340 Rods, speed pro L2316F “stock” weight is more than stock piston

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nick455440

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Rebuilding a 340, getting close to assembly and have a few question on rod/piston/balance weight. The rods are stock rods, have been reworked, new bolts etc.. I have the speed pro 10-1 compression pistons that are supposed to be a close stock replacement weight. Have heard and looked up that stock 340 pistons weigh 719/720 grams, the speed pros weigh 745 = 25 grams over stock. The rods mostly vary from 759 to 767 grams (averaging 763 grams) and one of the rods is almost like a copper plated “have heard domestic auto companies did this to fix flaws to still use a rod” but that one weighs 774 grams. I am not trying to get them spot on back to stock weight just close to avoid vibration. If they were a few grams off I would not be worried about it, even just to get the rods to match would mean cutting 14ish grams off of the copper rod, even more to make up for the 25gram over stock piston. I did some grinding on the copper rod just to see what it would take to just get it even with the lightest stock rod, it is taking grinding/rounding off the tip bulg on the small end of the rod and a little off of the big end. Will this make the small end weak? Is grinding the rods to match the way to go? Have looked at scat and other 340 rods that have the small end rounded off but know that’s how they were designed, have over thought this to death. Any and all advice is greatly appreciated
 
The Speed Pro is the old TRW. Old school tech. They were never meant to be lightweights. I would advise getting the bottom end balanced, from what it sounds like.
 
Stop grinding on your rods until you know what your doing.
The rod is balanced on the top and sperately on the bottom.

Top go up and down.
Bottom go round and round.
 
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Going to try and not touch the pistons, I knew and didn’t plan on the speed pros being light pistons just didn’t expect them to be 25grams heavier than stock, figured they would be close to stock as I have read a lot of people run those with stock rods to avoid balancing. Knew the rods should be within a few grams of each other, but 14 grams total weight difference between the lightest and heaviest seems like a lot especially since the pistons are 25grams heavier than stock
 
The first 2 1/2 minutes explains the rod balance.



They balance the rods to within 1 gram.
You need to have your motor balanced if your going to run it hard.
I have + .060 2316s with stock rods. I had it balanced because I know my foot will be thru the floor board on occasion.
 
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Going to try and not touch the pistons, I knew and didn’t plan on the speed pros being light pistons just didn’t expect them to be 25grams heavier than stock, figured they would be close to stock as I have read a lot of people run those with stock rods to avoid balancing. Knew the rods should be within a few grams of each other, but 14 grams total weight difference between the lightest and heaviest seems like a lot especially since the pistons are 25grams heavier than stock

25 grams. Bout the weight of a sack of weed. Not a whole lot. Yeah, slingin around at 6K it's a little bit, but it can be made up somewhere else. It's not the end of the world for sure. There ARE actually advantages to a heavy piston. I wouldn't worry about it too much. Lots and lots of engines were built and very successfully with those "old boat anchors" lol.
 
been putting the TA 340 engine back together...stock rods and TRW 2316 pistons....dont drop one on your foot...
that combo can take a hit from a nuke and still survive.....lol...

and remember...the 340 is/was a high winding engine...LOL....with those slugs....
 
Forged alum is denser than cast alum. So if the new pistons are fgd, that likely accounts for the extra weight.
 
There are balance pads on the top and bottom of a factory rod. These pads are routinely ground down when balancing a motor, and no, doing so doesn't weaken the rod. Based on my limited experience, factory rods will vary in weight a lot more than aftermarket rods, which typically don't have those balance pads.

Heed the advice above. Don't grind on the rods yourself. Balancing is more than just making all the rods and all the pistons weigh the same, although that is part of it.
 
