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Piston Swap and Engine Balance

74Sebring

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Location
Topeka, KS
Ok, I'm probably going to sound stupid here but, I'm tired of guessing. I'm hoping that someone here can lead me to my personal come to Mopar moment. I've got a 1974 Satellite Sebring Plus, 360LA BBL, 727 Torqueflite. I've rebuilt both the engine and transmission. I have a lot of vibration in the engine now. I am using the factory crankshaft and rods. I swapped out the stock pistons, the ones with the ashtray tops, for a set of speedpro hypereuctic, flatops w/ 5cc valve relief. I also replaced the harmonic balancer with a badger performance unit. The factory flexplate has been shelved in favor of a b&m flexplate for externally balanced 360LA engine. I reused the torque converter that was in the car when i bought it. All main and rod bearings are new. I swapped out the new badger balancer for the old factory balancer and it seems to have smoothed out a bit. I'm considering swapping the flexplate out with a more standard design to see if that improves things any more. My questions are, 1. does the piston swap make enough weight difference to require a machine shop rebalance? 2. Can I make changes to the rotating assembly via the balancer/flexplate and possibly torque converter to correct the vibration and 3. is there any way to identify if the previous owner swapped the crankshaft to an internally balanced type from the factory externally balanced type? Bonus question for extra credit, am I nuts? Any assistance will be GREATLY appreciated!! I'm dying to dive my car!
 
I'm not an expert on balancing, BUT
Are you sure it's running on all 8?
If not try pulling a plug wire off one at a time.
if you get no change on one or two, look further.
 
It's worth a shot and it doesn't cost any money to try. Thanks!
 
Should have said while its running.
If you have rubber gloves it helps...
But if you are kind of sleepy this will fix it.
 
Not sure but is the original flex plate balanced? I think just the torque converter is. You might have a double external balance thing going on. I think the counter weighted flex plate is for neutral balanced torque converters
 
You asked if the piston swap made enough of a weight difference?
Well, what is the weight difference? You need to actually weigh the old pistons with the rings,
and the new pistons with the rings to know. Also, you changed too many things at once to
make any sense out of what the problem actually is. Balancing the rotating assembly doesn't cost
that much, but you have to take the engine apart to do it. (With all of the parts that will be on the crank)
 
1) With other than stock oem replacement pistons, a balance job is highly recommended.
2) The aftermarket B&M flexplate more than likely has a weight on it, so you are doubling the out of balance with the stock converter
3) The Badger balancer is probably a neutral balanced job. Hard to say without a pic or part number.

Either reuse the old flex plate with the old converter, or a new neutral balance converter with the B&M flexplate. Use the correct balancer for an externaly balanced 360, and finally, have the ENTIRE rotating assy balanced.
 
I agree with 19polara64....the torque converter is weighted for external balancing and if your aftermarket flywheel is set up to externally balance the 360 also you probably doubled up the weight. either put the original flex plate in or see if you can remove the weight from the flexplate for an internally balanced motor as in 318-340
 
OK, we agree that there is a balance issue.
B&M makes two different flexplates, one will have a scallop cut out, the second is a complete ring. The scallop cut is for external balance, 360 only.
bmm-10230_yj_m.jpg
bmm-10236_m.jpg

Stock converters are either neutral balance for the 318 and 340, or weighted for the 360.
The scallop cut B&M must use the neutral balance converter. The complete ring must use a weighted converter on a 360.

In all cases, balancing is a requirement prior to assembly of the engine if any internal components have been changed or modified. Varying any components of the rotating assembly will change the bob-weight causing vibration and harmonics. To keep the engine happy for a long time, it is necessary to correct any imbalance. Failure to address will lead to failure of one or more components of the rotating assembly.
 
It's good to know I'm on the same page with the rest of you. artr8, the flexplate i bought has the scallop. since i don't know how abused the converter has been prior to my buying the car, i was considering replacing it with this B&M converter. https://www.summitracing.com/parts/bmm-10416/overview/ It looks like it would match up with my flexplate. Thanks everyone for sharing your knowledge!
 
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