Fuel Nozzle Head Cracks
Properly treated, fuel injector nozzles need not contribute to a cylinder head crack. However, an area where cylinder head cracks appear is between the threads of the fuel injector nozzle and the threads of the spark plug. The area is structurally irregular. It is a natural metallurgical environment for a crack to develop - when excess stresses are imposed. The pointed thread grooves allow stresses to concentrate. When irregular forces are applied to the fuel injector nozzle, they are transferred to the threads, which in turn leads to a concentration of forces and eventually a crack. The situation can be avoided. Refer to Drawings A, B, and C.
RAM Recommends The Following:
1. Do not apply external forces to the air reference line “A.” See illustration below. The force is passed through to the fuel injector nozzle threads “B.” See illustration below.
2. Do not over torque the fuel injector nozzle “C.” It has a tapered pipe thread which exerts excessive stress the further it is tightened into the cylinder. See illustration below.
3. Cowling side doors should never be tied to the air reference line “A” during service work. If the cowling side door is to be tied to something, it can be tied to the large fuel injection line support brace. It is six to eight inches across and securely attached to the metal baffles of cylinders three and four. Never use the air reference line “A” as a place to tie the engine cowling back out of your way. See illustration below.
4. Fuel injector nozzles are only to be installed using a torque wrench. It is a RAM practice to torque the nozzle to 55 inch pounds. (Continental allows 55 to 65 inch pounds when using Continental Anti-Seize #646943 on the male threads only.)
5. When evaluating the significance of fuel stains around the fuel injection nozzle, consider these steps:
a. Check nozzle to be tight between 55 and 65 inch pounds. Look for positive signs that a sealing compound was used.
b. Remove and clean nozzle and threads. Use a soft and fine wire brush to avoid damage to the threads.
c. Visually inspect the port area for cracks visible to the naked eye. Note: A previously welded area can present a metallurgical pattern change which is not necessarily a crack.
d. Consider swapping the nozzle with one from another cylinder. They may both fit better.
e. When reinstalling the nozzle, apply Continental Anti-Seize # 646943. Apply to the male surface only, then torque the nozzle to 55 inch pounds. Increase the torque to 60 inch pounds if required, and never more than 65 inch pounds. If the threaded area runs out, you should replace the nozzle.
