Here's what I use for what it's worth.
I have a 74 CR125 and a 75 CR with a modified MR175 motor both running stock CR125 gears and 428 chain and stock suspension travel with no problems with excessive wear or stretched chains. With this setup I feel the 428 chain is more than adequate.
I also have a 76 CR125 with long travel suspension that needed to run a 520 chain setup with it's larger diameter rear sprocket to clear the swingarm pivot at full extension. I added a bracket made from a piece of angle iron bolted to the lower rear engine mount with a skate board wheel to support the chain and the stock rear chain guide on the swingarm.
To summarize, stock suspension travel use 428 chain and stock guide. Maintain it and keep it adjusted.
Long travel suspension may need 520 chain for clearance at swingarm pivot.
Hope this helps.
The 76 CR125 has been sitting in the garage for years so it's looking pretty crusty, so use the pictures for ideas and not to judge me on my bike maintenance.
Thanks,
Coll
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File comment: 76 CR125 with stock chain guide on swingarm
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File comment: 76 CR125 with skate board wheel under swingarm pivot
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File comment: 76 CR125 with angle iron bracket on lower motor mount bolt
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