Upgrade pistons are exact diameter?

[er82]

Does anyone know if aftermarket pistons are exact diameter? For example advertised 85mm then piston would measure the same and not less. I think oem measures a bit less than 85mm.

 

[Colt4G63 also]

They aren't always 100% exact. It's better to get the cylinders bored/honed to match each piston, number them, and keep them in that order.
However, that being said, I did have my block bored .5mm over BEFORE I had my new 85.5mm oversized CP pistons delivered, and mine is still running strong after nothing but all out beating all summer long since 2007ish. I have several hundred timeslips, and a shelf full of autocross trophies which show that it can work.

 

[bastarddsm]

Generally the clearance is built into the piston. So a piston for a 85mm (3.346") bore that specifies a .003" clearance will measure 3.343" I'm assuming your looking at wiseco's I'd suggest calling them to make sure, there customer support is pretty decent.

 

[er82]

They aren't always 100% exact. It's better to get the cylinders bored/honed to match each piston, number them, and keep them in that order.
However, that being said, I did have my block bored .5mm over BEFORE I had my new 85.5mm oversized CP pistons delivered, and mine is still running strong after nothing but all out beating all summer long since 2007ish. I have several hundred timeslips, and a shelf full of autocross trophies which show that it can work.

Generally the clearance is built into the piston. So a piston for a 85mm (3.346") bore that specifies a .003" clearance will measure 3.343" I'm assuming your looking at wiseco's I'd suggest calling them to make sure, there customer support is pretty decent.

Thanks for the input. The reason I asked, the cylinders are 3.347" (85.02mm) which is almost outside spec for stock piston. If upgrade pistons are 85mm exact then with honing the clearance should be good, I think, instead of boring .020" over.

 

[bastarddsm]

What pistons are you planning on using? Also keep in mind that the cylinder won't be worn perfectly round. It will be egg shaped. That's less than optimal. If it were me I'd be prepared to go .020 over, and have the shop hone one cylinder to clean it up and see what the clearance would be with selected pistons. If it's decently round, not tapered, and the clearance isn't huge, run it. Otherwise go .020" over.

 

[er82]

What pistons are you planning on using? Also keep in mind that the cylinder won't be worn perfectly round. It will be egg shaped. That's less than optimal. If it were me I'd be prepared to .02, and have the shop hone one cylinder to like 3.348", which would put you about perfect clearance for a set of standard bore wisecos, for a performance engine.

I'm not sure yet on the piston. I'm weighing my options with different combinations.

 

[BogusSVO]

Bastard, your math is a tad off.

If the block is honed .02 (3.366) then he will be ready for some .020/.50mm pistons

If you meant honed .002 (3.348) about any STD piston will have about .005 PTW

@er82 Just what are your goals for the engine? the car? What is your budget?

With this info, we can help and suggest a set up

 

[bastarddsm]

Sorry Dale, I just wasn't very clear. I meant be prepared to go .020" if it doesn't clean up honing it .0005"-.001" bigger. .005" is a bit on the loose side for most pistons, but it'll be fine. I think I'd rather have that than a egged/tapered hole. Maybe it's not a big deal though.

 

[er82]

Bastard, your math is a tad off.

If the block is honed .02 (3.366) then he will be ready for some .020/.50mm pistons

If you meant honed .002 (3.348) about any STD piston will have about .005 PTW

@er82 Just what are your goals for the engine? the car? What is your budget?

With this info, we can help and suggest a set up

My goals are 350-400 whp mainly because I want to keep it mostly stock appearing and limited to 190 lph fuel pump. This is street car, but not a daily driver. And budged is small, but will pay more for the right stuff. I don't want get myself carried away and never finish it.

So it's a 6 bolt that I'm rebuilding. From what I've read the rods with arp bolts should be sufficient for my goals. I'm just unsure of piston size, oem or upgrade, or machine rods for 2g piston.

 

[BogusSVO]

For that kind of power, Just run the stock rods and pistons. Not even a real need for the ARP rod bolts in the 6 bolt rods.

Bore .020/.50mm and hang new pistons and go on.

 

[Kendric Steed]

If you are going that deep, just take the time to take the engine out and take it to a machine shop to get the block bored / honed.

I have a 1997 Dodge Avenger ES 2.5 v6 bored 0.020" over and crank shaved 0.010"

 

[er82]

I was able to source new wiseco 85mm pistons for a good price locally. They measure 0.0025" less. So that would be their recommended piston to wall clearance.

 

[BogusSVO]

That is correct.

I see you posted about a 6 bolt block, did you get 6 bolt wiseco pistons?

6 bolts have a 21mm wrist pin
7 bolts have a 22mm wrist pin.

 

[er82]

Yes I have 6 bolt. And
wiseco pistons and eagle rods are 21mm pin. Maybe not the best but the price seemed good for new items.

 

[bastarddsm]

Make sure they have the updated .200 wall piston pin. Other than that your good to go man!

 

[BogusSVO]

Thats a fairly common set up, and a solid foundation for you ro work with.

 

[er82]

Make sure they have the updated .200 wall piston pin. Other than that your good to go man!

Per the catalog it has the s709 pin. It doesn't show an upgrade for it, but that pin is an upgrade for other pistons. So I'm confident it will be good.

 

[bastarddsm]

The older wiseco's came with a .150" pin, which will get all deformed and break at 400+whp.