TSB #70 - June 23, 1967
Amendment to TSB No. 65 Article 1053
Rough Idle, Poor Performance, Burning Valves, Improperly Seat Valves, Excessive Valve Guide Wear - (390 GT Engines)
The previous article did not cover the 390 GT engine. This engine is equipped with cylinder heads in which the exhaust manifold bolt pattern does not conform to the conventional 390 CID exhaust manifold. No "stress-relieved" cylinder head has been released for the 390 GT engine, so when dealing with rough idle and/or poor engine performance problems, use the GT cylinder head (Part No. C60Z-6049-AA1).
When defining the problem for GT engines, follow steps 1 through 4 as stated in Article 1053 of TSB No. 65.
The GT head will "stress-relieve" itself with use, therefore, when step #5 in the article is reached, it should read as follows in regard to GT engines:
5. If valve guides indicate excessive wear that cannot be corrected by using the available oversize valves, the cylinder head should be replaced using a like GT cylinder head (Part No. C60Z-6049-AA1), C0AZ-6507-N intake valves, C2AZ-6505-A exhaust valves, and the C7AZ-6524-A valve spring oil baffle. If the valve guides are within specifications, use the existing GT head and install the intake and exhaust valves plus the oil baffle as stated above.