I Agree....get that reciprocating assembly balanced.!! :) You'll need to take the following to the machine shop. Your 8 pistons, 8 rods, crank dampener, flywheel or flex-plate, one set of rings, one set of main and rod bearings. Let the skilled machinists handle the rest. Locally by me, Holbrook Racing charges $250.00 for balancing the reciprocating assy. Remember some old school machinist's wisdom.., Skilled labor isn't cheap...and Cheap labor isn't Skilled.
HR20
 
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25 grams. Bout the weight of a sack of weed. Not a whole lot. Yeah, slingin around at 6K it's a little bit, but it can be made up somewhere else. It's not the end of the world for sure. There ARE actually advantages to a heavy piston. I wouldn't worry about it too much. Lots and lots of engines were built and very successfully with those "old boat anchors" lol.


A friend wants to know how you know what a sack of weed weighs. He is the curious type!
 
There are balance pads on the top and bottom of a factory rod. These pads are routinely ground down when balancing a motor, and no, doing so doesn't weaken the rod. Based on my limited experience, factory rods will vary in weight a lot more than aftermarket rods, which typically don't have those balance pads.

Heed the advice above. Don't grind on the rods yourself. Balancing is more than just making all the rods and all the pistons weigh the same, although that is part of it.
Yep

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I’ve done the same but mine were 30 grams off. Put it together runs very smooth. Balancing is always best. At the time couldn’t afford it. Motor has been shifted between 6500 and as high as 7200 for 10 yrs. just freshened it up. Probably could have reused the bearings. Mopar are tough.
 
Just went through this. Ordered Speed Pro 10:1 L2316F forged pistons and found weight to be 727g. Eagle rods are almost 100g less than stockers. And you can buy a set of SIR6123CB forged rods for around what it will cost to refurbish yours and replace the bolts with new ARP bolts.

51020148376_a9fca49a49_4k.jpg
 
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As some have mentioned many of those motors have been put back together using your parts and called it good with good success. But in my opinion a good balance job is very important for a high RPM motor for longevity and peace of mind.
 
As some have mentioned many of those motors have been put back together using your parts and called it good with good success. But in my opinion a good balance job is very important for a high RPM motor for longevity and peace of mind.
WRT balancing...you are starting with a completely different set of pistons that weigh more than the stockers. Whether or not you replace the rods as well, either way I would recommend a balance of the recip assembly. You might get away without doing it, but why go through all the BS of assembling a complete motor then find out it has vibration issues down the road? Do it now and you'll soon forget about the couple hundred the balancing job cost you.
 
Additionally, you haven't said said whether this is a forged or cast crank 340 (internal or external balance). As was mentioned, if a cast crank, you need to send your damper and flywheel or flex plate with the recip assembly for balancing.
 
Think I am going to spring for a balance job, if I wanted to go cheap would have went Chevy right lol.. it’s forged crank, summit balancer for internally balanced motors
 
Think I am going to spring for a balance job, if I wanted to go cheap would have went Chevy right lol.. it’s forged crank, summit balancer for internally balanced motors
I HIGHLY recommend pricing out new forged rods before you send out for balancing. If you choose to use new ARP bolts you will have to have the rod and cap bolt bores bored oversize. That and reconditioning the rods is gonna put you a stone's throw from the cost of new rods. The thought of using 50 year old rod bolts was a no-brainer for me...chuck a rod bolt and you've scrapped an engine.
 
I HIGHLY recommend pricing out new forged rods before you send out for balancing. If you choose to use new ARP bolts you will have to have the rod and cap bolt bores bored oversize. That and reconditioning the rods is gonna put you a stone's throw from the cost of new rods. The thought of using 50 year old rod bolts was a no-brainer for me...chuck a rod bolt and you've scrapped an engine.

Been there, done that. Broken rod bolt ruined a pretty much stock but high mileage 440. Have never re-used rod bolts since.
 
One thing you can do before you deliver your parts to be balanced; weigh all 8 pistons and mark them with indelible marker on the crown. Do the same with the rods. Try to average them out to the closest mean weight and number the pistons (with marker) to match the rod numbers.
 
those pistons can be lightened by machining under the top inside . 25 grams isn't much to remove, it doesn't hurt piston. years ago, I would lighten my sealed power/trw this way . made them same weight as my cast pistons, didn't have to rebalance anything. i ran them in my 340, turned engine 7000 RPM , in traps many times. just food for thought.
 
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