Ok, since there is a bunch of talk about strokers, I decided to put my write-up on here. YMMV but these are the things I found, how I fixed issues that came up, and how to do all this on a budget with off the shelf parts and expanded on the fantastic research done by Rob Bryce.
I did this to a 1990 XJ 4.0 and stroked it to 4.6.
I hope it helps you with your build.
There really isn't any method to my madness here. I took pictures as I built the engine and as I measured things. I have noted some problems I ran into and given solutions to those problems. If, to solve one of the problems, there is a special part number and/or manufacturer, I have included it in the text.
Parts, parts and more parts. What isn't replaced will be painted, sand blasted, or both. Even some of what is new will be painted. I want it to look good, and be easier to get the mud off.
Ok, I have seen a lot of pages that deal with making a stroker with a manual trans. Mine is an automatic. There sometimes are problems with the pilot hole for the manual having to be modified. I was worried that I would run into a larger problem since, as you can see from the picture, that the AW4 torque converter has a large funky pilot thing sticking out of the back of it. This slides into a large hole in the rear of the 4.0 crank.
I then looked at the rear of the 4.0 crank and the 258 crank. There is a raised area around the pilot hole on the 4.0 (blue arrow). This raised portion is missing on the 258 crank but good news, the pilot hole is the same size in both cranks. In this picture, it looks like the pilot hole in the 4.0 crank is set back further than the 258. It's not, the 258 has a stepped down area (green arrow).
I was then worried that when I bolted the flywheel on, and the converter, that it would bind, or run out of room, but as you can see in this picture, the pilot ends up being flush with the flywheel face. Great news! The crank I used is a 1978 258 crank.
Next we have to worry about the length of the nose of the crank. The 1978 258 crank has the extra counter weights that I want for low RPM crawling (the crank is at least half again as heavy as the 4.0 crank), but in 1978 they had v-belts, not serp belts so the nose had to be longer. The diameter of the nose is the same on the 4.0 as it is on the 258, just the 258 is longer. I measured, and 1 cm must be trimmed off the 258 crank to make it the right length so the pulley will bolt tightly (red arrow). I thought about adding washers that were slightly larger than the diameter of the crank end to kinda shim it up, but my machine shop said no problem about cutting it off. There are plenty of threads.
On to the pistons. I have chosen to go with the newer Hypereutectic pistons with graphite coated skirts (arrow). I chose these for a few reasons. First being that they are strong. They fall between cast and forged as far as strength, but do not have the price tag of forged. The second being that they have the graphite coated skirts that will eliminate the dreaded piston slap than many 4.0 owners have had. I never had this problem until I hit about 250,000 miles, but I didn't want to get it now. These pistons also have a .010 inch higher compression height. This makes it so I have to mill less off the block to get the magical quench height correct.
The Pistons are Sealed Power Part #H802CP
The Rings are Sealed Power Metric Part #E925K
You can see by the arrows, there is allot of crap on top of the piston. There is the arrow to tell you the front of the piston, the compression height is also stamped into the top, there is a ring ridge, and allot of casting bumps. For performance reasons, and to reduce spark knock from sharp edges and hot spots, I chose to remove all of this from the top of the piston. With a Hypereutectic piston, the metal has less pores than cast, almost as tight as forged, thus allowing easy polishing. This removes sharp edges that can promote spark knock and by polishing the top, it reflects heat back into the combustion process to help with a more complete burn. Polishing the piston top also reduces carbon build-up because there is nowhere for it to take hold.
Here is the top after I went over it with a 3-M disk on a die grinder, then sanded it with 600 wet paper & oil. The next step will be to polish it with a polishing wheel and compound. This has long been a trick in racing and has been proven to give good results. It takes very little time, and costs nothing.
The Mysterious Quench Issue
The quench distance is the compressed thickness of the head gasket plus the deck clearance (the distance your piston is down in the bore). If your piston height (not dome height) is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power.
I got the block back from the machine shop, installed the crank and 1 piston. Then I measured the deck height. I came up with .025 inch. Add this to the compressed height of the head gasket (.040) and you get .065 inch. This takes the quench way out of spec. Keep in mind, if I had gone with stock pistons, this would have been .075 inch. I believe this is the major reason for many complaints of spark knock in a stroker. You have to remember that a 4.0 stroked to 4.6 will end up at about 9.5 or 9.6:1 compression. At the high end of pump gas. Proper quench, according to an engineer I spoke with, and a spec sheet he faxed me, should be .040 to .050 inch. So I chose to have .020 inch milled off my block to put quench at .045 inch. This should do the trick along with polishing the pistons.
For the fuel system, I have chosen to go with 22 pound injectors to feed the beast. I snagged a set of stock injectors off a chevy forum for 40 bucks. They are always replacing these things to put in 28's and 30's for their TPI motors. The stock non-HO injectors flow at 19 pounds. This is light for a stroker at WOT.
