The 0a4 Transmission, a component familiar to many Volkswagen and Audi enthusiasts, shares a surprising amount of commonality with other gearboxes in the VAG family, particularly the 02J. For those delving into transmission work, it’s reassuring to know that the bearings are largely interchangeable. Specifically, while technically the differential bearings are listed as different between the 0A4 and 02J, practical experience shows that 02A/02J differential bearings fit and function correctly within a 0A4 transmission. When undertaking bearing replacement, it’s advisable to pay close attention to the 02J preload setting for the differential bearings, which is generally recommended over the lighter 0A4 preload, potentially offering enhanced durability.
Beyond basic maintenance, the 0A4 transmission platform offers exciting avenues for performance enhancement, especially concerning gear ratios. For owners of models like the BRM in the US market, readily available bolt-on upgrades can significantly transform the driving experience. Two compelling options emerge from Volkswagen’s range of petrol engines, offering a noticeable improvement in gearing.
The QSY and NZA code 5-speed transmissions, originally paired with the 1.8 TSI gasoline engines in the Mk6 and early Mk7 generation vehicles, present an excellent, direct-fit upgrade path. Similarly, the 02S 6-speed gearboxes found in the 2019 and newer 1.4 TSI powered cars (Mk7 and later) provide an even more advanced option.
To appreciate the potential benefits, let’s compare the stock gearing of a typical GQQ code 0A4 transmission with these upgrade options. The factory 0A4 gearing is structured as follows:
- Final Drive: 3.39
- 1st Gear: 3.77
- 2nd Gear: 2.11
- 3rd Gear: 1.36
- 4th Gear: 0.97
- 5th Gear: 0.744
In contrast, the 1.8 TSI 5-speed gearboxes (QSY and NZA codes) offer a more performance-oriented setup:
- Final Drive: 3.39
- 1st Gear: 3.77
- 2nd Gear: Approximately 2.11 (similar to 0A4)
- 3rd Gear: 1.27 (closer ratio)
- 4th Gear: 0.87 (closer ratio)
- 5th Gear: 0.66 (closer ratio)
This 5-speed upgrade delivers a noticeable improvement by tightening up the gear ratios, effectively addressing the common complaint of a significant gap between 4th and 5th gear often encountered when fitting a taller 5th gear in a standard TDI 5-speed gearbox. The result is a more responsive and engaging driving experience throughout the gear range.
However, for those seeking the ultimate gearing enhancement, the 02S gearbox from the 2019-up 1.4 TSI models stands out. This 6-speed transmission, while part of the same MQ250 family as the 0A4 5-speed, incorporates an additional gear for even greater flexibility and performance. Intriguingly, it even shares 0A4 case half part numbers and internal components with the 02A, 02J, and 0A4 families, simplifying the upgrade process. The 02S features extended input and pinion shafts and a unique rear case cover to accommodate the sixth gear, but crucially, it retains compatibility with essential components from your original 0A4 5-speed setup. This means you can continue using the same clutch, flywheel, starter, and even shift linkage, minimizing the complexity and cost of the swap.
The gear ratios of the 02S 6-speed gearbox are configured as follows:
- Final Drive: 3.39
- 1st Gear: 4.11 (noticeably shorter for quicker acceleration)
- 2nd Gear: 2.11
- 3rd Gear: 1.36
- 4th Gear: 0.97
- 5th Gear: 0.77 (slightly shorter than the stock TDI 5th)
- 6th Gear: 0.625 (an overdrive gear for relaxed cruising and fuel efficiency)
The 02S 6-speed provides a compelling combination of shorter lower gears for enhanced acceleration and an added overdrive 6th gear for improved highway driving. The practicalities of this swap are highlighted by real-world examples. One enthusiast successfully integrated a TKR code 02S gearbox from a 2019 Jetta, utilizing an 02J bellhousing, into a Mk4 Golf. Remarkably, this swap retained the original Mk4 axles, 5-speed clutch and flywheel, and even the factory Mk4 shift linkage and clutch hydraulics, demonstrating the straightforward nature of this performance upgrade. Such a modification unlocks a new level of driving enjoyment and illustrates the versatility and upgrade potential inherent within the 0A4 transmission family.
