Machine shop questions on 454 mag
 

I am building a pair of hopped up 454 mags and have some machine shop questions. I have brand new GM/merc/quicksilver replacement pistons. I dropped my blocks off to be bored out and they asked what I wanted my cylinder wall to piston tolerances to be. I brought a mercruiser manual to them that says that says production is .0025-.0037 and max service is .0075. I told him to put them wherever he thought he thought was appropriate and they told me that I need to tell them and they do not want to make the choice in case of issues later on. This is the most highly recommended shop in the area but I feel like I can not get a straight answer or advice from them. On top of that, I asked if I needed to balance my rotating assembly with the new pistons, they said if i was within 20 grams that I was fine. This was after i asked him about buying a scat stroker assembly that was balanced to within 5-7 grams and they said it was not close enough. Question A, where do I want this piston to wall clearance? Question B, does a 454 mag with stock oem .030 pistons need to be balanced? Question C, do I need to ditch this shop and find someone else? thanks

If the pistons are cast aluminum they need to be set up at .0015"-.0027" clearance with .0035" max. If their forged aluminum they need to be .0045" to .0065" with a max of .0075". If these pistons are 454 mag replacement pistons than they should be Forged Aluminum. I would tell him to set them up at .0055" myself, provided that they are forged. Balance will probably be fine.

I understand what he is talking about with the pistons and balance. I'm sure stock pistons are all over the place weight wise and it's probably just within a stock spec before things get out of shape. IMHO though, balance the motor. You change major pieces even a stock motor it's needs to be done.
As for the bore, my guy wants the pistons and always asks if for a boat or car. Boats are bored to be a little looser is what he tells me.

For a performance engine I set piston to wall clearance at the maximum spec given by the piston manufacturer, if they have a spec for build something similar to my application. I would aim for .003” CLR in your case. In a boat loose pistons are always better than tight, unlimited cooling and heavy engine loads make it easy to stick a piston if one hole is running hot.

since it’s apart you should balance the rotating assembly. However, if you weigh the pistons that come out (undamaged, with rings, pins, retainers) and they are all the same, and you can match the new piston assemblies to that weight, you could get away without balancing. Not a smart choice in my opinion, but you could.

Still have not come up with a solution here. I get different answers from everyone and can't seem to find any other shops near me that I feel comfortable using. I am starting to look out of state for other options, does anyone know any reputable shops around lake of the Ozarks or maybe Kansas City? I agree with everyone on the balancing, that is why I was shocked when he told me that I didn't need it. Especially after telling me that scats 5-7 was not good enough. This build has frustrated me a ton, this went from doing rings, bearings and gaskets on good standard bore blocks to boring with new pistons to stroker builds back to boring these blocks. .003 seems to be my best guess as of right now, I figure that's in the middle of what Merc says is ideal.....?

Is the machine shop building these engines for you or are they just machining the blocks for you to build ? If they are just machining to your specs and not building the engine they might have a valid point.

Ok, here is my take on it.

1. For the guys that say the machine shop should not be asking the customer what clearance he wants I am going to say it depends. This is the decision of the engine builder, period. If the guy does not normally build performance marine engines he might not feel comfortable suggesting a clearance. If you had purchased an aftermarket piston it would be a simple phone call to the piston manufacturer with as much information on the engine and the piston manufacturer could give him a clearance. But . . . since you bought factory pistons you are pretty much going to have to go by the factory specs however. . . . if you are modifying the engine for more power then you might have to go with different specs.

Like I said, this is the engine builders (you) decision, not the machine shop.

Personally I would probably put them at the large end of the spec.

2. As far as balancing goes, like someone else said unless the new piston-pin-ring pack weight is within a couple of grams of the old ones then it needs to be balanced. 20 grams, no way. As far out as most of the stock ones are I would probably say balance it no matter what. I cant figure out why he would be turning away a balance job ??? Is he that busy or was he just trying to do you a favor and save you a few bucks ???

3. As far as ditching him and finding another shop, that is your choice. If you feel that the work will be done properly then stick with him. Just tell him you want it balanced no matter what he thinks.

Thats my take and I own a machine shop :)

What shop??

