Using the 848 spec Ohlins forks on a 998 requires a brake disc modification. The FGRT forks place the calipers farther apart by 5mm than the OEM 748-998 fork units do. The 748R/996R and 998R models shared this increased spacing that eventually became standard on 749/999 and 848/1198. The plan is to keep the stock 998 wheels which means discs with 5 bolt holes are required. The Ducati 749 and later models all used 6 bolt discs, meaning the only direct fit for 5bolt with increased spacing is from the 748R-998R. There are options still out there thankfully, but they’re not cheap. The discs that came on this 998 are still in great shape, so in the interest of being cheap, I bought these nice CNC 5mm spacers from Motowheels.
Secondly, the bolts that hold the brake discs need to be 5mm longer to ensure proper thread engagement. I bought some from McMaster-Carr. They’re stainless instead of zinc-coated, so they don’t look quite right, but won’t corrode either way. McMasterCarr sells multiple types of stainless fasteners. I used the ‘High Strength Stainless Steel Socket Head Screws’ specifically because they have a higher tensile strength of 110 ksi vs the regular stainless’s 70 ksi. I think either would work fine in this application though; titanium bolts are readily available as replacements and have less tensile strength than either type of steel.