I have been researching and learning about balancing engines and when its needed/not needed. To my understanding, all merc black engine does not come balanced from the factory, they have tolerances on each part +/- but are not done per motor like on blue motors. I am throwing together a couple of 454 mags built up with crane 731 cams, gil exhaust, dual plane air gap intakes, holley 800s.... shooting for close to 500HP. If I swap out pistons to stock replacements but .030 over with the stock rods and cranks, am I ok not balancing? thanks

New piston's I would always re-balance the assembly.

IMO +/- 5g would be fine. If they are more than 5g out I would balance it.

Weigh your old and new pistons and rings. Weigh old ones first. Try and find new ones that are ideally same but slightly heavier. Assuming your using same crankshafts rods you should be ok. You can always remove a little material if new ones are heavier. Run hard long distance balancing is key. I built an engine some years back and had to remove a little weight from pistons and left everything else the same and it's still running today. Scales are pretty reasonable.

I have found parts that say factory balanced were a joke, so yes definatly balance

it is $150 well spent to have it balanced

I just talked to the local shop, they said that its a minimum of 200 to balance. They also said if the pistons are close that it won't make a difference. He also said that 90% of people don't balance their motors on stock rebuilds. for 200 I almost say for peace of mind is worth the extra. I am starting to lean towards stroker kits that I can order balanced from the shop

And they will probably be internally balanced and will require a neutral balancer and flywheel, you see it never ends.

exactly and then who in the right mind would trust the rotating assembly balanced correctly. They spit those stroker kits out like peanuts. So yes all the fun and expense just keeps adding up. I believe some are advertised as externally balanced but most interenally balanced. Regardless I'd have it balanced if buying kit.. Anything I ride hard and put away wet is balanced. In my opinion it is a no brainer and definitely keeps everything in check especially in a marine app that is under a load and more rpm than a normal street engine.


My only other take on this would be as mentioned other post if they were for a couple of engines thrown together for spares and Pistons were same weight after some work go for it. All depends on usage and budget. I don't care for externally balanced but there are a ton of them out there running daily. Again if Pistons weigh out the same...

Shop I use charges 300.00 but is what it is.

If I do go to stroker kits i will absolutely have them balanced being custom assemblies at that point. I guess what I was getting at was that if I am going to bore it out, buy pistons, pay for balancing... I am not that far away money wise from stroker motors. I am not shooting for crazy power on these, I am trying to build something better than stock with a bunch of stuff I have laying around the garage already, it happens to be I have 2 454 mag rotating assemblies with balancers and flywheels, 2 gen VI blocks needing bored, 2 pairs of 088 heads, 2 sets of serpentine accessories.... I currently have ford 460s in the boat that I don't mind but they are better off left stock and I really want engines to play with so these were looking like budget builds but I am not so sure now. I also have a hydraulic steering setup that I really want to get on the boat but cant with my king cobra drives. Maybe I am better off finding a couple standard bore blocks instead.