Switching From Head Bolts to Studs

[PLohr]

Hello, guys.

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

I did a little digging and found:

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.

[modok]

The aluminum head expands more than the fastener with heat.
It gets tighter when hot.
Just because the low class 8.8 will hold at room temp does not mean it's good enough.
Clamping load bolt VS nut should be similar unless the head of the bolt or nut is much different size.

[mag2555]

The expansion rate of a non water cooled head like that is enormous, especially when the motor is making 100% more hp then stock.

The deck of the head needs to be mirror smooth so when it does expand and contract at it’s different rate then the air cooled block it sheers the gasket the least amount possible.

Before good head gaskets where available for a blown motor I was pressuring to 12 psi I found that dimples on the deck of the block outside of the fire ring helped to keep the gasket in place.

I made the dimples with a punch who’s nose was very short and sharp and left about a .030” OD dimple.

I spaced them .060” apart.

A composite gasket is needed for this to work well.