I would use Merc's spec and hone to .0035". I don't see any problems with that. Definately have the setup balanced.

I ran across this thread about balance that I found interesting. I never considered the weight of oil in balancing. I am not sure I agree with the shop on not needing to balance yet but I am getting a better understanding of what he is saying about it being theoretical. I have read a million threads on marine 454/mags and never heard of a failure due to balance which has me thinking about this even more. With all of that said, and correct me if I am wrong, pretty much any mercruiser black engine and gm crate engine probably has a pretty big window on balance and yet they do not seem to have issues. I am not saying that I am ruling out a balance job, I more want to understand this process than anything and figure out what is total overkill on a under 500hp application. I am going to buy a scale to see how close my pistons actually are to the old ones, purely out of curiosity at this point.

https://www.offshoreonly.com/forums/general-q/241310-balancing-pistons-how-much-too-much-3.html

I have never actually checked one after taking it apart but I would bet they are a lot closer than you think. Next on I get in I just might do it for conversation sake. It is time consuming but would be interesting to see.


I dont know if "theoretical" is the right word or not. It is based on known science accumulated over many years. A lot of engines that are going to be turning higher rpm are balanced slightly differently. Harley actually has three different bobweight percentages they recommend depending on the rpm range of the engine and they are significantly different. But they shake no matter what you do :)

If you want the absolutely smoothest running, longest lasting engine then you should have it balanced.

delete

That's not what the mercruiser manual says as my first post had in it. They are forged quicksilver labeled GM pistons that are the exact part number that mercruiser specs out. Am I looking at the wrong info?

The manual I am looking at is mercruiser technicians handbook 1997 gasoline 90-80653597

IDK but doesn't seem right having .0025-.0037 but then a max over twice that at .0075. Look at the spec for the 7.4 next to it. .0018-.003 but yet max .0018. How does that work? I have seen obvious mistakes in these merc manuals before. Need to call whoever makes the pistons for verification.

Daly's is one of the best in the area. I have not used them for long time and never for marine. I used Charley's the last time I had anything done and that was quite a while ago.

If your manual is for 1997 gas engines which are EFI and run a 160* Tstat, the specs between those and older carb engines running a 140* Tstat could have some slight variance. I do not know for sure, but looking at picklejim's post (I can still go back and see it), that may be the case or Merc changed their specs.

I know the pistons in my 1993 454mag were TRW's