Engine balance after new pistons

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trebor75

trebor75

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After talking to some friends I'm a bit confused about what I need to do to get my engine balanced and correct.

I am putting together a 360. My plan is to use the stock crank, rods and damper. The flexplate I will be using is the B&M Flexplates 10236. I have bought Speed Pro Hypereutectic (H116CP) pistons. They all came out the exact same weight upon weighing them. I have been told I'm fine just putting it together like this, and I have been told I need to get it to a machine shop to have the crank balanced.

For the engine, I have the XE268H cam & Edelbrock RPM heads. I'm not putting together a race engine, purely street. But I want to do it right so I need some help to get this straight before doing anything else.
 
You need to weigh one of the original pistons and compare it to the weight of the new ones. If there is much difference you will need to balance it. I think the Hyper pistons tend to be lighter than stock.
 
qkcuda said:
You need to weigh one of the original pistons and compare it to the weight of the new ones. If there is much difference you will need to balance it. I think the Hyper pistons tend to be lighter than stock.
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I see, thanks for clearing that up.
 
Another question. Will the shop need my old pistons to be able to make all calculations?
 
According to what I was able to find on the internet, your stock 360 pistons weigh 584 grams and the speed-pros weigh 567 grams, which is only a difference of 17 grams. I think that is insignificant and you may be able to get away without balancing on a street engine, but others may chime in.

If you do get the assembly balanced then you don't need the old pistons, but you will have to bring then a new piston, rod, bearing and rings so they can calculate the bob weight.
 
trebor75 said:
After talking to some friends I'm a bit confused about what I need to do to get my engine balanced and correct.

I am putting together a 360. My plan is to use the stock crank, rods and damper. The flexplate I will be using is the B&M Flexplates 10236. I have bought Speed Pro Hypereutectic (H116CP) pistons. They all came out the exact same weight upon weighing them. I have been told I'm fine just putting it together like this, and I have been told I need to get it to a machine shop to have the crank balanced.

For the engine, I have the XE268H cam & Edelbrock RPM heads. I'm not putting together a race engine, purely street. But I want to do it right so I need some help to get this straight before doing anything else.
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Do it right the first time and get the entire rotating assembly balanced! You will need all the rotating components for your build to balance it correctly. That includes the flex plate if an automatic or flywheel if a manual. You should also include the damper. 65'
 
stixx said:
Am I safe to say that a good balance job frees up horsepower?
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Yes. The bearings will look better. I've seen some weird stuff from harmonics that you can't always feel. Had a 408 done by another shop that would back off the rocker arm adjusters for no paper ant reason.

I fought it for awhile and then pulled the engine to see what was going on. It was a steel crank with good parts but the balance job was done with Mallory. After weighing all the stuff, hanging bobweights and spinning the crank it was over balanced by a bunch. Like 61% over balance (rather than the 50% it should have been). I know the shop that did it and I told them what was going on. They weren't happy they didn't get a chance to fix it. After talking to him we agreed he made a math calculation error. It can happen. He should have caught that a fairly light bobweights needed heavy metal.

I fixed the balance job and the adjusters backing off stopped. The crazy thing is it didn't shake. You couldn't feel it out of balance. I know some single cylinder stuff I'd over balanced that far but I've never seen that before.
 
The H116CP's are SUPPOSED to be stock weight replacement pistons. So really you can 'probably get by with it' if you want to. But, there is more to just balancing the new and old piston + pin + lock weights. To do it properly, you also take all of the rods into the shop and they weight the large and small end weights on each, and then grind on the heaviest ends make them all equal. Once that is done, then the bobweight can be computed and then any crank weight adjustments can be done. They won't need the old pistons. They will need rings, pin locks, and bearings (at least one set of these) and the flexplate and damper.

Now, if you buy new SCAT rods for example, then they come weight matched end to end and the shop does not have to do the rod work. Since the rod end to end balancing is usually the most time consuming part of the work, you should be able to pay for a substantial part the new rods with the balance work savings.

With the heads you have, you're gonna be able to turn some good RPM's so balance should be done IMHO if there is any question.
 
Just remember crankshaft assemblies can be internally or externally balanced. Your stock assembly is externally balanced. If you have your crank and new pistons internally balanced you will not be able to use your current damper and flex plate as they are weighted.
 
Thank you guys for the help. I feel I have a much better grasp on this now.
 
