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Old 02-15-2017, 09:46 AM   #1
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"Stroker" motor question

From what I understand, a stroker motor for our old Wranglers consists of a 4.0 block with a 4.2 crank. With the longer stroke of the 4.2 crank the end displacement is 4.6 or 4.7.

The bore of a 4.2 is 3.75"
The bore of a 4.0 is 3.88"

Wouldn't boring a 4.2 block by .13" to 3.88" be the same?
Assuming there is enough material in the 4.2 cylinders to safely bore them to that size. Then (assuming again) that the builder can use 4.0 pistons.

My current motor is a 4.2 with 180K miles. I can pick up a tired 4.2 and I have all the necessary tools and experience to rebuild a motor, but I don't want to reinvent the wheel.
So, basically my question is,
1)- does anyone know if there is enough material in 4.2 cylinders to allow safely boring to 3.88" and
2)- are 4.0 pistons the same as 4.2 pistons except with a larger diameter.
If I had a 4.2 and a 4.0 apart and could compare piston deck height, skirt length and wrist pin size, I might be able to answer #2 question myself.

Being that the reason I bought my current Jeep is that it is carbureted and I want to keep the carb, plus I am currently running a HEI distributor, I want to keep my current fuel pump setup. The 4.0 block has no accommodation for a mechanical fuel pump.

Thanks in advance for any thoughts and/or input.

Good Luck, L.M.

EDIT: That would be a bore of "a hundred and thirty thousandths" over. The largest available pistons for 4.2 are forty thousandths over. Still need an additional 90 thousandth of material in the block for a "safe" overbore.
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Old 02-15-2017, 12:30 PM   #2
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I'm no engine guru, but going over bore by a little more than 1/8th of an inch sounds like it might be way too much to be safe. I don't think you'd have enough sleeve left. If you're really curious, find an engine shop near you and ask a machinist!

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Old 02-15-2017, 03:03 PM   #3
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Quote:
Originally Posted by KyleB1996 View Post
I'm no engine guru, but going over bore by a little more than 1/8th of an inch sounds like it might be way too much to be safe. I don't think you'd have enough sleeve left. If you're really curious, find an engine shop near you and ask a machinist!
What KyleB1996 mentions is why I ask. In reality the machine shop would be cutting .065" off the cylinder wall (.065" X2 = .130) or a little more than 1/16". Sure seems like a lot to me. Even if the engine would run there is the possibility of hot spots in the cylinder wall that would ultimately cause the engine to fail. Without a bare block to measure the cylinder walls the next thing to do is ask here if anyone has done such a thing.

I certainly will go talk to a machinist at an engine building shop.
The most successful outcome is based on good planning and I'm just starting to think about this project.

I found out that the wrist pin sizes are .03" different.
The 4.2 pin size is .9313 and
the 4.0 pin size is .9310
Shouldn't be a big deal to ream the 4.0 piston wrist pin hole by .03".
I still need to compare a 4.0 piston to a 4.2 piston.

The really BIG deal is such a dramatic overbore. Of course, I could just go with the largest standard overbore that's recommended for a 4.2 and be safe.

We'll see. I'll keep the forum posted.

Thanks, and Good Luck, L.M.
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Old 02-15-2017, 04:53 PM   #4
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LM,
I too am curious about possibly building a 4.7 in the near future out of my 4.0. Keep me updated on your findings!
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Old 02-15-2017, 09:14 PM   #5
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LM going by the numbers you said the difference is not .03", It is .0003 or three tenths of one thousands of one inch. When I had my 4.2 Rods reconditioned with ARP Studs and the 4.0 pistons installed on them there was no reaming needed as this is a press fit [heat shrink] in the rods.
I don't think anyone will overbore a car engine that much, for some reason most engines can only be bored 0.060" total and some small newer ones only 1.0mm.
when doing this 4.2 rotating parts in a 4.0 block the block needs to be cut down about .050" if using stock over bore 4.0 pistons, to get the deck height down or you can use custom pistons like some Keith Black makes. I know of some one on here that did not deck the top of the block on his engine and said all was well, { But, I know of No performance engine builder that would the pistons .110" to .120" or so, down in the holes when put back together. You may not need to deck a 4.2 block if it can be used. Maybe You can have a 4.2 block resleeved to the correct size, but that cost a lot of $$$.
$.0 blocks are every where Cheap! I paid $400.00 for a complete 1997 XJ 4.0, brackets, headers, intake, every thing, it had 87K on the clock.
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Old 02-15-2017, 10:07 PM   #6
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For my brother we found a 4.0l long block from the pick and pull for $85. He took my old 4.2l crank and is using the 4.0l connecting rods. The 4.0l rods are much stronger the the 4.2l rods so many stopped using them. Here's a link to a site that has a bunch of options of easy to build proven stroker recipes: http://jeep4.0performance.4mg.com/stroker.html We're dong this recipe of the stroker because he wants low rpm torque in the v8 range
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Old 02-16-2017, 01:19 PM   #7
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The Keith Black pistons designed for the 4.2 crank with 4.0 rods are KB944's. They're custom forged low top pistons specifically for use with the 258 crank and the longer 4.0 pistons.

