I figured I’d start a proper thread documenting this build and hopefully get some input from people who have pushed the AJ126 platform further than the usual pulley/tune setups.

### How This All Started

One night I was driving home and noticed my coolant temperature starting to climb. It wasn’t immediately catastrophic, but it was clearly higher than normal.

I pulled over, shut the car down, and let it cool for a while.

After sitting for a bit, the temperature dropped back down and the dash was showing normal operating temps again. Thinking everything had stabilized, I continued driving.

Not long after getting back on the road…

Boom.

The engine let go.

Anyone who’s experienced that moment knows exactly the sound and feeling I’m talking about. Instant sinking feeling.

---

### The One Lucky Break

Luckily, at the time I was working as a Jaguar technician, so I had access to the shop and resources.

I had the vehicle towed back to the dealership and started diagnosing the engine the proper way.

First step was a compression test, and the results confirmed what I already suspected:

The motor was basically cooked.

At that point the original engine wasn’t worth trying to save internally.

---

### Finding Replacement Parts

Fortunately I was able to locate:

- A spare AJ126 block
- A spare set of cylinder heads

Those are the components I’m currently building from.

Right now the heads are at the machine shop.

Initial inspection showed:

- Warped deck
- Valve sealing issues

Nothing unusual for a motor that experienced an overheat event, and thankfully all of it is correctable through machining.

The plan is to bring everything back to proper Jaguar tolerances and essentially start with a fresh foundation.

---

### Where the Build Stands Now

Currently:

• Heads are at the machine shop
• Block is in my possession and being evaluated
• Majority of rebuild parts have already been ordered

The only components I’m still debating are lower end upgrades.

---

### Lower End Decisions

This is where I’m looking for input.

I'm currently considering:

- ARP head studs
- Possibly forged pistons
- Possibly forged rods

The forged setup obviously adds cost, and I’m not sure it’s absolutely necessary for my goals.

My realistic power goal is somewhere around:

500–600 horsepower

Not trying to build a full race engine—just something strong, reliable, and noticeably faster than stock.

If the consensus is that stock pistons can survive at that level with proper tuning, I’m open to staying factory there.

If forged pistons are strongly recommended for safety at that power level, I’d rather do it now while the engine is apart.

---

### Current Resources

One advantage I have is location.

This project is happening in Las Vegas, and I have access to a lot of:

- experienced technicians
- reputable machine shops
- performance resources

So the goal is to build this engine the right way the first time.

---

### Build Goals

Ultimately I want to end up with:

• A fully refreshed AJ126
• Properly machined heads and block
• Reliable 500–600HP capability
• Strong cooling system and supporting mods

Essentially a brand new engine that performs far better than stock.

---

### Looking for Advice

For those who have built or tuned these engines:

1. Are forged pistons recommended for 500–600HP, or can stock internals survive that with a safe tune?
2. Are ARP head studs worth doing while the engine is apart?
3. Any known weak points in the AJ126 bottom end I should address before assembly?
4. Any recommended supercharger upgrades or airflow improvements that pair well with this power target?

---

I’m excited to get this engine back together and will be updating this thread with:

- machine shop updates
- parts arriving
- assembly progress
- and hopefully dyno numbers when everything is finished.

Appreciate any guidance from people who have experience pushing these motors further.

 

No help but I do see a neat trick?
The hood struts are disconnected and a longer rod is used to hold the hood open. The longer rod pushes the hood almost vertical. This improves working room and the ability to see better in and around the engine.
Brand and cost?

Never thought of that and it's a good suggestion. I know I banged my head repeatedly on the hood when I was changing injectors and that blasted rear water manifold!

Good luck on the rebuild.
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Good eye — that’s actually exactly what I did. I disconnected the hood struts and used a longer prop rod so the hood sits almost vertical. It makes a huge difference when you’re working deep in the engine bay. Way more room and you can actually see what you’re doing around the back of the motor.



I don’t remember the exact brand of the rod — it’s honestly just a longer prop rod I sourced locally — but it was inexpensive and completely worth it for the extra workspace.



