andrew112_uk said:
So, do you set your wrench for 28N/m or should you start lower?
Andrew,
General practice is to start off all nuts/bolts with a figure far lower than the final required torque. With cylinder heads, start in the midddle with a rotating or criss-cross pattern till all have been tightened, in the case of a Lav triple, the 9mm nuts only. The outer7mm and 8mm studs can be tightened during this procedure after the 9mm studs have been done. The 8mm studs have no influence on the seating of the head gasket but be sure the cam bearings seat properly without squashing themselves on the edges of the locating slot in the head, the resulting alloy filett under the bearing will cause the cam to jam. Increase the wrench setting in 2-3 steps till the required torque is attained.
The theory is to settle the head slowly and evenly on the gasket instead of tightening completely in one spot, then the next. The head undergoes quite a bit of bending if this is done. Starting from the middle allows the head to "squeeze" the gasket uniformly to both ends of the cylinder. If tightened at the ends first, a bulging of the gasket would result in the middle, stressing both the head and the gasket and leaks are quite likely.
I stated 28N/m maximum as there are varying figures floating around, some of these will have you ripping out your studs long before you reach them!
HTH,
piet
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