Article by: Paul Axford
Applies to: all BMW models with the M30 engine (this article was written with a 730i).
This initially looks like a formidable job but once you get the splash guard off the lower-front engine area it’s really very simple.
Not so simple is getting the correct lengths. Check the part numbers currently installed rather than relying on the application book at the discount parts shop.
In the case of the A/C compressor, if an 13X0850 or 13X0855 is installed check where the adjuster is positioned and get a longer one if there is adequate adjustment range.
It seems that the optimum lengths are fairly easy to position over the pulleys with the tensioner at the slack end.
For my particular car the Bosch belts that should be used are underlined as follows.
On the Water Pump/Alt I used a 1055 which is too long and at the end of adjustment, while on the PS the 0850 I used was too short and hard to get on.
Water Pump/Alt 13X1040 or 13X1045, Power Steering 10X0860 or 10X0865, A/C 13X0865
FYI, it seems that some brands refer to widths “10” as “9.5” and “13” as “12.5”
You only need a 10mm socket with 6″ extension, and a 13mm and 19mm combination (open and box end) wrenches to do the job.
I raised the front of the car for better access but that does not appear to be essential. Tighten the tensioners very lightly – the geared design make it easy to over-do it.
A 1/4″ of total back and forth flex should be easily obtainable with a single finger applying about 10 lbs force to the belt span.
The basic steps are the same for each tensioner:
(1) loosen the 13mm nut on the tail end,
(2) turn the 19mm screw head to change the tension,
(3) while holding the desired adjustment re-tighten the 13mm nut to lock the position.
Below: 13X1055 (too long)
Below: 10X0850 (too short)
Below: 13X0865 (just right!)
