Pistons ? quick question

[amceaglesx4]

Are all jeep 4.0 pistons the same ? On my build its a 99 block bored 60 over using the 258 crank and stock rods. Does it matter what 4.0 piston I use that's 60 over ? Thanks

[jsawduste]

No they are not all the same.

[amceaglesx4]

you knew this was coming lol. Why ? With my 99 block should I just used oversized pistons for the same year block ?

[Russ Pottenger]

If you're going to use the longer ( more desirable ) 4.0 connecting rod your choices will be a forged piston.

All 4.0 blocks come with the same standard bore size of 3.875 and block height of 9.450ish

[amceaglesx4]

Hey Russ I'm going with the 258 rods on this one.

[amceaglesx4]

Good news today as well I found out the top of the block is straight and in stock tolerance within 3 thousandths if that sounds right.

[Russ Pottenger]

Hey Russ I'm going with the 258 rods on this one.

But if your going to use the 258 rods you're most likely going to want to deck the block

[Cheromaniac]

Good news today as well I found out the top of the block is straight and in stock tolerance within 3 thousandths if that sounds right.

What you're forgetting is that if you use 258 rods with dished 4.0L pistons (OEM or aftermarket replacements), you'll need to have the block deck milled anyway to bring the quench height down.
If you use 4.0L rods, you could specify custom forged pistons that'll achieve the desired quench height without decking the block. This would save you the headache of looking for shorter pushrods and concerns about altered valvetrain geometry.

[amceaglesx4]

How much should I have this guy deck the block then ? I don't want to drop thousands into this project. I want to stick with as much shelf parts as possible. Thanks

[Russ Pottenger]

If you're trying to do this on a budget, have your machinist measure the deck height of the block. You shouldn't "have to" deck the block.
Lastly we'll need to what your chambers are. Guessing you'll be looking at something around 58.

[amceaglesx4]

Well its going to be a stock head. Either the 7120 or the 0630. I think they are bouth 58 cc's. I don't know why th e block people are telling me it needs to be decked. I imagine I'll be in the 9.1 compression with I guess the .043 head gasket. Idk there is SO much online its confusing. Thank you for your help friend.

[Russ Pottenger]

Ask your machiniest what the deck height is now.
You're going to need to know that before you're going to make a piston choice.

[amceaglesx4]

Its stock deck height. He said the top of the block was in factory spec of I believe 3 thousandths and he didn't have to shave anything.

[Cheromaniac]

How much should I have this guy deck the block then ? I don't want to drop thousands into this project. I want to stick with as much shelf parts as possible. Thanks

I'll spell it out for you in simple terms.

1. If you use the 258 rods and off-the-shelf 4.0 pistons, you'll need to deck the block and you'll need shorter pushrods.
Parts required: Set of 258 rods, OEM replacement pistons, shorter pushrods.
Machine shop work: Cylinder bore/hone, press pistons on rods, decking of block

2. If you use the 4.0 rods and custom pistons, you won't need to deck the block and you can use the stock pushrods.
Parts required: Custom forged pistons
Machine shop work: Cylinder bore/hone, press pistons on rods

Option 2 gives you the desired longer 4.0L rods and forged pistons at little to no extra cost than option 1.

[Russ Pottenger]

^^^ This