4.2L Stroker Build


Hello all, here’s the current state of my 02 mustang build.

Non- engine stuff is quick and simple. Everything is stock except the following: Suspension (upgraded shocks / struts, lowered 1”, added rear sway bar), Automatic transmission (modified valve body), Gas pedal modded, and that about it for non-engine related stuff.

The engine is the current project. It’s a stock Ford 3.8L Essex V6. It’s currently out of the car on an engine stand, completely torn down. Found a bit of bearing damage in the engine. All of the rod bearings on the passenger side of the car were semi pitted / scored. I'm thinking the pitting came from dumping coolant in the engine and letting it sit for a month before finishing the tear down. (can coolant do that to a bearing?) I added a pic of the worst bearing, that happens to be missing a chunk as well, next to one of the better looking bearings.


The plan is to stroke it to 4.2L and do whatever else I can think of / afford. So far the only things that’s really been done to the engine is I polished the lower intake runners.

While I would love to do the entire engine at once, I’ll be splitting it into two stages: Bottom end, and top end. The bottom end is what I’m working on first. This includes everything below the heads. Most of the top end stuff will happen after the engine is back in the car. I know I’ll need to get the engine back in the car before I can finish everything, so I plan for 2 stages, but hope for 1.

I need a bit of advice on a few things:


218 at .050 Intake, 226 at .050 Exhaust, 112 lobe separation


Think this would be a good cam? Too big, too small? I’m not very knowledgeable when it comes to cams.

Pistons / Cylinder bore:

I have no idea what to get. I figure it might be good to get it bored, even a little, to eliminate any sort of ridge that may have formed in the bottom section of the cylinder. I don’t know if this matters, or if the ridge at the bottom is even a thing that happens, but doesn’t the piston go further down into the cylinder when it’s stroked to 4.2? Just don’t want to murder my rings right off the bat.

I’m thinking about over-boring 0.020” – 0.030”, then throwing in some semi cheap hypereutectic pistons from summit. Again, I don’t know if it’s worth it though.

4.2L stroking:

I’m not 100% sure what all I need to make this work. I know I need the crank, don’t know about other parts. I’ve heard 2 different things on this, and can’t seem to find a solid answer.

Path 1: just change crank, and harmonic balancer and reuse everything else.

Path 2: change crank, con rods, pistons, harmonic balancer, flex plate (for auto), and remove windage tray.

Honestly just kind of making it all up as I go at this point, until someone can offer some sound advice.

I added some random pics but don't really know what to show. anyone want to see anything specific? it's just a bunch of parts right now.

Comments, questions, concerns, advice, and criticisms are always welcome.

List of parts I have:

4.2L crankshaft

(wow such a long list…)

Parts I still need:



Gaskets / seals


Harmonic balancer

Timing chain tensioner


Valve Springs

Push rods


Lots of machine shop work

More will be added as I remember

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New Member
4.2/3.8 connecting rods are the same so you may be able to reuse yours or just get factory replacements. You will need 4.2 pistons and matching rings as they are not the same as 3.8 pistons, 3.8 pistons may work, but your compression ratio will be off. Also, i don't know if summitracing is selling the hypereutectic pistons as an upgrade, but the factory pistons are already hypereutectic and are actually fairly robust up until around the 400 hp range. Over boring 0.030" would actually give you a 4.3L stroker, don't over bore more than that, you'll risk weakening the block too much. The cast factory connecting rods are really the weak link in these engines. They don't handle revs well, 5500-6000 rpms is really their reliable limit. Beyond that, and the big end of the rods will oval out over time. So that being said, that camshaft you have listed is really too aggressive for an engine without ported heads and intake manifolds. Its powerband (up to 6200 rpms) is beyond what you could safely run without forged rods. The 44-702-9 cam is probably a good option. You will need upgraded beehive valve springs with that cam, part#: 26918. That's all you have to upgrade valvetrain wise but roller rocker arms and hardened pushrods are always a good idea too. This link below should help you out.

V6 Stroker Build - Muscle Mustangs & Fast Fords

6 Shooter

Well-Known Member
3.8 pistons may work, but your compression ratio will be off.

3.8 pistons will not work with a 4.2L crank. The 3.8 piston skirts will be too long and smash into the crank counter weights. So, if going with a 4.2 crank, you will need pistons with shorter skirts. Suggest you get a hold of Tom at Super 6 motor sports who has and can get Comp to make off-set forged pistons for the 4.2L crank that correct for the geometry of the longer stroke of the crank. The pistons can or cannot come with the piston rings. And, if you decide to bore out, you should FIRST get the pistons in hand and have the bore cut then honed afterword to perfectly fit to the purchased pistons.


Okie dokie, thanks for the advice so far.

This week I will be ordering more parts, I want to get my pistons before sending the block off to the machine shop. I'll check with SuperSix on pistons before I make any purchases.

