regalman4925

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

Crude Intentions

picklenjim

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.

f_inscreenname

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.

sailtexas186548

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.

regalman4925

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.....?

bck

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.

BillK

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

Griff

What shop??

regalman4925

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.