After the water pump tensioner was completed, I had to determine the length required for the drive belt. The most common way I've heard to measure it 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.
For the alternator belt, I adjusted the alternator swing tensioner to it's loosest position and measured around the crank, alternator & idler pulleys. That measured 950mm.
I then remembered that had kept one of the original drive belts from the Starlet engine so I cut it in half and remeasured the two above lengths using an actual belt. I measured the lengths of 725mm & 960mm - so using the cloth tape to measure was within 1% of the actual lengths.
Drive belts are generally available in size graduations of 5-10mm (Dayco Drive Belts) so it would be easy to dial in an accurate belt length if I chose an unsuitable size. I purchased a 725mm belt (4PK0725) for the water pump - this belt was a bit tight to get over the pulleys but there's not a huge amount of adjustability in tension with this setup. For the alternator, I purchased a 960mm belt (4PK0960). Once installed, both belts had a suitable amount of tension so I did not have to swap either belt our for a different size.
With both belts fitted and tensioned :
I moved the water pump tensioner pulley to it's loosest position and measured around the crank, water pump & tensioner pulleys. It measured 720mm.
For the alternator belt, I adjusted the alternator swing tensioner to it's loosest position and measured around the crank, alternator & idler pulleys. That measured 950mm.
I then remembered that had kept one of the original drive belts from the Starlet engine so I cut it in half and remeasured the two above lengths using an actual belt. I measured the lengths of 725mm & 960mm - so using the cloth tape to measure was within 1% of the actual lengths.
Drive belts are generally available in size graduations of 5-10mm (Dayco Drive Belts) so it would be easy to dial in an accurate belt length if I chose an unsuitable size. I purchased a 725mm belt (4PK0725) for the water pump - this belt was a bit tight to get over the pulleys but there's not a huge amount of adjustability in tension with this setup. For the alternator, I purchased a 960mm belt (4PK0960). Once installed, both belts had a suitable amount of tension so I did not have to swap either belt our for a different size.
With both belts fitted and tensioned :
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