Just asked thumper racing about the leaking issues with there steel sleeves. They said it was a thing of the past. Here is what was said. The DRZ sleeve is wet vrs the YZF sleeve which is dry. What that means is to install the DRZ sleeve you have to completely remove/machine out the inner cylinder and the sleeve which has the potential to leak- although we haven’t had one leak that I know of in over 4 years. The problem was mainly due to the way the sleeves were installed back then. We now use a different sealing process on the DRZ sleeves that has eliminated this issue. The DRZ is a completely different sleeve compared to what is done to the 426 cylinder. The YZF is a dry sleeve which fits inside the cylinder and the sleeve is similar to plating because the water jacket is not compromised when the sleeve is installed. The only safe way to big bore the YZF is to sleeve the cylinder. The Stock 426 cylinder becomes too thin if you just over bore and re-nicasiled it and can crack so to maintain strength and cylinder integrity a steel sleeve must be used for this kit. That being said what do you guys think or know from experience about the 444 kits on a 426 without a steel sleeve, just the bore and recoat.