I had no crank to block, or crank to piston clearence problems. Everything fits like it belongs there.
Oh damn...my first real snag. I have an early 1990 XJ. In Jeep's great wisdom, they randomly chose some parts of some years to install tapered springs, where they are wider at the bottom than at the top. These springs have to have more coils to have the same spring rate as a straight spring. The Crane Cam I have chosen has too much lift for these valve springs and will bind the coils, so reuse is a no-go, but with 300,000 miles on them and a higher lift cam, they should be replaced anyway. It's hard to see in the pic to the left, but the shorter spring is the tapered one out of my 4.0 non-HO
I purchased a set of Crane Lifters to go with my cam. They came with the cam via UPS. they looked great, nice heavy springs to make the valves slam shut fast. One problem, they don't fit on the head right and the retainers from my 4.0 don't fit them. Bummer. A call to Crane confirmed that to use them, I would need to buy their retainers and keepers, AND machine the head to fit the springs. I think not...so, back to the research part. The retainer on the right is out of the 4.0 with tapered springs.
THE SOLUTION: To keep it short, I sent the Crane springs back and started doing some real nasty research. The 4.0 has 8mm valve stems, so the options are very limited. I found 1, count it...1 solution.
Mopar Performance valve springs and retainers that are designed for a Magnum V-8!
The valve spring on the left is the Mopar Performance spring. The right is the stock spring. The MP spring is very stiff, taller, and has a vibration damper inside the coils.
The retainer on the left is the MP Chrome Moly Retainer. The right is the stock retainer. Even though the MP retainer is larger and stronger, it actually weighs LESS than the stock retainer.
The next thing to do is to make sure that there is no problems with valve to piston clearance. With a large lift cam, I may have problems with clearance. Claying the piston is the best way to find out if there is any problems.
Go to the hobby shop and get some modeling clay. Smash a bunch on top of the piston. Fill the dish of the piston up fully. Install the lifters, then put the head gasket and head on. You only have to put about 4 bolts in and all they have to be is snug. DO NOT TORQUE THEM DOWN. Install the rockers for #1 cylinder and roll the engine over 2 times. Remove the head and inspect the impressions left in the clay by the valves. A depth caliper is the best way to measure the clearance. Just stick it down through the clay where the impressions are. Min spec is .120 inch. If there aren't any impressions, like when I did mine, then you have at least .150 inch so you are fine.
With all that done, and all the problems taken care of, it's time to put it all together. Here is a pic of the assembled head after sand blasting and painting and all that good stuff.
BUT WAIT! Yet another damn problem. I am about sick of problems at this point. The stainless header that I bought to replace my cracked in half exhaust manifold was designed to be used on many years, and will fit none of them perfectly. Great. The EGR tube does not even come close to fitting into the hole in the header, so had to use the persuader (the torch) to warm it up to the idea of bending in the right places to fit.
Yes you want to keep your damn EGR....it will reduce combustion temps thus reducing spark knock thus letting you run regular gas. Enough thus's?
This is just a shot of how my manifold was cracked in half. It has been leaking for at least a year and a half, and I kept putting it off because it's a pain in the ass to replace. Much easier when you have the head off to do it, but I wouldn't recommend pulling the head just to replace the manifold, unless you have lots of extra time to play around with.
Ok, all back together and looking perdy. I used Accel 8 MM wires.
I got rid of the damn emmision tube set-up, pulled it all out, re-routed the vac lines I needed for things like EGR and ported vaccum, and installed a push in style Mr. Gasket breather. That's the little chrome thing on top of the valve cover. Really cleans up that mess of plastic tubes. The PCV tube that runs from the rear of the valve cover to the intake manifold is still there. I mainly got rid of the rubber hose that runs from the valve cover to the intake box and fills your filter with oil. I removed the air box also and installed Rusty's air tube. If I had it to do again...I wouldn't do Rusty's tube. It sounds awesome, but sucked water bad and didn't fit correctly out of the box.
Without getting into too much of this, I modified Rusty's air tube so it will fit an XJ. Seems Rusty is a little fast on the building and not to fast on the measuring. I had to bend the holding bracket and drill out the mounting hole because it wasn't big enough to fit over the stud. I had to re-weld the bracket to the tube because it wasn't welded too good either. I think with vibration, off-road bumps and the large K&N that I choose (instead of the little one Rusty provides) it would have broken.
I filled it full of fluids and fired her up. This project was finally done. All in all I would have to say this is a 5 wrench job. By far the most time consuming and difficult project to date on my XJ. I wouldn't do this unless you know motors, or have a helper that you can bribe with beer that knows motors. I am a Master Mechanic and some things even stumped me, and I had to run for help.
I hope this article helps you with your project. Would I do it again...for sure! The difference was night and day. A way flat torque curve and way more power. Nice and smooth from idle all the way up.
April 19th, 2006, 10:54 PM
