Billyboy:
Found this on the web. Maybe it will help you and me (maybe others) measure our chains.
"Chain size is in inches, the three digit number used to describe the chain size is based on the "rule of eights". The first digit is for chain pitch. This is the distance between the pins. The second and third digits combine for roller width, with a decimal point inserted between the second and third digits.
For example, a 420 chain has a pitch of 4/8" = 1/2" (12.70mm) and a roller width of 2.0/8" = 1/4" (6.35mm).
A 525 chain (the size my bike uses) has a pitch of 5/8" (15.88mm) and a roller width of 2.5/8" (7.94mm).
A 530 chain has a pitch of 5/8" (15.88mm) and a roller width of 3.0/8" (9.52mm).
There are a number of chain sizes in use, and chain manufacturers may have slightly different specs for roller diameter, sprocket width, etc. But a 525 chain from one manufacturer will have the same pitch as a 520, 525, 530, or 532 chain from another and run smoothly on any 525 sprocket from any manufacturer. " credit (
http://everything2.com/title/Motorcycle+chain)
That is not my info, so I used it to measured the chain on my BS 200cc MKII SS.
Pitch (Pin center to Pin center) = 0.5 ". Because it's difficult to measure just one link, I measured 10 links, then divided by 10. So, 10 links on my chain = 5.0". So 5.0" divided by 10 links = 0.50" pin to pin. That makes sense to me. According to the info above, I've got a 400 series chain (4/8 = 1/2 = 0.5). I think we'll all agree on that.
Then, I measured my roller width as close as I could with my dial caliper.
My roller width = 0.30". According to the info above, that would mean that I have a second two digits of 24. Thus, 2.4/8" = 0.30".
Using this logic, I would conclude that I need a chain of 424 size. I think you mentioned that you were thinking it would be a 428. I could be off a little on my roller width measurement, but the difference between 424 and 428, according to the info above, would mean that my roller width measurement was off by 0.050". That seems unlikely to me. Unlikely, but indeed possible.
I also measured my roller diameter as closely as I could with my dial caliper. Roller diameter = 0.32" (in case that's worth anything.
After all that calculation, I check the CPC parts manual. It shows chain, master link, and sprockets for the DT, HS, SS, and RS Doesn't specify 175cc or 200cc. It specifies 428-108 links for the DT, HS, & SS. And 428-106 links for the RS. I think you're right going with the 428. And I think I might need a new dial caliper.
Good Luck,
Gary
