I have been working on a small air cooled Briggs engine (350777, 18 HP). Both of the head gaskets look to have been blown (just barely) at their narrowest location. I am thinking of switching to head studs to provide more uniform clamping force. The engine uses a composite head gasket. Another improvement might be to switch to a multi-layer steel head gasket.
OE bolts
- 8 mm diameter by 1.25 thread pitch
- Marked with a "10" on the head (Class 10.9?)
- Service manual calls for 165 in. lbs. (13.75 ft. lbs.) of tightening torque
- Threads are to be lubricated during torquing
Class 8.8 bolts typically get 14.09 ft. lbs. (169.1 in. lbs.) based on a torque coefficient (K) of 0.15. Is 0.15 appropriate for lubricating the threads? This is where I found the typical torque specification for Class 8.8 bolts: https://www.fastenal.com/content/merch_ ... 0Guide.pdf
Assuming 0.15 is reasonable, 14.09 ft. lbs. is close to the 13.75 ft. lbs. that the service manual calls for. So these may be similar to Class 8.8 bolts.
Should the torque coefficient for studs (lubricate the nut) be approximately the same as bolts with lubricated threads?
Should I stick with Class 8.8 and try to achieve the same clamping load with the studs as the OE bolts?
In working with a formula for calculating clamping load, I noticed that the torque coefficient has a significant effect on clamping load.
Thanks for any help.