I find that aftermarket parts are generally poorly engineered for fit. I bought a Wilwood pedal assembly that had course thread on the pedal side, and their pushrod that came with their master cylinder had fine thread, or vice-versa. One problem is trying to make things fit between manufacturers when you are mixing and matching. My machinist was complaining about BBC heads since every aftermarket head has different designs. Sometimes having lots of choices leads to fit issues that require mods to put it all together. You almost need a mill and a lathe to build a race car since nothing seems to fit well.