Proper Head Torque

[san dimas]

I have a 1997 Force 75hp outboard. Serial # OE261263. My Mercury shop says the head bolt torque is 120 in. lbs. My workshop manual says 120in. lb and then 90 degrees more. Which is right?

 

[Frank Acampora]

Re: Proper Head Torque

I have a 1997 Force 75hp outboard. Serial # OE261263. My Mercury shop says the head bolt torque is 120 in. lbs. My workshop manual says 120in. lb and then 90 degrees more. Which is right?

120 inch pounds is only 10 foot pounds which is well below what a 5/16 bolt can tolerate and probably below sealing pressure for combustion pressures.

I have seen older Fords which had a similar head tightening: Torque to a specified value then 1/4 turn more. The problem is that this torqued the head bolts to their yield values (stretched them slightly) and they were not reusable. If you removed them, or loosened them for any reason, you needed to replace them.

SO: I am suspect about 120 inch pounds then 1/4 turn more. I would stick with the old tried and true 240 inch pounds in three steps. My reasoning is that it is easy to miscalculate a quarter turn and slip. I would much rather trust an accurate torque wrench.

 

[Jiggz]

Re: Proper Head Torque

I'm not really sure why those with serial number OE093699 and later were only required 120 in-lbs + 1/4 turn while those before and up to OE093699 were required 225 in-lbs. The only thing I can think off is that probably the bolts used were of different material and grade or the head material is of different material. Typically a Grade 5, 5/16-18 bolt has a max torque of 250 in-lb when fastened to a cast iron or steel material and 150 in-lb into an aluminum material. So it would seem the newer engine heads were most likely aluminum as indicated by the torque specs while the early ones are steel or cast iron. I'm just surmising based on the torque requirements.