And yeah, that rear water manifold area is brutal to reach with the hood in the normal position. This setup definitely saves a few head bumps.



Appreciate the encouragement on the rebuild — I’ll keep updating the thread as the machine shop progresses and parts start going back together.

 

Donor Motor Teardown & Bottom End Inspection

Quick update on the build. I’ve started tearing down the donor AJ126 engine that I sourced after the original motor failed from the overheat incident.

For anyone following the thread, the original engine that was in the car when it overheated is most likely completely cooked. When it happened, there was a significant amount of smoke coming from the crankcase area on the side of the freeway NHP actually helped push the car safely off the road and I was able to get it towed from there.

Because of that, I decided it really wasn’t worth sending the original motor to my machine shop. With the level of heat that engine saw, there’s a high likelihood the block or internal components could be warped or compromised. I didn’t want to waste my machine guy’s time or money chasing a block that might ultimately be unusable.

So the plan now is to use this donor engine as the foundation for the rebuild.

---

### Initial Inspection

After opening up the donor motor and inspecting the bottom end, here’s what I’ve found so far.

The crankshaft journals do show some wear, but overall they actually look better than expected. The wear marks are visible, but when running a fingernail across the journals they still feel smooth and pass the fingernail test, which is encouraging.

The rod bearings themselves definitely show wear and will obviously be replaced during the rebuild. I included photos above showing the condition of the bearings and crank journals.

At this point the crank appears very usable, but I will still have my machine shop inspect and measure everything to confirm tolerances.

---

### Bearing Options

One thing I’m currently researching is aftermarket bearing options for this engine.

I would ideally like to run something a bit stronger than standard OEM bearings if possible. I’m curious if anyone here has experience sourcing performance bearings for these engines.

Possible options I’m looking into:

- King Bearings
- ACL Race Bearings
- Custom bearings if available

If anyone has successfully run aftermarket bearings in an AJ126, I’d love to hear what you used.

---

### Parts I’ve Already Ordered

I did recently order piston kit from eBay that arrived in what appears to be legitimate Jaguar Land Rover packaging. But the ring lands were crushed. So I sent them back.

That said, I’ve been warned that some sellers are offering lower quality replica components, so I’ll be inspecting everything carefully before installation.

One of the suppliers I spoke with is EuroAmp out of Houston, who mentioned they manufacture some components for these engines and claim to offer higher-quality aftermarket parts.

They mentioned items like:

- Connecting rods around $300
- Pistons around $200

That said, if the price difference isn’t too dramatic, I would definitely consider going with custom forged pistons or rods for reliability and performance.

---

### The Challenge With This Engine

One thing I’m realizing as this build progresses is that the AJ126 platform is somewhat rare in the aftermarket world compared to more common performance engines.

Because of that, finding performance parts isn’t always straightforward.

If anyone here has experience sourcing performance components for this engine platform, especially:

- bearings
- rods
- pistons
- machine work specs
- bottom end upgrades

I would really appreciate the input.

---

More updates to come as the machine shop finishes the heads and I continue inspecting the block and crank. The goal is still to build a solid 500–600HP setup while the engine is apart.

 

 

Great pictures of the horrible damage!
Not to go off topic again but I have found that prop stick and have ordered one.
They are VIM V387. VIM adds a letter after the numbers to indicate the color of the stick. I have seen green, red and black so far.
Price ranges from around $30 to over $50.

I found a black one on EBay for $27 including shipping. VIM has more than one style of this tool too.

So thanks again for the suggestion!
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Wow that looks rough! I have a stock AJ126 and my understanding was that it already had forged internals. Some others need to chime in.

I have just "graduated" to the VAP phase 3 with ported supercharger and should beat 558 hp. I did run phase 2 at 450HP for 15,000+ miles with absolutely no issue,

Best of luck!

 

Appreciate that info 👍



From what I’ve been seeing, Phase 2 seems like the sweet spot for these AJ126 motors — solid power without pushing the bottom end too far. Good to hear you ran ~450hp for that long with no issues, that’s reassuring.



