Is it cost effective to cut down pistons?

Moparfeind

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At some point I need to lower my compression ratio as well as re-cam my motor. Was wondering if cutting pistons (as opposed to buying a bee set) was cost effective given shop time and possible questionable results!?!

Whats your opinion? I KNOW you have one
 

451Mopar

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How much needs to be removed, and how thin will the piston top be after?
Might affect engine balance too.
Thicker head gaskets usually is easier, but may need longer pushrods and intake might sit lower unless thicker gasket used there too?
 

Moparfeind

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How much needs to be removed, and how thin will the piston top be after?
Might affect engine balance too.
Thicker head gaskets usually is easier, but may need longer pushrods and intake might sit lower unless thicker gasket used there too?
Not sure would beed to measure and run the calculations but I would assume balance would be included in the machining as its internally balanced 340. Pistons are forged TRWs. Probably going to refresh the bottom end as well.
 

RemCharger

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Are the pistons domed, or flat original replacements.
Which heads.
Balancing is not a normal machining operation. It's extra.
 

Moparfeind

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Are the pistons domed, or flat original replacements.
Which heads.
Balancing is not a normal machining operation. It's extra.
Domed 12.5:1 yup but I like my internals balanced
 

RemCharger

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Should be able to mill the dome off , not sure what the flat deck height is in those.
I agree with getting it balanced.
 

MIKESPOLARA

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Depends on how much needs to be removed. Most piston have a shallow dome, meaning they're not thick straight across under the dome so too much removed will result in too thin of a piston. Plus depending on the shop, most may not have a jig to hold the piston on a mill so cost could get up there which might be better off buying new set of pistons
 

Kern Dog

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OP will have to Un-ignore me to read this.
Dwayne Porter gave me some great advice once. I was having trouble with detonation with my 440/493. He suggested to have my pistons milled .040 on the non quench side. This would have the effect of creating a quench dome style piston without buying new ones. My machinist didn’t think he could do it with precision so I went with thicker head gaskets. The knock is gone but I’m sure I am leaving power on the table. I would like to just go ahead and replace the pistons someday... 440 source does have some quench dished pistons that allow a decent ratio with the benefit of quench.
 

33 IMP

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I am not at all familiar with small block mopars, but...
I have done it myself, had it done, and have a set that someone else did. The "someone else" are 2295 trw's that had the dome machined off, but I think done poorly, the compression height varies more than I would like.
The set I did at work, on a lathe with a fixture I made, were 400 pistons used in a stroker 440 (before there were stroker kits everywhere). They would have been slightly out of the hole, and the compression too high, so I took off .070. I wish I had done it in a mill the way Dwayne suggested to Kern.
Third set was a dished forged set for a short rod 400 chevy. I had the outer ring of the dish removed, they became a really light zero deck flattop for a long rod 406. Other than the custom work on the pistons, a bright chimpanzee could have put it together. (Those forged dished trw's were $100, plus $50 for the piston custom work. I think the lightweight tool steel pins cost more than the pistons did.) It ran very well, but nitrous was never contemplated, material above top ring land was way too thin for a power adder.
Edit: the Mopar 400 forged flattop trw's had an unbelievably thick piston head for balance, I think I remember well over .400 thick. Really heavy, and heavy pins too!
 

Runcharger

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If your piston to wall clearance is good you should be able to mill those old ashtrays. It will require a rebalance though, just as changing to modern, lighter pistons will.
Done it once on a 426 Hemi when all you could buy was the old 12.5 hightops, worked great.
 

33 IMP

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Agree with runcharger. If they are otherwise usable, just too high a compression, you can either invest a bit into making them usable, and lighter, or throw them away (I can't see anyone wanting trw's that heavy and that much compression.)
 

RemCharger

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Back in the day we were checking for thickness in stock 440/400 pistons for valve reliefs.. so punched holes in the and found to be ~.340 or so.
I had a look at my 2293s and they appear to not be flat on the inside piston head.. so yes, they should be measured..
 

What’s up dock

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I think you'll find that the dome is "hollow". Look at this one, last pic of the underside. Save the old pistons, sell them, and put that money towards a set of new, lightweight ones. It will offset the cost. You'll be running a much lighter bobweight as far as reciprocating mass, so definitely rebalance. Depending on the year, make sure you take into consideration that later 340's use weighted balancers, flywheels, or torque convertors. If you have this, you'll need to balance as an assembly.
https://www.ebay.com/itm/324266801037?epid=1240187577&hash=item4b7fceb78d:g:fM8AAOSwyPRe1ZkY
 

RemCharger

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20211215_150450.jpg
20211215_150510.jpg
 

zyzzyx

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Yes, I chucked a set of TRW's in a lathe and indicated the top of the piston to within 0.001
and took 0.070 off to knock the compression ratio down to 10.3:1. Had the engine re-balanced.
Milling is fast and gives a nice finish but you have to have the fixturing to do it. Probably took
me three hours to do all eight. Watch so you don't make the tops too thin!
 

Moparfeind

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I think you'll find that the dome is "hollow". Look at this one, last pic of the underside. Save the old pistons, sell them, and put that money towards a set of new, lightweight ones. It will offset the cost. You'll be running a much lighter bobweight as far as reciprocating mass, so definitely rebalance. Depending on the year, make sure you take into consideration that later 340's use weighted balancers, flywheels, or torque convertors. If you have this, you'll need to balance as an assembly.
https://www.ebay.com/itm/324266801037?epid=1240187577&hash=item4b7fceb78d:g:fM8AAOSwyPRe1ZkY
Hum I thought all 340’s were internally balanced. Mine (72 dated block) had been rebuilt so its not stock anyhow. I am using a zero balancer and flex plate on it as I had with my stock 70 318.
 

33 IMP

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Yep, obviously the first step is to determine if they are hollow dome, or solid dome. If they are hollow dome, sell them as-is. Are they standard or oversize?
I have (or had) three sets of domed 440 pistons. The Arias 12.5's were hollow dome, the trw's solid, and the giant ten pound domed pistons that came out of my max(only a slight exaggeration) were also solid (Grant's? Unknown brand, but ancient).
The domed BBC pistons I've had are all hollow dome.
 

WileERobby

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Before I can give a realistic opinion, I have to know: what is the CR now, and what do you want it to be ? You can do all the things everyone mentioned, but why ? Let's not jump to solutions before we know what the problem is.
 
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