View Full Version : What stock replacement piston / Shop problem?

09-21-2004, 12:33 PM
Ok, here's me trying not to rant.

I took my engine into a machine shop for a hot-tank, mag, cleaning of cylinders, and balancing of rotating assembly(Fly-wheel, BHJ, crank, rods, pistons, fly-weel, and pressure plate).

The problem comes in that one of my pistons was smaller than the rest (.005 i think). So the shop recommended that I get a replacement. (if the replacement came in as a different weight than the others, then he recommended getting a new matching set of pistons).

So far, he's had to return 2 pistons because they were not the right ones?

this is the first time i'm having this kind or work done, so can some of you with experience let me know if it's time to pull out from this particular shop, or maybe tell me what part I need this "GUY" to get? So that I can finally put my car back together?

Oh, time wise, this is going on just over a month now. needless to say, the rest of the rebuild is done, and awaiting the motor).


09-21-2004, 01:19 PM
Sealed Power H675P are stock replacements according to this thread:


09-21-2004, 01:50 PM
thanks, any thoughts on my "undersized" piston?

09-21-2004, 05:26 PM
Not sure what "undersized means". It's not the piston that's small, it's the cylinder bore that's bigger.

Have all pistons been pulled? Have they and the bores all been checked to ensure they are round?

I've not gone through such a process, only read about it. From what I understand, usually if the cylinder bores have worn past a certain point, new, oversized pistons are in order. Then the block is bored oversize to get the bores round again. But you don't bore the block until you have pistons in hand so you can match the size to the piston size.

So if you have one cylinder that's a little large, it may be time to even it all out and replace all the pistons.

Hopefully someone with more rebuilding experience pipes up.

09-21-2004, 06:20 PM
Nope, he was pretty clear that the piston was undersized. I think he's referring to the skirt area of the piston.

No mention was made in regard to cylinder size being an issue. The cylinders actually looked quite good.

I am trying to NOT overbore the motor. But maybe if this new piston ever comes in, I'll have a little more information.

09-21-2004, 11:02 PM
Mike, the skirt of that one particular piston has probably "collapsed", causing the smaller dimension. John

09-21-2004, 11:10 PM
i'm using ross forged pistons for a 351. they run about 500 bucks though. you get an oversize piston from advance auto for 20 bucks a piece. they only list one size for the sc engine.
check it out.120 plus tax isn't to bad for a set of new stock style pistons.
good luck,

09-21-2004, 11:18 PM
Fellow I used to work with was a quality assurance engineer and had a job years ago to examine the best way for a production line to ensure that a block+piston assembly didn't have to be made as a set. Turns out that the factory was open to the air with great big windows at the top of the roof and that the temperature inside the factory would vary from season to season. So as long as the pistons were made and block bored on the same day, no problem, but if a block was bored on a different day, it had to be adjusted due to the difference in the piston size from the temperature.
His paid conclusion was to buy an airconditioner. :D However, due to the cost of running a factory size A/C, they concluded that they would just keep building them the way they were.
Moral of the story: the size can vary based on time of year. This is evidently why you have to have a piston fit and why they recommend you replace all of them with 1 soze over. I suppose you could keep trying individual ones until you got a match, but that takes time and luck. :)

09-21-2004, 11:24 PM
Good Point!

09-22-2004, 12:48 AM
Thanks for the input guys.

Well, I talked with the guy tonight, and he said that the second piston already came in, and that it was 20 grams heavier than the rest of the set. Then he tells me that he had ordered a complete new set, $30 a piece, and that they were sitting on his desk. Wow, wish I was kept in the loop and told that the second piston had already arrived and was weighed.

He says that he got the Sealed Power H657P pistons. So, that's good.

He also mentioned that the one cylinder was oversized as compared to the rest because of the collapsed piston. Is that what happens? Therefore, he recommended to bore it "20" over. (0.020 right?)

My only concern with the over bore is that now I'll have to get another set of rings to match the new bore. (anyone want a stock set of rings?)

Also, what about the new C.I.D. size? Do I need to get an EEC tuner and compensate for the change in cylinder volume and (I'm assuming) change in compression?

1. does a colapsed piston necesitate an overbore?
2. what are the ramifications of an overbore regarding engine performance?

09-22-2004, 12:56 AM
No tuner is needed, and the compression shouldn't change, only the cylinder size.


09-22-2004, 02:28 AM
ok, but the only reason I mentioned the tuner was that (from my research) I've found that the EEC-IV has a setting for engine size.

09-22-2004, 10:28 AM
1. does a colapsed piston necesitate an overbore?
2. what are the ramifications of an overbore regarding engine performance?

1) It all depends. The only way to be sure of anything with the engine is to measure the cross sectional diameter of the cylinders in multiple locations to check for out of round. Any change in the piston can influence the wear pattern in the cylinder. The point is, there should be no guessing. Your machinist should have hard numbers for what the issue is in the clyinders. Personally I guess I'd like to hear what the actual dimensions are that he came up with.

2) Shouldn't be an issue unless you are trying to get max power. The volume change shouldn't be outside the ability of the EEC to compensate.

Here is a good thread regarding rebuilding a motor that might have valuable info for you:

09-22-2004, 12:51 PM
that was an excellent post. Thanks for referring me to it.

Can I voice my desire that we have a FAQ, member's only or not, that walks us through the machine shop experience. It would be nice if we could all chime in regarding our experiences with different machine shops too.

09-22-2004, 06:14 PM
Here's the deal, I want to make an SC engine strong enough to handle lotsa power...I plan on doing an entire rebuild myself. A few questions...

1) Can I do this myself (with proper tools of course)?
2) I'd have the heads, crank, and bearings done by a shop.
3) I do have experience building an engine, just not a V-6..only have V-8 experience.

Not trying to hijack the thread or anything, but I thought this would be a good place to start instead of starting a new thread. Thx in Adv.


09-22-2004, 07:23 PM
it's pretty much a v8 with the back two cylinders cut off. if you can do an 8 you can do a six. same thing. also a word of advise, do it right. don't cut corners.
good luck.

09-22-2004, 08:31 PM
A "guide to rebuilding your 3.8 SC motor" would be a great idea. Probably the biggest issue is the things that are outside of your control. I.E. the stuff the machine shop is doing.

Machine shops, especially performance oriented ones are much more familiar with the v8 motors than the 3.8. So I can imagine that it will really help if the person dropping off the motor can give the machinest very specific direction on what you want to see done.

I have a great book that goes over the Cologne V6's from Ford (2.8, 2.9, 4.0) and it has very specific information on what works and doesn't when improving the block.

For those asking about rebuilding their motor themselves. Basically you can do it if you are a sort that is detail oriented. You have to be willing to take your time and be careful. Measure twice, bolt down once.