Piston / rod / cylinder matching when rebuilding engine

[IDIBronco86]

Im in the process of my first ever rebuild on my motor and i made the mistake of not stamping / keeping an order to the pistons when cleaning them to know which cylinder they came out of, i know which connecting rod goes to which hole and a few of the pistons due to unique wear but i am unsure of a few of the pistons and where they go. I am installing new rings and honing the bores. how bad did i mess up or should i just guess as to where they go?

 

[IDIBronco86]

one other thing to mention the piston skirts on the unknown pistons look almost new, like 0 scuffing or anything. are the rod and piston assemblies even balanced in these motors with how low rpms they turn?

 

[ihc1470]

I would not worry about it.

 

[KansasIDI]

how low rpms they turn?

*laughs in Cummins*

 

[Clb]

To late now, but studty the pin bore fitment for clues.
Mic each cyl bore/piston to see if you find anything telltale.
Look at the crowns for identifying marks corresponding to the heads marks.
Didja keep the rod caps / rods together?

 

[Nero]

Wrist pins wear to the piston/rods, try micing them.

 

[IDIBronco86]

Kept the rods and everything together and know where the rods go, the only thing I don’t know is where a few of the pistons go.

 

[FrozenMerc]

You are now in a situation where you need to measure carefully and be able to calculate proper clearances. Do you have a micrometer set, bore gauge, caliper etc. to accurately measure these parts to figure out what would fit best and where? How much machine work was done, or is this just a gasket and ring job? How confident are you in your bore geometry?

Or.... just throw it back together and let it buck. You will never know anything was wrong until it is.

 

[IDIBronco86]

You are now in a situation where you need to measure carefully and be able to calculate proper clearances. Do you have a micrometer set, bore gauge, caliper etc. to accurately measure these parts to figure out what would fit best and where? How much machine work was done, or is this just a gasket and ring job? How confident are you in your bore geometry?

Or.... just throw it back together and let it buck. You will never know anything was wrong until it is.

No machine work, just gaskets and rings. I do have measuring tools and am confident in the roundness of my bores, which I have honed. Would I just measure the piston to wall clearance (what’s the spec?) and just fit based on that?

 

[Nero]

Since you honed, I wouldn't worry about piston to hole as the issue, I'd focus on piston to rod.

Something I've had to do in rare pinches is if you put a red scotch pad on a drill, use it with a liberal amount of clean engine oil to hone the wristpin bore, then use that same used scotch pad on the pin with tons of oil as well. Do not do it on the aluminum pistons.
I hope you at least left the pins matched to the pistons. Make sure they go in the same direction they came out. Make sure the pin moves freely in the piston and rod after install, no small hangups or anything. You'll probably be okay.

I have had to do this on 3 engines in my entire career building Cummins, two on an old mechanical 8.3 and one on a natural gas 8.9. All ran the dyno full load for break in.

 

[FrozenMerc]

General rule of thumb is 0.001" to 0.002" of RADIAL clearance per inch of bore size. So a 4.110" inch bore 7.3 should have 0.0041" to 0.0082" of RADIAL clearance at the skirt. That means the piston diameter should be .0082" to 0.0164" smaller than the bore at the skirt. More clearance needed for turbocharged and higher duty cycle applications.

For example, I don't have the 7.3 IDI specs in front of me, but I do have 6.4L specs. The Bore spec on a 6.4L is 3.8661" +/- 0.0004" and the pistons are a nominal 3.8425" in diameter at the skirt. That is a diametrical clearance of 0.0236", or a radial clearance of 0.0118". A bit higher than the rule of thumb, but not unusual for a turbocharged application.

Nero has you covered on the pins.

 

[IDIBronco86]

Since you honed, I wouldn't worry about piston to hole as the issue, I'd focus on piston to rod.

Something I've had to do in rare pinches is if you put a red scotch pad on a drill, use it with a liberal amount of clean engine oil to hone the wristpin bore, then use that same used scotch pad on the pin with tons of oil as well. Do not do it on the aluminum pistons.
I hope you at least left the pins matched to the pistons. Make sure they go in the same direction they came out. Make sure the pin moves freely in the piston and rod after install, no small hangups or anything. You'll probably be okay.

I have had to do this on 3 engines in my entire career building Cummins, two on an old mechanical 8.3 and one on a natural gas 8.9. All ran the dyno full load for break in.

I have pin to rod matched still, no direction for the pins though. god i feel dumb for making this mistake.

Should i just put the pins in and make sure they spin freely and call it good or am i screwed?

 

[Nero]

I would suggest still measuring them. Cleaning the pin and rod bore kind of 'resets' them, just make sure when you put them together they are lubed up really well. Should still fully free spin with no burrs or hangups in any position. I think you'll be fine.