RHS’s New Aluminum LS Race Block Is Built for 4.600-Inch Stroke: When a Factory LS Block Stops Being Enough

The factory GM LS architecture earned its reputation because it can make serious power without turning every build into an exotic-engine project. But once the goal moves beyond a conventional stock-block combination, the block itself stops being a background part and becomes part of the engineering plan.

That is why the August 2026 Race Shop feature from Performance Racing Industry caught our attention. PRI highlighted the new RHS aluminum LS race block, including the standard-deck 54903U-904 configuration. RHS, part of the Edelbrock Group, designed the block around the kinds of problems that start showing up when stroke, rpm, cylinder pressure and valvetrain demands move far beyond a typical production-engine build.

SMG Speed Shop currently carries both the RHS 9.240-inch standard-deck LS aluminum race block, 54903U-904 and the RHS 9.750-inch tall-deck LS aluminum race block, 54901U-904.

There Is No Magic Horsepower Number for “Needing” an Aftermarket LS Block

One of the easiest mistakes in engine planning is asking, “How much horsepower can a stock LS block take?” as if there is one universal answer. There is not.

A block does not experience a dyno-sheet number. It experiences cylinder pressure, crankshaft loading, main-cap movement, heat, detonation, rpm, bore distortion and the leverage created by the rotating assembly. Two engines that both make 1,000 horsepower can place very different demands on the block depending on boost response, fuel, tune, stroke, piston weight, rpm range, vehicle use and how often the engine sees full load.

For a moderate street build, a production block can still make excellent sense. For a purpose-built engine where the owner is already investing in a large-stroke rotating assembly, serious cylinder heads, high rpm, substantial boost or repeated competition use, the better question becomes: Does the block architecture support the combination we are actually trying to build?

Why the RHS Block’s Raised Cam Centerline Matters

PRI and Edelbrock’s current RHS technical information call out a raised cam centerline as one of the block’s key features. The reason is packaging.

As crankshaft stroke increases, the rods and counterweights need more room to move through the crankcase. RHS moves the cam centerline upward and engineers additional rod clearance into the block, allowing clearance for up to a 4.600-inch stroke with standard 2.100-inch rod journals. That creates room for large-cubic-inch LS combinations without asking the engine builder to solve every interference problem by grinding away structural material.

The 54903U-904 is also machined around a 55 mm roller cam bearing arrangement and can accommodate an oversized 60 mm camshaft configuration. Larger cam cores can improve stiffness, which matters more as spring pressure, valve acceleration and rpm rise.

Priority Main Oiling Is About Keeping the Bottom End Alive

The RHS block uses an outboard priority-main oiling layout. In simple terms, the oiling system is designed to prioritize the crankshaft main bearings before feeding less critical areas of the engine.

That matters because the main bearings are carrying the rotating assembly while combustion is trying to drive the crankshaft out of the bottom of the block. At high load and rpm, oil control becomes every bit as important as the strength of the crank, rods and pistons.

RHS also moved the side oil gallery outward to improve rod clearance and increase windage space around the rotating assembly. Those details do not create a flashy dyno number by themselves, but they address the systems that determine whether a high-output engine can repeat that number without hurting itself.

Six-Bolt Head Clamping and a Full Water Jacket

The RHS LS race block uses a six-head-bolt design with a full water jacket around the cylinders. The extra clamping provision gives an engine builder more options for controlling head lift and gasket sealing in high-cylinder-pressure combinations, especially when the rest of the cylinder head and fastener package is designed to use it.

That does not mean simply adding more bolts automatically fixes a marginal combination. Head-gasket material, bore size, surface finish, fastener type, torque procedure, cylinder-head casting, tune and peak cylinder pressure all work together. If you are choosing heads for a serious LS build, our recent guide, Peak CFM Is Not the Winner: How to Choose LS/LT Cylinder Heads That Actually Make Power, explains why the complete combination matters more than one headline specification.

Why RHS CT-Scans Every Block

One of the more interesting manufacturing details is RHS’s use of CT scanning for casting quality control. The company says each block is scanned to check casting precision and consistency before it becomes the foundation for an expensive engine.

That is especially relevant with an aluminum race block. Internal casting quality, wall consistency and material around critical areas are not things the customer can evaluate by looking at a finished block sitting on an engine stand. The inspection process is intended to catch problems before machining and assembly money gets invested in the casting.

Standard Deck vs. Tall Deck: Which One Makes Sense?

SMG carries both current 0.904-inch-lifter-bore versions, and the right choice depends on the engine plan rather than which block looks more “race.”

The 9.240-inch standard-deck 54903U-904 keeps the build closer to conventional LS deck geometry. That can simplify intake, accessory, header and vehicle-packaging decisions while still providing the raised-cam, priority-oiling and large-stroke architecture of the RHS block.

The 9.750-inch tall-deck 54901U-904 creates additional room for combinations that need more deck height and longer cylinder liners. The tradeoff is that tall-deck engines can affect intake-manifold width and position, accessory packaging, headers, hood clearance and other vehicle-specific details. It is a combination that should be planned as a complete engine and chassis package, not ordered one part at a time.

What “Ready to Hone” Really Means

The 4.120-inch bore is supplied for final finish honing before assembly. That is not a defect or an inconvenience. A serious engine builder wants the final bore size, geometry and surface finish established around the actual piston, ring package and intended operating conditions.

The correct final hone depends on piston-to-wall clearance, ring material, power adder, fuel, expected heat, bore target and the manufacturer’s specifications. An aftermarket race block should be treated as the start of precision machine work, not as a crate you open and immediately fill with parts.

The Block Is the Foundation, Not the Whole Build

An RHS block can provide more room and more structural capability, but it cannot rescue a mismatched rotating assembly, inadequate fuel system, poor oil control, excessive intake-air temperature or an unsafe calibration.

For a high-output LS, the block decision should be made alongside the crankshaft and stroke, rod length, piston and compression ratio, cylinder heads, cam core and valvetrain, head sealing, oiling system, power adder, fuel system and intended rpm range. Even the harmonic damper deserves to be part of that conversation; our guide Your Crankshaft Twists Every Time a Cylinder Fires explains why torsional vibration becomes more important as the combination gets more aggressive.

Planning a Serious LS Build?

SMG Speed Shop can supply the core engine and supporting components, including the current RHS standard- and tall-deck LS blocks. If you are not sure which block, deck height, rotating assembly or supporting parts match the goal, contact SMG before turning a pile of expensive parts into a compatibility problem.

For customers who want the complete engine and vehicle handled as one system, North Texas High Performance in Justin, Texas performs engine builds, forged-assembly planning, valvetrain work, fabrication, forced-induction integration and calibration. Finished combinations can be verified on NTHP’s Mainline ProHub 3000CCS hub dyno with controlled-load and repeatable before-and-after testing.

Bottom line: the RHS aluminum LS race block is not about chasing a single horsepower threshold. It is about giving the engine builder more structural margin, more stroke clearance, better oiling architecture and more valvetrain flexibility when a production block is no longer the right foundation for the job.

Fitment, machining requirements, emissions legality and component compatibility vary by application. Verify the complete engine combination before ordering.