Bulletin 74-016: Improper Machining of Pistons in Colt Industries (Fairbanks-morse) Diesel-Generators


                                      Directorate of Regulatory Operations 
                                      RO Bulletin 74-16 


We recently received information from Northeast Nuclear Energy Company 
(NNECO), Baltimore Gas and Electric Company (BG&E), and Colt Industries 
describing a deficiency in the machining of pistons in a Fairbanks-Morse 
emergency diesel generator, type H7B. 


NNECO reported that the four capscrews retaining a piston insert in an upper
piston of the diesel engine had broken away and were found lying in the 
engine crankcase. The capscrews were found during a routine field inspection
prior to operating the equipment. 

NNECO's and Colt Industries' investigations indicated that the cause of the 
failure was improper machining of the underside of the piston crown. The 
improper machining left an elevated, ring-shaped surface inside the piston. 
This prevented the piston and piston insert from mating properly. As a 
result, during the compression cycle, the resultant space between the 
surfaces of the two parts allowed the piston crown to flex and be forced 
down on the insert surface. This subjected the capscrews to an alternating 
stress, causing their eventual failure. 

During the inspection of the remaining pistons, two other pistons were found
with the same machining error. However, the capscrews in these pistons had 
not failed. 

BG&E reported that they had found two incorrectly machined pistons. In 
addition, they found that several locking strips, which are attached to each 
pair of capscrews, had cracked at the bolt-hole locations. 

Colt Industries stated that the probable cause of the failure of the locking
strips was an improper torque sequencing of the capscrews during 

Colt Industries indicated that they have initiated a program, directed by 
their service representatives, to inspect all diesel generators in the field
that could have these conditions. 

                                      Directorate of Regulatory Operations 
                                      RO Bulletin 74-16 


1.   Notify this office in writing within 20 days whether engines of the 
     make and type described above are used at your nuclear power facilities 
     with operating license or construction permit. 
2.   If engines of this make and type are used, your written reply to item 
     1. above should include your schedule for examination of the 
     potentially affected engines. 

3.   Upon completion of any required examinations, provide to this office, 
     in writing within 30 days, the results of the examinations and a 
     description of any repairs found to be necessary. 

Page Last Reviewed/Updated Tuesday, September 01, 2015