1978 CB400A Front Brake Disc Replacement

YumaTheTanuki

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Sep 9, 2025
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To preface, I'm not the brightest crayon in the lunchbox sometimes so forgive me if I'm missing something obvious. I have a 1978 CB400A that needs a new brake rotor, and the only results I can find are a couple used listings on eBay (used rotors seem like a weird proposition to me) and a couple $600+ nos parts. Looking at the diagrams I see 2 part numbers, 45251-410-000 and 45251-410-305, but I can't find dimensions on either to look for an aftermarket replacement if any exist. If anyone could point me to some specifications, reasonably priced nos parts, or good aftermarket alternatives I would be in your debt (for whatever that's worth to a stranger on the Internet lol).
 
If you have a decent core and can measure the thickness of the existing rotor with a caliper then there is a thickness measurement in the inner black spider web section. It will be in mm's (like maybe 8 mm's) stamped into the webbing on one side. If it is well within spec then TruDisc In Michigan can Blanchard grind (machine) the original core back to new. That is the correct way to refinish a rotor, as long as it still has enough "meat" left to have the service done. He only charges around $50 per rotor plus shipping.
 
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