I’m currently refreshing the timing system and top end, so just trying to make sure everything is solid before even thinking about stepping into that range. Probably going to stay conservative at first, then maybe pulley/tune later on once everything’s proven.



That Phase 3 setup sounds serious though — ported blower and 550hp is no joke on these. Curious how it holds up long term.

 

Picked up a full set of metal coolant components:

• Front and rear crossover pipes
• Metal thermostat housing
• Rear Coolant outlets
• New seals/hardware

(Pics attached)



Main reason I went this route is pretty simple — didn’t want to deal with the plastic stuff failing later, especially if I end up doing a pulley/tune down the road.



Trying to eliminate the weak points now while everything’s easy to get to.





Engine is currently:

• Heads resurfaced
• Valves cleaned/sealed
• Timing system getting refreshed

Just trying to build a solid baseline before putting it back together.







Curious if anyone here has been running the metal cooling setups long term:

• Any fitment issues?
• Any leaks or weird expansion issues?
• thermostat ? How to check for function

Not trying to overbuild it, just don’t want to be back in here for something dumb later.

 

At this point where I need to decide what I’m doing with the bottom end while it’s apart.

• Heads done (resurfaced, valves cleaned up)
• block being picked up tommrow by machine shop.
• Still have all my stock rods, only one had a bearing issue
• Sent rod bolts + head bolts out to ARP in CA. Awaiting head and deck measurements to pass on to them. (Thank you ART)

Piston options I’m looking at:

I’m awaiting measurements first but best case scenario if there is no warp

• Brand new OEM pistons (~$1200)
• Found a full set of OEM pistons + rods (~70k miles) for $400 (seems like a steal)
• Mahle pistons (~$360 range)
• if warped, only EUROAMP option I believe $200 +/-
• sleeve experience anyone ?

Part of me thinks:

• OEM stuff is proven and already matched
• These engines seem picky with tolerances

But then the Mahle option is cheap enough to consider



Rod question:



Since I already have my stock rods and only one was questionable:

• Would you guys just replace that single rod and run it?
• Or is it worth upgrading rods altogether at this level

Not trying to go crazy — probably staying around Phase 2 power later on.





Main thing I’m wondering:





Is it better to keep the OEM piston/rod combo together

or does mixing Mahle pistons with stock rods not really matter on these?



Just trying to do this once and not regret it later.



Appreciate any input from people who’ve been inside these motors.

 

Figured I’d throw these up for reference before the I got the block machine work started.



This is how the cylinders and crank journals looked as it came apart from the donor block. No cleanup or machining done yet — just straight out of teardown.



(Pics attached)



Initial observations:

• Cylinder walls looked decent overall, no major scoring
• Some normal wear but nothing catastrophic
• Crank journals looked clean, no obvious damage

Didn’t see anything that immediately screamed failure, but wanted to document it and get some extra eyes on it before everything got sent out.





Machine shop already went through it, but curious what you guys think just based off initial condition:

• Would you have run this as-is?
• Or still go through full refresh like I did

 

My bias would be to go OEM on the internals. The engine will make the power you are looking for without getting exotic.

VAP phase 2 and in fact the setup I am running now with the ported supercharger would cost you near zero labor right now. If your Supercharger is a bit rough, they have a rebuild service.

 

Contact or website?

 

https://www.velocityap.com/product-c...jaguar/xf-xfr/

Sales | VelocityAP <sales@velocityap.com>

I would suggest not putting the Symposer back on....

 

I agree. when I was a Jaguar technician, one of the most common issues with the 3.0L AJ126 V6 was a rich/lean running condition, often traced to vacuum leaks at the rear of the intake. The hoses in that area are known to become brittle and crack, especially during or after supercharger removal.

 

Quick update—block has been sent out to the machine shop and we’re currently waiting on measurements. In the meantime, I’m trying to stay ahead and start lining up parts.



