something on the pistons that indicates front of engine.

Rick

This is true if the wrist pin bores are offset/not in the center of the pistons. If they are centered,they can go in any hole in either direction.

 

[earlbrown]

The pistons will tell you which way faces front, and the rods will tell you what side of the journal pair they should be on....


I've been drinking for quite a few hours but I'm pretty sure that's not enough to be able to identify each piston with each hole.



Just to make sure we're all on the same page, you pulled the pistons/rods out of your engine without marking them and now you don't know which is what. Is that correct.

and to your original question, are the big ends resized and round now? If so, you need to spend a LOT of time making sure there's no honing stone grit in the wristpin area. If so, your engine will eat itself with a quickness after you put it together.

 

[TURBOTIMMER]

The pistons will tell you which way faces front, and the rods will tell you what side of the journal pair they should be on....


I've been drinking for quite a few hours but I'm pretty sure that's not enough to be able to identify each piston with each hole.



Just to make sure we're all on the same page, you pulled the pistons/rods out of your engine without marking them and now you don't know which is what. Is that correct.

and to your original question, are the big ends resized and round now? If so, you need to spend a LOT of time making sure there's no honing stone grit in the wristpin area. If so, your engine will eat itself with a quickness after you put it together.

Earl, yes they were resized. I marked each piston with a sharpie before I took em out. I asked the machine shop to stamp the rods before he cleaned em up after the resize. He didn't do that, so it's now a guessing game. He said it would not matter.

 

[TurboBuRick]

This is true if the wrist pin bores are offset/not in the center of the pistons. If they are centered,they can go in any hole in either direction.

Roger that, I should have clairified.

 

[Ttype6]

Roger that, I should have clairified.

We don't know what kind of pistons he is using,but I think he is using the stock ones. I thought it couldn't hurt to add a little bit more info just in case for his benefit.

 

[seedling]

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