I am in the process of rebuilding 2 gen 6 502's,all I am really changing as far as weight is concerned is I am going a little more than .030 over on piston size.For you guys who have done this did you have your roating assemblies re-balanced.I say re-balanced because I'm sure they were balanced by Mercruiser when built new.Thanks in advance

It's just like putting old spark plugs in a new motor, you can but why would you ? Spend the extra little bit and know that it is right. Inside the engine is not the place to cheap out.

Balancing is always a good thing to do when you have the engine down. A good balance job from a reputable machine shop will add life to your engine / investment. Sounds like your aware of that and on the right track.

(rush mode) If you have run this particular engine in the past and didn't experience any problems after your tare down then you can weigh your old pistons and then your new ones. Be sure the old ones are clean and free of any carbon build up. Weigh them with the rings on. Even though your new pistons are larger that doesn't always mean they will weigh more. They may possibly weigh less. You can lose a little weight if need be. If you don't have a good gram scale them take them to a shop to get weighed.

You really don't think Merc balanced that engine do you? I would rebalance and not chance what someone else has done, stock balance bet it's not even close to even 10 grams, stock rods not less than 2 grams within each other large and small ends. That rotating assembly was balanced down the line at GM.

Thanks Mer, we've got to get these engine owners educated here!
First performance boaters need to realize and remember that Mercury Marine has two engine types offered.
One is Mercruiser, where they just take standard GM power train engines and add marinizing.
The other is Mercry Racing, where they handbuild basically blueprinted engines from special parts and then marinize them.
THEY ARE NOT THE SAME!!
All engines produced or marinized by Mercury (both divisions) are balanced to a point. at Mercury Racing probably to within 1-2 grams, at Mercruiser, the GM preassembled engines vary between 2-10 grams depending on engine type and model.
If you are rebuilding or building a marine high performance engine you SHOULD and MUST balance your rotating assembly for ultimate life and reliability.
Also remember the value of balancing is also based on the quality of the balance and the person or shop doing the balancing. A bad balance job is as BAD as no balance at all!!

The thing I notice here on the net more than anything is that performance boaters with engine plans are asking some questions that kind of indicate that they are either dealing with shops or people who are NOT fully capable or qualified to be doing good marine performance engine, build or machine work since the answers to the questions should come from the persons or shops doing the work in the form of-You must , or We must! do this as part of your engine work if the JOB is to BE DONE RIGHT!
If your budget is causing you to ask these questions, THEN GET A BETTER BUDGET! before starting, or of the shop or persons doing the work are asking these questions, then find another qualified shop or person!
I REALIZE THAT THERE ARE REALLY NO DUMB QUESTIONS IN THAT A DUMB QUESTION IS BETTER THAN A DUMB MISTAKE, BUT AN EXPERT SHOULD NOT BE ASKING DUMB QUESTIONS!!!

Best Regards,
Ray @ Raylar

Stock motors are balanced but they are done externally. Spend the extra time and money and have a shop balance it internally.

Raylar,
Very well said. I could not agree more.

If you are using the stock crank and rods, do not internally balance it. You will spend more money in Mallory than the crank and rods are worth. The counterweights on the crank are not heavy enough to offset the weight of the piston and rod combo. That is why the engine is externally balanced. It will do just fine externally balanced........just make sure it gets balanced. I would absolutely NEVER consider boring a block .030 and not rebalancing. If it doesn't need any Mallory (heavy metal), than the balance job should only run $200-250. It is money well spent. Your stock assembly is actually not balanced. They do not balance every one. They simply get the rods and pistons close is weight to each other. All of the stock balancers and flywheels have the same counterbalance. In theory, the engine should be in balance, but there is certainly a margin of error. Hope this helps, Eddie.

Thanks for all your replies gentlemen.I couldn't agree more with what you said, the only other hi-performance motor I was involved in building did get balanced. That motor ran 3 plus years under a set of Edelbrock heads & B&M 250 with no problems,I just recently sold the short block to fund these rebuilds,it looked as good inside as the day we built it, and I am in no way going to do something half @$$ to try and save time or a couple of bucks.Spend it now and stay happy much longer. And for you guys wondering, yes I did have the blocks bored WITH deck plates.I was very suprised to learn Mercruiser would be that far off on their balancing,thats alot of weight at 5500-5800.Glad I didn't bet,I would have lost.Thanks again guys

Just for the record "stock" engines are almost ALWAYS off by far more than 10 grams from the factory. In fact, I don't know that I've ever had an assembly in the shop that was that close from the factory.

I would venture to guess Mercrury Racing's production line balance is somewhere between 5-10 grams... It's still a production shop.

As for the OP's question it depends on how much they vary by... but more than likely the answer is Yes, they need to be balanced.

Brian

I remember when I first heard of the term "blue print and balance" back around 1981. I guess you have to ask yourself...Do I really trust the factory? Basically, all blue printing and balancing means is to take all the slop out a mass produced factory engine or parts and bring it within a certain set of tolerances/specifications. Most of these are done/corrected with accurate, competent maching processes.