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ACL trimetal connecting rod bearings Porsche 991 Turbo MA1.71
Set of 12 trimetal connecting rod half-bearings for Porsche MA1.71.
- Brand and treatment: ACL Race
- Compatibility: 911 (991) Turbo, 911 (991) Turbo S
- Engine codes: MA1.71
- Available sizes: standard to +0.25 mm, increased-clearance version (HX)
The Porsche MA1.71 flat-six engine
The MA1.71 is the 3.8-litre twin-turbo flat-six of the Porsche 911 type 991 Turbo and Turbo S. ACL lists this bearing set for this engine only.
From the factory, this engine is fitted with standard production connecting rod bearings, sized for the loads of the original engine: as soon as power, engine speed or boost pressure increase, they reach their limits.
Trimetal connecting rod bearings
Three layers make up a trimetal bearing: the steel back for stiffness, the copper-lead intermediate layer for load capacity, then the anti-friction running layer that protects the crankpin. This set includes 12 half shells, that is 6 pairs.
Where an aluminium bi-metal bearing, designed for low to medium loads, withstands about 8,000 psi, a performance trimetal bearing goes up to 12,000 psi: 50% extra margin. That is what keeps it durable on an engine boosted by a turbo or supercharger, or pushed to high rpm.
Technical specifications
| ACL bearings Crankpin Ø 52.971 / 52.990 mm Housing bore Ø 56.000 / 56.013 mm Width 15.75 mm |
| Reference | Brand and treatment | Size | Wall thickness (mm) |
|---|---|---|---|
| 6B2484H-STD | ACL Race - Untreated | Standard | 1.495 |
| 6B2484HX-STD | ACL Race - Untreated | Std, increased clearance (HX) | 1.482 |
| 6B2484H-.025 | ACL Race - Untreated | +0.025 | 1.508 |
| 6B2484H-.25 | ACL Race - Untreated | +0.25 | 1.620 |
Running clearance
Between the crankpin and the bearing there is a small gap, the running clearance, in which the oil film circulates.
In operation there is no metal-to-metal contact: the crankpin turns on this oil film, which supports the whole load. The clearance determines its thickness. A clearance that is too tight hinders oil flow, heats the assembly and promotes seizure; one that is too wide lets oil leak away, weakens the film and makes oil pressure drop. The correct clearance balances oil flow and film strength.
| Size / grade | When to choose it | ACL Race Series |
|---|---|---|
| Standard (STD) | Standard journal, not reground | 0.055 to 0.066 mm |
| Increased clearance (HX) | Wider clearance: high oil flow, high rpm | 0.080 to 0.091 mm |
| +0.025 mm | Polished journal | 0.055 to 0.066 mm |
| +0.25 mm | Journal reground to 0.25 mm | 0.055 to 0.066 mm |
Checking on assembly: lay a calibrated strip (Plastigauge) on the journal, torque the rod cap to spec, then compare the crushed strip against the gauge to read the clearance.
Technical guide
Designed for engine building and motorsport, these ACL trimetal connecting rod bearings carry much more load than an original bearing, whether the engine gains power, revs higher or receives a turbo or supercharger. They are available in several sizes: the standard size, a +0.025 mm size for a crankpin that has only been polished, a +0.25 mm regrind size and the HX version with increased clearance for high-revving engines or engines needing a high oil flow. That makes it possible to put a reground crankshaft back into service without changing range and to set the clearance for the oil and use.
The brand
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ACL Bearing Company has been making high-performance engine bearings in Australia since 1949. Its Race Series offers precise trimetal bearings, available in many sizes and proven in tuned and racing engines worldwide. |
Vehicle compatibility
This connecting rod bearing set fits the MA1.71 flat-six engine of the following Porsche models:
| Brand | Model (chassis) | Engine code | Power | Years |
|---|---|---|---|---|
| PORSCHE | 911 (991) 3.8 Turbo | MA1.71 | 520 hp | 2013–2020 |
| PORSCHE | 911 (991) 3.8 Turbo S | MA1.71 | 560 hp | 2013–2020 |
| PORSCHE | 911 Convertible (991) 3.8 Turbo | MA1.71 | 520 hp | 2013–2020 |
| PORSCHE | 911 Convertible (991) 3.8 Turbo S | MA1.71 | 560 hp | 2013–2020 |
Why replace the bearings
When a connecting rod bearing gives way, the damage quickly spreads beyond the part itself: a scored crankpin, debris carried through the whole oil circuit, a crankshaft to regrind at the very least, sometimes a rod and block lost. A bearing set costs far less than a regrind and is nothing compared with a complete engine rebuild. It is the cheapest part of the rotating assembly, yet the one whose failure is most expensive.
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Brand
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Reference6B2484H-STD
