How does it do that?

[ftpiercecracker1]

How do you go from 4.0 liters to 4.5 liters just by adding a 4.2 crank? Never really understood that. Something to do with the bore? stroke? Rod length? Compression?
What is it?

thanks for all the help.

[Antifreeze2]

It's basic engine math. Going from 3.413" stroke to 3.895" is a pretty big jump. It's also why you need new connecting rods or pistons, there isn't enough room in the engine. There's a compression ratio calculator on the website if you want to plug in random numbers and see what it equates to.

[ftpiercecracker1]

new rods? you can reuse 4.2 or 4.0 rods, no need for "new" ones.

[Eaglefreek]

You may need to spend a little time researching the net to get an understanding of the basics. There is a lot of info out there. Here's one site that may help you understand. http://www.fordmuscle.com/archives/2003/09/stroker/

[Sharp Rock]

Your choice of 4.0 or 4.2 rods dictates your choice of "pin height" needed, which then dictates your choice of pistons. 4.0 rods at this time require either the Keith Black forged pistons (IC944 or 945) or custom forged pistons. Where as the 4.2 rods give you a choice of cast pistion or the Hyperutectic <--(I can't spell'em) pistons.

Hope that helps, and I have it right.

Jerry

[SilverXJ]

If you used 4.0L rods and 4.0L pistons with the 4.2L crank, the piston would sit above the block deck at TDC. E.G, you couldn't even bolt the head down. You can use 4.0L rods but you would need either the KB/IC 944 or 945, the Bulltear pistons, or a custom piston. Those are your 3 choices if you would like the longer 4.0L rod. The 4.2L rod is shorter and can be used with a 4.0L piston so the piston sits below the block deck at TDC>

[Cheromaniac]

How do you go from 4.0 liters to 4.5 liters just by adding a 4.2 crank? Never really understood that. Something to do with the bore? stroke? Rod length? Compression?
What is it?

This page explains it all:

http://www.jeep4.0performance.4mg.com/stroker.html

A stock 4.0 has a bore of 3.875" and a stroke of 3.413" giving it a displacement of 3958cc (4.0L). The 4.2 crank has a 3.895" stroke and with the stock bore size, displacement would increase to 4516cc (4.5L). If you then overbore the cylinders +0.030" or +0.060", displacement increases to 4587cc (4.6L) or 4657cc (4.7L) respectively.

The 4.2 crank has a 0.482" longer stroke than that of the 4.0 which makes the crank throw (1/2 the stroke) 0.241" longer. In a stock 4.0 engine the pistons sit ~0.022" below the block deck at TDC. If you then add a 4.2 crank with the same 4.0 piston/rod combination inside the 4.0 block, the pistons would come ~0.220" above the block deck at TDC and that would not be a good thing. To compensate, either the connecting rods or the pistons would need to be at least 0.220" shorter to make a stroker work.
The 4.2 rods are 0.250" shorter than the 4.0 rods so you could use those with stock 4.0 pistons (or off-the-shelf stock replacement pistons), but that would leave the pistons at least 0.030" below the deck at TDC. Not a problem but not ideal either. Some people opt to shave the block deck to bring the pistons closer to the deck at TDC.
If you use the 4.0 rods in a stroker, you need pistons that are at least 0.220" shorter than stock (which have a compression height of 1.601"). Fortunately Keith Black have forged ICON 944 and 945 pistons that fit the bill with a compression height of 1.353". Slightly less than ideal but again, you can shave the block deck to bring the pistons closer to the deck at TDC.