28-09-2018, 12:00 PM
My thoughts on head gaskets:
If the surfaces of head and block are sound and flat and the stud and nut threads in good condition there should never be any problem with head gasket sealing. The nuts should run smoothly on the studs by hand.
A smear of grease lubricates the gasket and allows it to settle correctly as it is tightened. It has no sealing effect.
Soft gaskets lead to problems. They tend to compress around the studs causing the head to bow and in time also the block to rise up so that there is a high spot around the studs. This has the effect of reducing the compressive force in the areas between the studs.
Tightening the head further tends to increase these effects and make it worse.
On my engines, when reassembling, I ensure the block face is flat by removing the studs and rubbing the mating surface on two sheets of emery cloth glued to a flat piece of kitchen worktop - home made surface plate. The high spots around the stud holes show up straight away and it can be surprising how bad they can be. Treatment continues until an even colour across the face. Same treatment to the head.
When fitting the head I tighten it down progressively in the correct sequence and use a ring spanner of normal length done up firmly by hand. I don't use socket spanners here. After a few miles I retighten by hand with the engine cold.
I've never had any head gasket problems over many thousands of A7 miles. I reuse head gaskets, lightly greased, many times provided there is no mechanical damage.
A firm gasket on flat surfaces gives best results. This is the reason modern engines use metal shim gaskets and stretch bolts.
The shim gasket ensures that it is an even thickness all over. Stretch bolts, at the point they start to stretch permanently enter a stage when the force applied remains constant as they are further tightened. Hence the 2-part tightening. First they are torqued down to the start of the stretch phase, then a further rotation takes them into constant force. This gives consistent and even tightness to all the bolts.
If the surfaces of head and block are sound and flat and the stud and nut threads in good condition there should never be any problem with head gasket sealing. The nuts should run smoothly on the studs by hand.
A smear of grease lubricates the gasket and allows it to settle correctly as it is tightened. It has no sealing effect.
Soft gaskets lead to problems. They tend to compress around the studs causing the head to bow and in time also the block to rise up so that there is a high spot around the studs. This has the effect of reducing the compressive force in the areas between the studs.
Tightening the head further tends to increase these effects and make it worse.
On my engines, when reassembling, I ensure the block face is flat by removing the studs and rubbing the mating surface on two sheets of emery cloth glued to a flat piece of kitchen worktop - home made surface plate. The high spots around the stud holes show up straight away and it can be surprising how bad they can be. Treatment continues until an even colour across the face. Same treatment to the head.
When fitting the head I tighten it down progressively in the correct sequence and use a ring spanner of normal length done up firmly by hand. I don't use socket spanners here. After a few miles I retighten by hand with the engine cold.
I've never had any head gasket problems over many thousands of A7 miles. I reuse head gaskets, lightly greased, many times provided there is no mechanical damage.
A firm gasket on flat surfaces gives best results. This is the reason modern engines use metal shim gaskets and stretch bolts.
The shim gasket ensures that it is an even thickness all over. Stretch bolts, at the point they start to stretch permanently enter a stage when the force applied remains constant as they are further tightened. Hence the 2-part tightening. First they are torqued down to the start of the stretch phase, then a further rotation takes them into constant force. This gives consistent and even tightness to all the bolts.
Jim