Lewis-san,
Thank you for your concern.
There are many Japanese riders that they keep the condition of old bikes. For exhample, FZ750 and XJ900 XS650 FZR1000 non-exup, FZR1000 etc. Especially, FZ750 has very poor caliper and damn ventilated rotor.... Many people swap using FZR front end.
On this point YAMAHA doesn't change too much and somehow easy one to swap wheels or brakes. Most YAMAHA bikes have 83mm or 100mm(FZR750R OW01 has 90mm Nisshin). Of cource, newer developped parts have good spec and, quality but, 20 years ago, there was huge boom of racer replica bike. Still they have top class spec and quality.
Secret? No no .... We all geeks having huge JUNK box full of treasure. Once find part looks having possibility, bid at auction site like ebay, if it is cheap(around $10). Also aftermarket part maker provide some information. For exhample, Sunstar provide diameter of rotor or PCD. Ohlins sometime tell us collect length of fork spring for each bike. Now I know the YAMAHA Genuine parts destributor that he was R&D in YAMAHA. When I buy something, I can ask him details of spec. Also, I was in FZR owners club all Japan and, there was comunity to making huge database.
In the other hand, if you have friend in dealer you can check specs from service manual.
However, I don't have experience using Nisshin TZ250 caliper into FZ6. Need check more details (5mm or more thicker rotor acceptable or not etc.) I show just possibility(but possibility is very high). Actually, i reccomennded him to get junk caliper from auction site to check. Also you can use FZR400(3TJ) Caliper and FZR600 siries(need check). Additionally, I have outer tube of early FZ6 fork to install 83mm gap one when I have problem at FZ6 OEM caliper in long term use and OH front fork.
Any way, I never hesitate help someone who need information of GENUINE Yamaha parts. Send me PM when you have question.
What kind of information do you need? I need more credits play in casino of here(NONONO Just kidding)
Have nice weekend!
