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Magnum 360 piston replacement without rebalance

JoePole

JoePole

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Hey All. Has anyone upgraded 360 Magnum pistons without rebalancing the engine?
I am looking at replacing my factory 2001 piston with high compression KB107's. The cylinders have been honed and they are STD bore. I read an engine build article where ICON forged IC741's were used without rebalance but who knows about the long term engine health. They are also double the price and forged which I do not need at my hp level.
I am looking for real life, been there/done that experience. How much can you deviate from stock piston/pin weight without rebalance? +/-10% Ok?
My build is a street only motor under 6000 RPM and around 400hp.
Here are some numbers:

Stock: 474 +136 =610 grams Piston/pin actual weight.
ICON IC741: 530 + 118 = 648 grams from Icon website.
KB107: 524 + 132 = 656 grams From KB website.

https://www.hotrod.com/articles/build-500hp-capable-360-magnum-short-block-cheap/

Thanks
 
I think as long as your replacing all 8 the pistons from the same manufacturer they should be pretty close in weight +/-. 360's are externally balanced so a few grams here or there in a mild build won't be a deal breaker. It wouldn't hurt to just check all of them for reference and peace of mind. Remember to weigh the rings and wrist pin with the piston... but then you already know this...
 
The internal reciprocating mass weight difference will have minimal effect on a engine that is externally balanced and you should be OK.
The external balance however is quite a different story. If it was good before and you put it back the same way you will be fine.
 
Thanks maniac. Not so much worried about the new piston to piston weight but the old piston to new piston difference. I know the factory didn't do a great job keeping the weight the same across all 8. And yes I will consider ring weight as well however it's difficult finding that information.
 
You are looking at a piston weight difference of 46 grams. Per my "Engine Blueprinting" book, 'one ounce (28 grams) can have a dynamic weight of over 700 lbs. You are almost twice that at 46 grams. Spend the $200ish and have the entire assembly balanced. When it shakes the teeth out of your head, your gonna have to go back and do it anyway. Also, make sure you pay attention to the instructions for the KB pistons. They usually require a larger top ring gap.
 
Used the exact same pistons in my 360 build. Did not rebalance. Ran great. No unusual vibration. I say go for it.
Make sure I use the correct flex plate.
 
I'm not a machinist, but I do build my own engines. From my perspective, the rod/pin/ring/piston weight is in part balanced by counterweights on the crankshaft. With heavier pistons, I think you "need" to rebalance the engine.

While you're at it, this might be a good time to internally balance the engine (if possible) since it will make any future parts swaps much easier (ask me how I know this part)
 
I think at the least you should weigh the old pistons, pins, and rings, one by one and see how close they are.
Then weigh the new pistons, pins, and rings, and see what you have. 10% of 600g is two ounces! That's not
at the crank, that's reciprocating mass. IMO, that's too much so weigh everything and ask someone who actually
balances engines. Four cylinders are not that tricky because they're two up and two down. they always oppose
themselves. But v-8's are a different animal. find out what the weights are and ask someone at a machine shop.
Good luck with the build, and let us all know what you find!
 
The weight of the pistons actually balance each other out.
If they didn't every V8 engine every made would fly to pieces.
 
Another point is every engine manufactured by the Japanese or whoever the parts in that engine are made to a plus or minus tolerance. If you think they are precision balanced you have rocks in your head.
A part has to be pretty far out before you would notice it.
If the engine is externally balanced and you fit a wrong part you would notice straight away.
I have assembled many Japanese and american V8 engines were there was no way the owner was going to spend more money balancing it.
Never once was there any sign of a problem. Engines are more forgiving than you think at times.
You will get more vibration from a driveshaft out of balance than the engine in most cases.
When cars really vibrate it is almost never the engine itself.
 
Thanks to all. Question for D Stillie since he may have the same combo.
Is your 360 a Magnum? What heads and head gasket are you running? Thanks.
 
With .020 over speed pro replacement pistons I had to re-balance. Spend the little bit extra and get the rotating assembly balanced. Worth it IMO.
 
Just went downstairs and weighed some piston assemblies from a stock original 383. Rods, bearings, bolts. pins, pistons, and rings. Each piston/rod assembly was between 1920 and 1926 grams. Obviously they each wore differently over the miles, but that is pretty close for a 1970 Mopar 383.
 
Thanks to all for your contribution.

So the score is 4 to 3 in favor of rebalance if you count the build in the article. JK.
I contacted tech support at UEM (ICON,KB mfg) and the actual weight of the
KB107std is 520 grams. Now if I were to use their 118g ICON pin I would be at
638 grams total which is less than the build in the article (648).
Unfortunately UEM calculated that my static CR would be 10.7 and that's not
gonna work with my iron EQ 62cc heads on pump 91 octane.
Kinda odd because they were at 10.4:1 in the magazine build but heads were at 64cc otherwise same as what I am doing.
 
Last edited:
Jegs site says kb107 is 10.9-1 with 62cc heads. A really thick head gasket could lower that a bit, and if your heads are open chamber, it won't mess with your quench, because you already don't have any.
There are other ways to mess with the balance. You could use a lighter, stronger h-beam rod, or very high quality lightweight pins, or both. Either would make a nice addition, but would cost more than simply balancing what you have.
Fwiw, I'm not impressed with hypereutectic pistons in high compression hi performance engines.
 
Thanks IMP. The 10.7:1 stated above was calculated with a .050 head gasket. Not sure what else is out there that is thicker except the high dollar Cometic. I already have the EQ heads and Comp springs so would like to stay with them.
Not seeing many choices in Magnum slugs to get me at around 10:1 but still looking.
Anyone running a similar combo they would like to share?
 
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