On the original engine layout the alternator was used to tension the belt that ran around the main crank pulley, water pump and the alternator. Since the alternator has been relocated (standard for our engine swaps), a new tensioner had to be fabricated for the water pump.
I found a tensioner at an auto wrecker (I can't remember what car it was from) so I could adapt it for my needs.
I purchased the smallest 4PK pulley that my local auto parts store could provide (72mm OD), but this was way too big to fit between the other pulleys and subframe tower.
I outsourced some work and got a custom pulley machined at a 20% smaller diameter of 58mm OD and used the same bearing size (6201RS) as the pulley I bought at the wreckers. This fitted nicely and seemed like it allowed for some reasonable vertical movement to tension a belt.
I have a layshaft/jackshaft/intermediate shaft (whatever you want to call it) and the water pump tensioner bracket will share two of the mount holes in the engine block with the layshaft bracket. The layshaft bracket had two gussets originally and I removed one of those gussets
and the water pump bracket is be mounted against it
The final tensioner bracket design (rear view), made from 6mm steel which allows for approx 30mm of vertical adjustment. In the photo below, there is a custom spacer between the pulley and bracket. I had to make a few spacers of differing widths before getting the alignment with the other pulleys just right.
The bracket in situ. The layshaft bracket is not fitted here so I used some spacers behind the tensioner bracket to simulate the thickness of the layshaft bracket
The most common way I've heard to measure a belt size is to wrap a piece of string around the pulleys and measure the length of the string however because belt part numbers are coded and sold based on the "effective length" (outside circumference) of the belt, I think a closer approximation is to use something like a cloth tape measure because the tape does not sit in the pulley grooves like the string.
I moved the water pump tensioner pulley to it's loosest position and measured around the crank, water pump & tensioner pulleys. It measured 720mm.
I bought a 725mm belt (4PK0725) to account for the thicker belt compared to the cloth tape. It was a tight-ish fit to get the belt over the pulleys was once tensioned, it was spot on. I also measured the alternator belt (960mm) and here is both water pump belt and alternator belts fitted :
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-Alan
I blame my dad for my love of minis. I think I was conceived in the back seat of one
I also blame my Dad for me being 6' 1" - not really the optimum height for driving a Mini.