On August 29, 2011 ODI received a Vehicle Owner's Questionnaire (VOQ) from a Los Angeles school district indicating a 2003 Thomas Built Saf-T-Liner HDX compressed natural gas (CNG) school bus in their fleet had insufficient clearance between the CNG tank and mounting bracket resulting in damage to the outer tank wrapping.The Mathematical Analysis Division of NHTSA calculated that a sample size of thirty (30) buses would give a high confidence (90%) estimation of whether similar buses were experiencing tank abrasion.In the course of this investigation nearly 70 buses (over 300 tanks) were inspected; no tank other than the subject of the VOQ was found to have insufficient clearance.Busses manufactured before and after the subject vehicle were inspected.As a result of this investigation, Daimler Trucks North America (DTNA) developed an improved inspection procedure that addresses the concerns identified in the Los Angeles complaint.In early February 2012 DTNA will mail an owner letter along with the new inspection procedure.See the investigative file for copies of DTNA's bulletin and owner letter.This preliminary evaluation is closed.The closing of this investigation does not constitute a finding by NHTSA that no safety-related defect exists.The agency reserves the right to take further action if warranted by circumstances.The ODI report cited above can be reviewed at www-odi.nhtsa.dot.gov/owners/SearchNHTSAID under the following identification (ODI) number:10423400