As said I would weigh a piston/pin/1 ring pack & compare em (soak the old ones in carb cleaner for an accurate comparison). HF has a gram scale for $9.95 that is just as accurate as my buddys' high dollar scale. As said Ma's balancing was not the best, is this a DD or (I'm assuming) it will be jumped on a bit, to decide if to balance or not (I would). with some work you can grasp the procedure to weigh the big/small rod ends (the hard part) & give the grinder a bobweight # along with the crank/dampener/washer/bolt/plate/6 bolts for a cheaper bill. An option would be to get a neutral 318 dampener & neutral balance the assy with your OE flexplate (I have nothing to offer if that is cost or performance effective but others I'm sure do).
 
I see this is an older post but thought I'd chime in. I have built two 360 engines with the same set up as you are using. Same Speed Pro pistons, flex plate and xe268h cam. I weighed both set of original pistons and then the Speed Pro's. All of them were within a couple of grams of each other. When I talked to my machine shop he told me for a street car the factory balance job is fine. The H116CP pistons are OEM weight replacement pistons. I'm sure the factory balance job wasn't meant to be spun up to 7000 rpm's all day but one of my engines has done 6500 many times with no problems :) You will love the way it runs. Just did another one with same setup but with a Lunati Voodoo 10200703 cam. The Lunati has a little better street manners. If you are running an automatic you will need a torque converter with a 2400 stall or it will be a turd off the line with either these cams though.
 
Hellrats said:
I see this is an older post but thought I'd chime in. I have built two 360 engines with the same set up as you are using. Same Speed Pro pistons, flex plate and xe268h cam. I weighed both set of original pistons and then the Speed Pro's. All of them were within a couple of grams of each other. When I talked to my machine shop he told me for a street car the factory balance job is fine. The H116CP pistons are OEM weight replacement pistons. I'm sure the factory balance job wasn't meant to be spun up to 7000 rpm's all day but one of my engines has done 6500 many times with no problems :) You will love the way it runs. Just did another one with same setup but with a Lunati Voodoo 10200703 cam. The Lunati has a little better street manners. If you are running an automatic you will need a torque converter with a 2400 stall or it will be a turd off the line with either these cams though.
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Thank you for that information! Very nice to hear from someone with the exact same set up. I have a 10" 3000 stall Turbo Action converter and I have 3.55 gearing, I also have a set of 3.73 gearing on the shelf.
 
That converter and gear combo will work great. You may want to run and external transmission cooler. Remember the more converter you run the more heat it will make. Enjoy :)
 
I actually already run an external cooler, I had the same converter with my previous enginge. I hope I can stay with the 3.55's, I like that gearing.
 
trebor75 said:
I hope I can stay with the 3.55's, I like that gearing.
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I don't know why you can't. Your stall is higher than mine and I run that cam in my 318 with a stock stall and 3:55's. It would be a bit better with a higher stall, but doesn't run bad as is.
 
Hellrats said:
I see this is an older post but thought I'd chime in. I have built two 360 engines with the same set up as you are using. Same Speed Pro pistons, flex plate and xe268h cam. I weighed both set of original pistons and then the Speed Pro's. All of them were within a couple of grams of each other. When I talked to my machine shop he told me for a street car the factory balance job is fine. The H116CP pistons are OEM weight replacement pistons. I'm sure the factory balance job wasn't meant to be spun up to 7000 rpm's all day but one of my engines has done 6500 many times with no problems :) You will love the way it runs. Just did another one with same setup but with a Lunati Voodoo 10200703 cam. The Lunati has a little better street manners. If you are running an automatic you will need a torque converter with a 2400 stall or it will be a turd off the line with either these cams though.
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I'm back finally, bringing back an old thread, but here it goes. So the speed pro pistons are supposed to be stock weight, is that with or without the wrist pin C Clips? I'm not sure how much they weigh or if I would be better off putting them on adding weight slightly. Another member posted a difference of 17 grams? I know I should weigh them but for kicks I thought I would ask to make future information correct to the next guy.
 
Lots of good advice in this thread, don't matter if it's a little old.
And also, this is my opinion only.
With the exception of the above mentioned balancing math mistake, you can't go wrong having a new assembly balanced.
But,.... if the only New stuff in the assembly is pistons and rings....
and if you can weigh the old piston properly....
and if they are within 20 grams of the new.... (mopars original balance tolerance)
(Weigh everything normally hooked to the small end of the rod. If you are replacing pressed pins with full floaters, yes, weigh the clips too)
...then you probably can get by without.....
but I would balance it anyway.
 
Way old thread I know. Just thought I would mention that there are piston pin options that weigh different to help dial in the overall piston/pin/rings total weight if needed.
 
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