525 and 520 are the same pitch but the 525 is about 1.5mm wider, which makes more difference than you might imagine. Due to the 520 sprocket being narrower than the 525 chain you'll end up with excessive side play which will lead to greatly increased chain wear and, in the long run, possibly premature failure.

On a correctly matched sprocket and chain setup there is very little, if any side play, so the chain is always pulled inline from the front to the rear sprocket, which crucially means there is minimal, or no lateral load on the side plates.

If there is excessive side play (even a millimetre) the chain is going to wander round the sprockets and the driven front sprocket will end up pulling the chain back at an angle, rather than inline with the teeth. This will ultimately yank the chain about and put extra lateral pressure on the side plates, which they are not designed to take. In turn the riveted ends of the pins will suffer undue stress and could potentially fail with disastrous consequences. Use of a split link with this setup would almost certainly give rise to impending disaster!

I would not recommend the use of 520 sprockets with a 525 chain.

OTR do a range of sizes of 525 sprocket for the XT in all the sizes anyone could possibly want. Rears go from 44-48 and fronts from 14-16, which gives a huge range of possible final drive ratios to choose from.

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