Planning to run King XP connecting rod bearings. I haven’t ordered main bearings yet, but I’ll list part numbers here as I confirm fitment. I did order the rod bearings. From what I’ve found, King does offer bearings for the AJ133 platform—appears to be bimetal rather than tri-metal. This will work for aj126 v6 motor



Ideally, I’m hoping to stay within spec and avoid any crank cutting, but that’ll depend on what the measurements come back at.



The King main bearing part numbers for the AJ133 platform fall under the MB5856SI series, which is their bi-metal (SI) main bearing offering. These are available in multiple sizes including MB5856SI-STD (standard), MB5856SI010 (.010 undersize), and MB5856SI020 (.020 undersize). These part numbers cover the main bearing applications for the platform, with final sizing dependent on crankshaft journal measurements once verified by the machine shop.

 

While the block is out at the machine shop, I’ve been going all-in on the cooling system to eliminate the known weak points on these AJ126/AJ133 platforms.



So far, I’ve already upgraded to all aluminum cooling components.. including the crossover pipes, rear outlet pipes, and the “white pipe” assemblies that are notorious for cracking with age and heat cycles.

I’ve also ordered the metal thermostat housing, which is another common failure point on these engines. The goal here is to remove as much plastic from the system as possible and improve long-term reliability.



Taking it a step further, I’m now looking at upgrading the auxiliary coolant pump, specifically to a Pierburg unit.

These pumps are known for better durability and more consistent flow compared to aging OEM pumps. Typical cost is in the $120–$250 range depending on the exact model and source. This becomes especially important once you start increasing heat load from pulley upgrades or extended driving under boost.





On top of that, I’m exploring upgraded radiator and intercooler (heat exchanger) options, possibly through Mishimoto. I’ve reached out to see what might be adaptable with minor fabrication. Even if it’s not a direct bolt-on, a larger core radiator and higher-efficiency heat exchanger can significantly improve coolant temps and intake air temps under load.



As most know, the common cooling failure points on these engines typically include:

• Plastic crossover and rear coolant pipes becoming brittle
• Thermostat housing failures
• Auxiliary pump wear or reduced flow over time
• Heat exchanger limitations under higher boost

The idea is to overbuild the cooling system now while everything is accessible… so once the motor is back together, it’s not fighting heat, especially with future plans for increased boost or ethanol.



Interested to hear what others are running for upgraded radiators or intercooler setups, especially anyone who’s successfully adapted Mishimoto or similar solutions.

 

if you haven't gotten the pump as yet.

https://www.velocityap.com/product/w...3-velocity-ap/

 

Thank you, as I saw velocity has the harness for plug and play. Will grab that and the SC symposer delete as well.

 

Quick update on where I’m at with head fastening.



I reached out to ARP to see about getting a proper head stud solution together for the AJ126.



Key point they made: studs and bolts behave completely different in the block.

Bolts only engage part of the thread depth, while studs are designed to utilize the full thread engagement in the block. Because of that, they need actual measurements from both the block and the head to build the correct stud.



What they’re asking for:

• Block side:
  • Thread size and pitch
  • Total thread length (usable engagement)
  • Full depth of the bolt hole (deck to bottom)
  • Whether there is a counterbore at the top of the hole, and its depth
• Head side:
  • Counterbore depth and diameter
  • Thickness from the bottom of the counterbore to the deck surface
  • Bolt/stud hole diameter
• Additional:
  • Head gasket thickness

I also brought my old pistons in to seperate them from the rods. ARP will need a rod and a cap with bolts to create some custom rod bolts as well.

• 
  

They emphasized that the counterbore in the block (if present) is a big deal because it directly affects overall stud length. Also, since studs can bottom out if not spec’d correctly, they need precise numbers to ensure proper thread engagement and correct protrusion for washer/nut stack.



I’ve got the block and head at the machine shop right now, so I’m coordinating with them to pull all these measurements while everything is accessible.



Best case, ARP finds something off-the-shelf that works. If not, they said they can quote a custom kit through their special orders department.



Trying to do this once and do it right—especially with the direction this build is going.



More updates soon once I get measurements back.