B4x Forged Internals Guide
This guide covers the specifics required when building a B4x engine block. The B4x Platform shares around 90-100% of its engine parts with the B58 & B38, S58 part cross compatibility to be confirmed.
There are many forged parts suppliers available for the B4x platform. In the interest of this guide, we will only cover some of the popular options. Always research and consult with engine builders for your specific application.
Video Build Guides
These build guides cover concepts applicable to both N2x and B4x platforms due to their shared architecture.
N20/B48 Forged Build - Part 1
N20/B48 Forged Build - Part 2
N20/B48 Forged Build - Part 4
Factory Specifications
The B4x engines are made in the factory with the following specifications. This information is taken from BMW Technical Documentation and applies to B46, B48, B46TU, B48TU and B48A20T1.
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Parallel/Repair Sleeve Option
The "Parallel" or "Repair" Sleeve is the most common and cheapest sleeve option. Good for those that need to repair the block close to factory specification if re-applying the arc spraying process is not available.
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Known Issues with Parallel Sleeves
This gasket incorporates a "Seal Wire"/"Fire Ring" that is quite thick. During head torqueing, it places immense strain on the sleeve. If not installed correctly, the sleeve will begin to drop during the first engine heat cycle, resulting in a blown head gasket and loss of cylinder compression.
Most "Parallel"/"Repair" Sleeves are usually a "close enough" match from another engine application. Too large OD results in machining into coolant passages. Too small OD results in cylinder wall thickness being too thin and can crack under high horsepower or detonation.
Under high power, the immense cylinder pressures cause the block to warm up. The B4x uses quite a soft aluminium composition which, once warm combined with high power, will see the sleeve drop and head gasket fail.
Only recommend for OEM style rebuilds where engine power won't exceed standard Stage 2 OTS levels. If you have access to re-applying the arc spray process, that is preferred over a parallel/repair sleeve.
Flanged Sleeve Option
The "Flanged" sleeve is very common in the BMW engine building world, mostly in 6 and 8 cylinder engines. The flanged top of the sleeve has more surface area and helps the sleeve stay seated in the block, eliminating sleeve drop.
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Wall Thickness: NO LESS THAN 0.100" & NO MORE THAN 0.139"
If too thick, you risk machining into coolant passages between cylinders. If too thin, the sleeve can crack under high horsepower applications.
- Cracked sleeves in high performance builds if wall thickness is too thin
- Sleeve material may not handle high boost/nitrous applications
Have Darton Sleeves Inc or LA Sleeves create custom "Flanged" sleeves that allow maximum wall thickness without machining into coolant passages. Darton Sleeve's flanged sleeve material has been tested and yields great results.
Connecting Rods
| Rank | Part | Notes |
|---|---|---|
| 1 | CP Carrillo Conrod with Carr Rod Bolts | Strongest option |
| 2 | BC Conrods with CustomAge 625+ Rod Bolts | Excellent strength |
| 3 | CP Carrillo Conrod with WMC Rod Bolts | Strong option |
| 4 | BC Conrods with ARP2000 Rod Bolts | Good entry-level forged option |
Parts listed from strongest to weakest. Generally stronger/higher HP rated parts are more expensive.
Forged Pistons
CP Carrillo
2618 Forging Piston
Premium choice, extremely robust material
JE Pistons
2618 Forging
2618 forging used in top fuel engines
Piston Selection Notes
- •Piston Coatings are optional and can improve durability, recommended for 600+HP builds
- •Default piston pins are chromoly material - upgrade to heavy duty/steel pins for 800+HP range
- •Pistons come in 2 off-the-shelf sizes: 82mm and 82.5mm
- •Compression ratios available: 9.5:1, 10.0:1, 10.2:1, 10.5:1, and 11.0:1
- •For turbo setups, 9.5:1 or 10.2:1 compression recommended - 11.0:1 will limit achievable power on gasoline
- •Head gasket thickness and block/head resurfacing affects total compression
Head Gasket Options
| BMW Part Number | Thickness | Notes |
|---|---|---|
| 11128654273 | 1.0mm | OEM MLS Gasket - requires fresh block and head resurface with surface finish >60RA |
| 11128654272 | 0.8mm | OEM MLS Gasket - requires fresh block and head resurface with surface finish >60RA |
Bearings
Connecting Rod Bearings
Due to different sizing's, have your shop/builder source the correct sized bearings. Recommended upgrade: ACL Tri-Metal Racing Bearings. King Bearings also offers products but they are either OEM replacement or ACL rebranded.
Optional: Calico Coating (low friction coating) - reach out to Calico separately for this service.
Crankshaft Bearings
Currently only OEM or King Bearings available. Given the vast array of sizes, let your shop/builder order the correct size for your application.
Calico Coatings can be applied for a low friction coating/film on the bearing.
Head Studs & Main Studs
Head Studs
B58 ARP2000 and ARP CustomAge 625+ Head Studs are the go-to aftermarket options for B4x.
Torque Procedure:
- Follow ARP TQ specs for your studs
- Use BMW torque sequence (see diagram)
- Recheck TQ after initial procedure to release air pockets in ARP lube
- For added security, recheck after 500km break-in
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Main Studs
BMW B58 ARP2000 or CustomAge 625+ can be used in the B4x.
Torque procedure for Main Studs:
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Oil Pan Upgrade
Recommended: URO Parts aluminum oil pan with Brintech baffle kit.
The stock plastic oil pan is prone to cracking and is not suitable for high-performance applications. An aluminum oil pan provides better durability and heat dissipation.
The Brintech baffle kit helps prevent oil starvation during hard cornering, acceleration, and braking - critical for track use and high-performance driving.
Auxiliary Mods Checklist
When building a motor, ensure you have all the correct supporting mods for your engine/turbo setup. This is a checklist of what you may want to consider (not all required):