The advantage of using the low top KB's with 4.0 rods rather than a dished 4.0 piston with the 258 rods is that the 258 rods/4.0 piston combo allows the piston skirt to exit the bore farther and cause piston slap. I don't know of it causing anyone to blow the engine, but when the stronger combo of 4.0 rod/custom piston is available...why risk it?
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Old 02-16-2017, 02:09 PM   #8
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Originally Posted by c5wagner View Post
For my brother we found a 4.0l long block from the pick and pull for $85. He took my old 4.2l crank and is using the 4.0l connecting rods. The 4.0l rods are much stronger the the 4.2l rods so many stopped using them. Here's a link to a site that has a bunch of options of easy to build proven stroker recipes: Stroker We're dong this recipe of the stroker because he wants low rpm torque in the v8 range
Those are really good numbers for an inline 6. More power than my stock 5.0 V8...
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Old 02-17-2017, 05:32 AM   #9
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Love my 1st stroker so much I'm building another, and selling my current one to my buddy. I figured out a few years back that a properly built stroker makes way more sense than a sbc swap big time.
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Old 02-17-2017, 06:53 AM   #10
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Originally Posted by ov yj View Post
a properly built stroker makes way more sense than a sbc swap big time.
agreed
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Old 02-17-2017, 08:54 AM   #11
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Love my 1st stroker so much I'm building another, and selling my current one to my buddy. I figured out a few years back that a properly built stroker makes way more sense than a sbc swap big time.
x2
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Old 02-17-2017, 12:03 PM   #12
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Love my 1st stroker so much I'm building another, and selling my current one to my buddy. I figured out a few years back that a properly built stroker makes way more sense than a sbc swap big time.
It's probably cheaper but I'm swapping to a v8 mostly for transmission reasons and easier to find/cheaper parts that go with the v8
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Old 02-17-2017, 12:32 PM   #13
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It's probably cheaper but I'm swapping to a v8 mostly for transmissions reasons and easier to find/cheaper parts that go with the v8
That thought is tempting to me too for the transmission aspect. I'm looking at a probable AW4 swap in place of my AX15 somewhere down the road.
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Old 02-17-2017, 03:00 PM   #14
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Originally Posted by chris yoos View Post

That thought is tempting to me too for the transmission aspect. I'm looking at a probable AW4 swap in place of my AX15 somewhere down the road.
I love manual transmissions, but living in traffic filled southern California and taking very difficult trails, I'd rather have an automatic. An aw4 is fine and if my 4.2l didn't completely give up, I would've swapped all the xj fuel injection and aw4 in. I have an aw4 in my xj and it does okay, if you go that route, I suggest you get a very good transmission cooler as rhe aw4 makes a ton of heat.
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Old 02-17-2017, 04:57 PM   #15
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Originally Posted by c5wagner View Post

