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Old 03-25-2015, 01:13 PM   #1
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Default "Weight matching" OEM pistons

Hey guys, I did a bunch of searching before creating this thread but could not find anything definitive, so bear with me.

I have a set of OEM pistons that are going back into this motor, for a "mild" build, aka Vlad style. Anyways, the heaviest piston is some 5-6 grams heavier than the lightest, and I'd like to do some balancing.

All of the rods are almost exactly the same weight (ebay forged), so I can't really match the heaviest rod with lightest piston, etc.

So now I'd like to get these pistons closely matched, but I cannot figure out WHERE I should be taking weight from them. I've read a few different opinions on the matter, and from what i gather there are a few options.

1) take the weight from underneath the top of the piston. IE dril/dremel from underneath and remove material.

2) Remove "casting" marks from the inside of the piston.

3) "shave" the weight from the bottom of the piston skirts with a belt grinder.

4) don't take any material from the pistons themselves, but rather remove material from the inside of the wrist pins.

Whatcha guys think? Want to get rolling on some miata related stuff, and this would be the next step.

Thanks <3
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Old 03-25-2015, 04:25 PM   #2
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Can you match the lightest piston pin with the heaviest piston and get the weights closer?

I'd be comfortable removing casting flash on the inside of the piston...not sure about the other options.
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Old 03-25-2015, 04:51 PM   #3
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Don't know why you need balanced pistons for a mild build, but every time I've seen a piston balanced it was shallow drilled dimples on the underside of the piston.
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Old 03-25-2015, 05:18 PM   #4
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Quote:
Originally Posted by Efini~FC3S View Post
Can you match the lightest piston pin with the heaviest piston and get the weights closer?

I'd be comfortable removing casting flash on the inside of the piston...not sure about the other options.
I'm going to keep the wrist pin with the piston it came from, so that's not an option.

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Originally Posted by guttedmiata View Post
Don't know why you need balanced pistons for a mild build, but every time I've seen a piston balanced it was shallow drilled dimples on the underside of the piston.
Well, i guess.. because why not? If I'm doing a budget DIY build, why not throw some budget DIY balancing in there too?
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Old 03-25-2015, 05:41 PM   #5
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Opinions vary as to where weight ought to be removed from a piston, and I don't believe I've ever heard a single answer which was uniformly supported by a majority of reputable sources.


Some shave the inside of the skirt:






Some mill the underside of the face:






Some grind the underside of the wrist-pin area:






Some remove material from above the wrist pin:





And some simply use a Powercard:

Attached Thumbnails
&quot;Weight matching&quot; OEM pistons-piston.jpg   &quot;Weight matching&quot; OEM pistons-004.jpg   &quot;Weight matching&quot; OEM pistons-ebalance3002.jpg   &quot;Weight matching&quot; OEM pistons-melted_piston.jpg   &quot;Weight matching&quot; OEM pistons-mill.jpg  

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Old 03-25-2015, 06:07 PM   #6
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Chuckle was worth props.
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Old 03-25-2015, 06:07 PM   #7
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, classic JoeP post.

Thanks man, it gave me some ideas. I like milling the underside of the piston the best, i'll see how much weight i can get from there before trying the other methods.
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Old 03-26-2015, 12:58 PM   #8
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Just an update for you guys, I ended up pulling about 5 grams out of my heaviest piston to bring it back in line with the others. I basically drilled very shallow holes on the underside of the piston dome, and came back in with a dremel to smooth it all out.

I'll get some pics, but I'm pretty happy with how it came out. I doubt it harmed the structural integrity of the piston much, if it all, but time will tell I suppose.
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Old 03-26-2015, 01:04 PM   #9
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Could have just glued coins to the top of the lighter pistons.












/s
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Old 03-26-2015, 01:18 PM   #10
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Quote:
Originally Posted by M.Adamovits View Post
Could have just glued coins to the top of the lighter pistons.
Yeah, I guess.. but that would raise compression on those cylinders too.

The trick is to JB weld the coins to the INSIDE of the pistons, so that the CR is not changed in the process.
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Old 03-26-2015, 01:25 PM   #11
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Quote:
Originally Posted by Fireindc View Post
Yeah, I guess.. but that would raise compression on those cylinders too.
So it's a win-win!
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Old 03-26-2015, 01:42 PM   #12
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Quote:
Originally Posted by M.Adamovits View Post
Could have just glued coins to the top of the lighter pistons.
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Old 03-26-2015, 02:04 PM   #13
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Quote:
Originally Posted by Joe Perez View Post


And some simply use a Powercard:



powercard method is clearly superior as it removes the most weight.
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Old 03-28-2015, 02:50 PM   #14
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FYI, if you don't have the crank, flywheel, and pressure plate balanced you did nothing by balancing your pistons.
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Old 03-28-2015, 11:04 PM   #15
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Quote:
Originally Posted by guttedmiata View Post
FYI, if you don't have the crank, flywheel, and pressure plate balanced you did nothing by balancing your pistons.
Do you really think it will do nothing at all? I mean, it's certainly not the most effective, but wouldn't it be more important for the reciprocating rods/pistons to be balanced within spec of each other, than it is for the crankshaft to be balanced?

Oh well. Didn't really go too crazy with it so we'll see what happens.
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Old 03-29-2015, 12:40 AM   #16
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It can't hurt right?
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Old 03-29-2015, 04:33 AM   #17
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Quote:
Originally Posted by guttedmiata View Post
FYI, if you don't have the crank, flywheel, and pressure plate balanced you did nothing by balancing your pistons.
I'm not so sure about that.
I've no experience with balancing, but isn't the crank, the fw, and the clutch all balanced independently?
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Old 03-29-2015, 09:37 AM   #18
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Where is BogusSVO when you need him.
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Old 03-29-2015, 10:19 AM   #19
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Michael is right
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Old 03-29-2015, 10:31 AM   #20
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I have the entire rotating assembly balanced. For $50 the shop can balance the rods, rod bearings, pistons, pins, clips, and rings.

For $200 they send the pressure plate bolts, pressure plate, flywheel bolts, flywheel, crank, woodruff key, timing sprocket, front pulley, main pulley bolt, and the 4 small pulley bolts out to be balanced.

Of the engines I've built that I have recent experience with, 2 have been balanced, and 2 have not. It's a fairly obvious difference. A proper balance is the way to go for sure.
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