The question I have now is: What kind of difference is there between the custom pistons that super six can get, and normal 4.2L pistons from summit? Would it run different?

side question: how much power can a stock 7.5" rear end take? I've blown two up on stock power, but just curious how much other people have managed. (I'll be upgrading it soon after engine is done.)

6 Shooter

Well-Known Member
Call Tom at Super 6 and ask him to explain the 4.2L offset pistons. He is the best source for info and parts.

On the 7.5 rear, it is easier to tear one up with a stick while reving the motor and dumping the clutch.


I'm doing another parts order and I have another question.

Do i need the flexplate from an f150? I see they are different between the mustang and f150, and I can only seem to find info on flywheels not flexplates.

Any help would be greatly appreciated.

6 Shooter

Well-Known Member
Suggest you look into an SFI approved flex plate for a 2002-2004 3.8L Mustang. Think Tom at Super 6 either sells them or knows where to look. It is for a neutral balanced crank. Be sure to count the teeth on your stock flex plate and make sure that the one you find has the same number of teeth.


thanks for all of the help 6 Shooter.

I'll do a big update this weekend or early next week. I've assembled almost everything I need to go to the machine shop. So there will be pics of shiny new parts.

Phil II

Cone Destroyer
I have an SFI flex plate available. Used, then removed when I swapped to a manual transmission. PM me if interested. It came from SSM.


I was able to find a flex plate locally for dirt cheap. looks brand new.

I found it on Summit and added a link, will this work? While I would like to take Phil II up on his flex plate, I've already dumped way too much money into this engine. :)


Any opinions on whether this will work or not?


Alrighty update time.
I've taken pictures of all my parts in their current state before taking them to the machine shop. I've also included part numbers, if it turns out something doesn't work I will come back and correct.

ATP Flexplate P/N: Z-273


Harmonic Balancer:
Doorman P/N: 594-390

Balance Shaft Bearings:
Sealed Power P/N: 1807M

I may get the balance shaft eliminated at machine shop, got these just in case.

Cam Bearings:
King Bearings P/N: CS432BB

Main Bearings:
Sealed Power P/N: 7295MA.25MM

Rod Bearings:
King Bearings P/N: CR6712SI0.25

Piston Rings:
Sealed Power P/N: E-911K 75MM

Sealed Power P/N: H850CP

Valve Springs:
Comp Cams P/N: 26918-16
(accidentally ordered the 16 valve instead of 12 valve...)

Comp Cams P/N: 44-700-9

Refurbished 4.2L Crank from an 02 F150

Engine Block:





Rods + Old Pistons + Valves:






Push rods + Lifters/Rockers (in cups)

Balance shaft + timing parts:

Most of my engine that has been crammed in a cabinet for the last 6 months:

Misc Pics:

Last edited:


Awesome! Thanks you for keeping us updated. I've been looking at the same build for about a month. I'm about ready to head to the junk yard looking for a engine and crank. I'm pretty lucky in that department. I've got a really good junkyard close by that is ran by good people. They don't charge an arm and leg for things. They also good about refunds if the part is bad.

6 Shooter

Well-Known Member
If you decide to remove the balance shaft, suggest you drill out the two oil holes, tap the holes, and install steel plugs with red locktite. Also, when removing the balance shaft, the shaft holes can be sealed with the correct size of freeze plugs. And the shaft gear is no longer used, but my machinist had to turn down one of the gears to create a collar to take up the space. Don't recall which one but for the machinist/engine builder, he should notice the large gap and with this hint, he should know what to do. Plus, the gear is not so hard that it can't be turned down with a lathe.

What size of overbore pistons? Plus, the cylinder bore should be done after the pistons are in hand to get the correct bore size. BTW, very nice job with your organization of the tear down with the individual labeling of parts and locations.


Slithermgs - I was originally going to get my crank from the junk yard, but I just couldn't find the time so I just bought a refurb. The junk yard route would probably have saved me $100-$150.

6 Shooter- Thanks for all the balance shaft elimination information. I will likely discuss this with my machinist when I drop the engine off.

The pistons on 0.030" over sized.

I tried to be as organized as possible. I wanted to give myself the best chance I could to get everything back together.


I have no idea on power. I'm hoping to get a dyno tune once it's back together so I can let you know then.

I was going to do an at home port and polish on them, but I kind of lost interest.

I did polish my lower intake manifold though. Took about a week of grinding away and polishing, looks really pretty now, I'll post pics later today.

Really porting and polishing the split port heads doesn't seem very difficult. from what I was seeing all that needs to be done is knife edge the divider, match the ports to the intake, then polish it to within an inch of it's life. I just can't bring myself to dedicate a weeks worth of grinding for each head.


C'mon man that's free power lol. I understand though for sure. I'll most likely p&p everything I can. But my scenario is a little different than yours.

I'll be driving the car as is till I get the engine together. Then I'll do a weekend swap and hopefully a dyno tune the following week.

The tune is going to be problematic for me. I live in a geographically handicapped part of the country and dyno's are hard to find let alone find one that can tune as well.