It's probably cheaper but I'm swapping to a v8 mostly for transmission reasons and easier to find/cheaper parts that go with the v8
Get the trans thing, but stock 4.0 parts are plenty cheap, you don't need a cheat sheet to go parts shopping because you got a ton of mis-matched, different year parts. Only parts in my stroker not stock to my 91 are the 258 crank, pistons, and I did add 4 hole injectors. Open my hood and you wouldn't know there was anything going on. It's not till you feel the low end torque that you would even know there was a stroker under the hood.
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Old 02-17-2017, 06:25 PM   #16
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Get the trans thing, but stock 4.0 parts are plenty cheap, you don't need a cheat sheet to go parts shopping because you got a ton of mis-matched, different year parts. Only parts in my stroker not stock to my 91 are the 258 crank, pistons, and I did add 4 hole injectors. Open my hood and you wouldn't know there was anything going on. It's not till you feel the low end torque that you would even know there was a stroker under the hood.
I know, my brother took my crank and we're putting the stroker in his xj. If I had a 4.0l to begin with in my jeep I would have done the stroker. With the earlier jeeps like mine the v8 swap is too easy. I also am dissatisfied with jeep transmissions and I rebuild transmissions. The ax15 or nv3550 are fine but the gearing just isn't what I need for rock crawling on the black diamond trails in my area without spending $ on an atlas with 5:1 low. The aw4 has given my brother a ton of issues in his jeep because how hot the transmission gets even with an rv cooler. The shift points are also crap and it favors 2nd gear and the only way you can force it into 1st only is with a manual solenoid controller. My stockish xj does fine with the aw4 but it hardly locks up the torque converter when it should wasting fuel.
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Old 02-17-2017, 06:47 PM   #17
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... My stockish xj does fine with the aw4 but it hardly locks up the torque converter when it should wasting fuel.
That's what I'd have to learn about. The last automatic transmission I had to really speak of was an A727 in a big block '73 Sebring back in the early '80's. They didn't have them lock 'em up shtuffs back then. lol
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Old 02-18-2017, 08:52 AM   #18
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It's probably cheaper but I'm swapping to a v8 mostly for transmission reasons and easier to find/cheaper parts that go with the v8
x2 on this. You can't beat the aftermarket support for sbf's and sbc's when you want to get past the 300hp mark. I looked into building up my 4.0 before I went to a sbf, the cost-hp ratio is way too high for the 4.0. Plus, options are pretty limited for aluminum heads and high-flow intakes for the 4.0. My 5.0 has many, many options. And I can swap to a 5.8 later down the road with minimal modification and be able to use many of the 5.0 parts, even the heads.
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Old 02-18-2017, 11:43 AM   #19
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If I had the torque of some of the newer sbc's and sbf's I would have to do a serious upgrade to my front axle. Being an old gearhead I know that wheelies would be the norm for me, and I don't think the D30 is up to the constant barrage of "hard landings"! LOL
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Old 02-18-2017, 11:45 AM   #20
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Luckymac...no intention of hijacking your tread, but we seem to have gotten side tracked talking shop and having fun in the process.
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Old 02-18-2017, 02:28 PM   #21
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No problem Chris, I enjoy the input.

Since I am happy with the performance of my 4.2 in its current condition (180K miles) for my mostly daily driver use, I expect that I'll pass on the monster overbore. Unless the machine shop can convince me that a 3/8" overbore is a "safe" overbore, I will most likely go with a .040" overbore and new 4.2 oversized pistons. Not that the money is a big issue, but whatever money I spend on building more engine than I need, is money that I can't spend on other mods.

I'm still waiting for my full doors to get back from the paint shop.

Good Luck, L.M.
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Old 02-18-2017, 02:46 PM   #22
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Quote:
Originally Posted by chris yoos View Post
If I had the torque of some of the newer sbc's and sbf's I would have to do a serious upgrade to my front axle. Being an old gearhead I know that wheelies would be the norm for me, and I don't think the D30 is up to the constant barrage of "hard landings"! LOL
With a truss and in 2wd it would hold up, I've jumped my jeep a few times and that's how I destroyed the rear dana 35. Bent it then the gears failed later
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Old 02-18-2017, 08:41 PM   #23
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No problem Chris, I enjoy the input.

Since I am happy with the performance of my 4.2 in its current condition (180K miles) for my mostly daily driver use, I expect that I'll pass on the monster overbore. Unless the machine shop can convince me that a 3/8" overbore is a "safe" overbore, I will most likely go with a .040" overbore and new 4.2 oversized pistons. Not that the money is a big issue, but whatever money I spend on building more engine than I need, is money that I can't spend on other mods.

I'm still waiting for my full doors to get back from the paint shop.

Good Luck, L.M.
The AMC 6's are all great engines!
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Old 02-18-2017, 08:42 PM   #24
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With a truss and in 2wd it would hold up, I've jumped my jeep a few times and that's how I destroyed the rear dana 35. Bent it then the gears failed later


LOL

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