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Old 04-10-2012, 02:58 PM   #1
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Default How to R&R pistons and Balance Rods

How to R&R pistons and Balance Rods

The used rods and pistons, the pistons are being replaced due to cracked and broken ring lands.



I start off by marking the front of the rod with a center punch



Next is to find the proper sized press pieces to press out the wrist pins.



Due to the pressures used to press out the wrist pin, care must be used, the piston itself can bust sending pieces of it flying that can and will cause injury.


After the pistons have been pressed off, and the small end inspected for scoring, the rods need to be numbered on both the rod and end cap.



Now the rod and caps can be split and cleaned, I use a glass beader to remove all the grime



Once that is done I spray them down with a good brake cleaner and blow them dry


Chase the rod bolt threads with a nut to remove any dust left from the glass beading.

Now to match the rods back with the caps


Install the rod nuts.


Now the rods are clean and assembled, they can be balanced. Start off by weighing all the small ends

The weight will be matched to the lightest one.

Once the small ends are matched, weigh the overall weight of each rod, and match the overall weight by removing material from the bottom of the rod.

Now that is done, it is time to match the weights of the pistons WITH OUT the wrist pin installed.

Then weight match the wrist pins.

Now that all the weights are with in 1/10 a gram the pistons can be hung back on the connecting rods. The small ends must be heated so the wrist pin can slip through it.

This must be done very quickly so the wrist pin is fully in place before the small end of the rod contracts around the wrist pin holding it in place.
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Old 04-10-2012, 03:51 PM   #2
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Thanks for posting. Lots of information in your posts for noobs like me planning an engine build.

Now please clean your camera lense before the next one...
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Old 04-11-2012, 09:19 AM   #3
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So can you explain for somebody who has never seen an engine apart in person: What moves relative to what with the piston/rod/wrist pin? I guess I always assumed the wrist pin stayed stationary relative to the piston, and the rod kind of rotated around on the pin, but based on your heating description, that doesn't seem right. Also, on some other engine you were working on, the wrist pins were held in with clips, suggesting that they just kind of floated and everything was free to rotate. This seems like the method with the least friction...
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Old 04-12-2012, 02:41 AM   #4
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Skidude... You are on the rigt track.

There are 2 basic types of pistons, Press fit and full floating.

With press fit pistons, the wrist pin is gripped by the small end of the con rod.
Most engines were assembled this way years back. Floating pistons was for hind end racers.

But over the past 15 years or so, the OEs have been going to the full floating style.
The 1.8l has a full floating set up from the factory

With this set up style, the wrist pis is free to rotate in the rod and piston. The wrist pin is held in place some style of wire lock or sprio lock.
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Old 04-12-2012, 04:54 AM   #5
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Somebody buy this man a decent camera and teach him how to use it!
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Old 05-11-2012, 11:34 AM   #6
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When a person keeps the same crank but gets new pistons and rods same as the above procedure is showing, would this be advertised at a shop as "balance rotating assembly" ?

I'm assuming the crank doesnt need to be touched and that once I follow your procedures here I have DIY balanced my rotating assembly myself ??
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Old 05-11-2012, 12:18 PM   #7
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No, the rotating balancing is more than just this.

This is weight matching, or balancing the rod/piston combo.

This is the first step in spin balancing a rotating assembly
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Old 05-14-2012, 01:24 AM   #8
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If I'm paying a shop to balance the rotating assembly are they basically assembling the bottom end of my motor for me as well? Can you explain the process? Also I hear conflicting views on whether to include the clutch / flywheel as part that assembly?
Thanks
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Old 05-14-2012, 11:34 AM   #9
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Balancing the rotating assembly, has nothing to do with building the bottom end/short block.

First all the pistons will be brought to the same weight, commonly within .5g high to low

Then the small end of the rods will be weight matched, then the big end or overall weight will be done.

Next the crank will be put on a spin blancer, and brought into balance. By removing weight from the counter balance weights of the crank, or in some ultra lightweight rod/piston combos weight will be added to the crank.

This would be a "Basic" balance

Now, some go a bit further, I normally do, and add the H-balancer, the flywheel and pressure plate.

Now the flywheel and H-bal is installed on the crank, and spun up.

Any weight removed will come off the flywheel.

Now the Pressure plate will be added, and spun up again.

Now the machinist running the balancer machine, will rotate the pressure plate on the flywheel untill the balance gets close, then index the PP to the flywheel.

once that is done, any weight that needs removed will come off the pressure plate, normally 2g or less.

Now with done, it can all be cleaned again, and short block assembly can start.
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Old 05-14-2012, 01:23 PM   #10
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Thank you for the explanation. I was under the impression that they assembled the block then balanced some how.

How necessary is this step if one is replacing pistons and rods and balancing them as described above? I'll be keeping to the stock rev limit if that matters .
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Old 05-14-2012, 01:35 PM   #11
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I personally recommend on any rebuild to atleast weight match the rods and pistons.

Most 4 cyl cranks are normmaly close from the factory.

The closer you get to a "perfect balance" the better and the smoother the engine will run.

Now most aftermarket, forged, pistons and rods are generally fairly close.

OE replacement cast pistons I have seen as far off as 6g

If you havent read these threads, you will see what I found with new parts right out the box.

https://www.miataturbo.net/engine-performance-56/first-look-supertech-pistons-1-8l-dohc-65235/

https://www.miataturbo.net/engine-performance-56/first-look-manley-h-beam-connecting-rods-65527/
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Old 05-20-2012, 11:16 AM   #12
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If your interested in rebuilding a 1.8 long block to include fitting Flyin Miata rods and pistons pm me with your contact info.
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