Using a small radiator (cooler) to keep transmission fluid temperatures down increases the life of a transmission's clutches, bearings and other components. This is a fact of which BMW (with all of its racing experience and engineering prowess) is well aware, which is why it has included a cooler for every automatic car it has built for decades. However, these units are designed to work with stock power levels and performance and may need to be upgraded for high performance or racing use.
As with almost all cars, most BMWs use an transmission cooler integrated into the radiator. The good news is that, unlike other cars, the BMW's tranny cooler is removable and is easily serviced if you're simply doing a stock-for-stock replacement. The bad news is that because the cooler is integral to the radiator, installing a larger and better flowing tranny cooler often necessitates using a stand-alone aftermarket cooler attached to the front of the radiator.
There are two basic ways to get better cooling for your BMW transmission: replacing the radiator with one designed for a larger car or using an aftermarket one. Bentley radiators have huge transmission coolers and great flow and are the heat exchanger of choice for those who can afford them. However, any part designed for a $250,000 car is going to be expensive, so this is a costly route to accomplish a simple task. A good aftermarket transmission cooler such as those made by FlexALite and 4 Seasons can generally be had for less than $100 and are fairly easy to install. Measure your car's hard transmission lines before ordering your new cooler, so you know whether to purchase an adapter with it.
Removing the stock cooler is fairly easy: simply disconnect the clip at the top of the stock mount and pull it out. Some models may require a specialized line removal tool such as those made by Assenmacher, but others can be removed using an improvised equivalent. Then again, this may be an extra step considering the fact that you're probably (depending on the model) going to need to cut the stock metal lines behind the radiator core support to attach new rubber lines.
Most aftermarket transmission coolers come with a 2 to 3 foot length of coolant line for both the input and output side. Most models of BMW use the metric equivalent of 3/8 line, so the kit tubing should fit over it, but other cars may require an adapter.
Simply attach your kit's cooler lines to the stock hard lines with the supplied hose clamps, and follow your manufacturer's recommendations to mount the cooler. Although some people dislike using the zip-tie-like plastic retainers that come with most coolers on an aesthetic basis, these retainers will work just fine and are often used on race cars. If you're concerned that your cooler is going to block radiator airflow, you can compensate for any rise in temperature by using a radiator coolant additive such as Redline's Water Wetter.