Griff, its dalys

I have taken the mercruiser manual with all of the specs and the pistons to them, specs are in the first post. They responded with I need to ask around to figure out where I want it, so I am asking around. They did not seem to like my stock gm pistons either, basically told me they were poor quality. It is very hard to find a piston that has the correct dome for a 454 mag so I was pretty excited when I found these pistons, maybe they are junk? The short block is being built to exact stock 454 mag specs. I get it, they don’t want any blame on them if I mess it up but when I bring them a manual and the pistons, I would think that should cover it.

As far as balance, I asked for it all to be balanced and they told me that it was not needed and explained that it's all in theory and that bolting it all together alone could throw off the balance. Told me a story about the balance king from California where the stuff all started…. He said the factory was probably up to 20 grams off and all was fine so if my setup is close it will be fine. I have done a ton of research on balancing engines and believe that it is money well spent. Sure I am trying to keep this build within reason on the budget but that is not where I want to cut costs.



I do plan to assemble my rotating assembly but I was considering letting them do it so I asked how much for them to do it but never got a answer as he went back into the balance talk. My old shop only charged me a little extra for assembly if he was balancing it so it was worth while in those cases, otherwise I have built all of my engines.

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

Can you post a picture of one of the pistons ? Maybe top and bottom ?

Were the factory pistons forged?

I will get pictures of the pistons tonight.

The manual is 1997 and listed both carbed and EFI info. The rotating assemblies are a mid to late 90s when new, both are going into gen VI blocks. If I recall correctly, the date on the piston boxes is late 1996. I am not sure if any of that makes any difference but in my mind the manual was correct for when all of my parts are from. I am going to look around some more online today and see if I can find any other merc specs that may be more current.

thanks for the help so far guys

my diamonds are all .008-.0085. i dont have any piston knock ever and no blowby. these are also probably a better forged material than mercruiser and require more room to grow. not sure if this helps at all just showing that you can open them up pretty good and its ok.

https://cimg6.ibsrv.net/gimg/www.off...d6c8daea40.jpg
https://cimg5.ibsrv.net/gimg/www.off...bb6f24cc12.jpg
https://cimg8.ibsrv.net/gimg/www.off...a11ceedd12.jpg
https://cimg1.ibsrv.net/gimg/www.off...982d8a7d91.jpg

If they are cast then mercruiser is passing off cast as replacement for the 454 mags, 420s and hp450s. That would surprise me

That was actually back in 87-88ish if I recall correctly. I am fairly certain these are forged. The machine shop told me they were trw forged. Maybe we are all mistaken

That is definitely a forged piston. I forgot how bad those old "TRW" forgings looked compared to modern pistons :) I can look them up monday and see what clearance they suggest. I might have a book here at home somewhere I can look in over the weekend.

I am almost afraid to ask what you had to pay to get those things in Mercruiser boxes :(

I got a absolutely amazing deal on the pistons :). Sure they may be ugly but there is probably millions of merc and GM crate engines built with them and worked fine. These are going to be at tops 500hp builds, they should be fine. If I was building anymore than 500hp the whole rotating assembly would be scrapped anyways. I still fight my self on going stroker but the parts I currently have would not be enough for the cubes

Those were the very early (late1980's) "454 magnums" that were mated to Alpha drives and were actually 330's.

Well I did a lot of digging into this and apparently those specs are correct for those pistons, .0025"-.0037" with a max of .0075". On all the hot rod forums there were a lot of guys questioning it as the instructions with their new power forged TRW's said .0015. I could see Merc wanting to open that up a little for marine use. Lot of guys said they set them up at .003" and had no problems. And yes their good up to .0075 max. Funny thing one guy said he honed an old block and put a used set in it and had .017" clearance! Said it ran fine for years! I seen those are NLA from Merc now but one place, MarineEngine.com still had a set for $200! What a steal!

Most wont notice blow by or burning oil with wet exhaust.

I believe that is 200 per piston.

so does .003 sound like a good place to put these? That's what I have been leaning towards

thanks everyone for the effort and research. It is appreciated

Thank you I got my time period wrong :)

Perfect!

interesting comments here,
sorry i dont know what piston to wall clearances were set up at.on my 547s
But, as others mentioned,, thats why u / i go to a good shop with experience in marine and race motors
its their job to know this and advise YOU.
i can stay my rods are at 2.8 thou, mains 3.5 thou