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3.8L 24v - MA1.71
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3.8L 24v - MA1.71
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Set of 12 trimetal connecting rod half-bearings for Porsche MA1.71.
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.
A trimetal bearing stacks three layers: a steel back, a copper-lead intermediate layer with high load capacity and an anti-friction running surface that protects the journal. The complete set contains 12 half shells, six pairs for a six-cylinder engine.
| 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 |
The running clearance is the gap between the bearing and the journal where the oil film flows.
In operation, the crankshaft journal does not rub against the bearing: it rides on this oil film, which carries the load. Clearance sets the thickness of this film: too tight, oil flows poorly, temperature rises and the risk of seizure increases; too loose, oil escapes, the film loses load-carrying capacity and oil pressure drops. The right clearance is the balance between adequate lubrication and a well-maintained oil film.
| 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.
The ACL rod bearings shown here are trimetal bearings, designed for engine building and racing. Compared with an original bearing, they handle far higher loads: they cope with increased power, high rpm or the addition of a turbo or supercharger, where a standard bearing quickly reaches its limit.
 | ACL Bearing Company, an Australian manufacturer of high-performance engine bearings since 1949, offers with its Race Series precise trimetal bearings, available in many sizes and proven on built engines worldwide. |
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 |
