Milling the head for more compression?

[bobgroger]

Why has no one mentioned a thinner base gasket? That is the cheapest and easiest way to move the piston closer to the head and increase compression. Measure the deck height and valve clearance first, of course. Base gaskets are available as thin as .005 for the 1100e

[GregT]

Why has no one mentioned a thinner base gasket? That is the cheapest and easiest way to move the piston closer to the head and increase compression. Measure the deck height and valve clearance first, of course. Base gaskets are available as thin as .005 for the 1100e

Because the OP is working with an 8 valve GS1000. The different thickness base gaskets are used on the 16 valve motors to set the squish clearance. Not applicable to the 8 valve engines as they don't have a squish band to speak of.

[stetracer]

Why has no one mentioned a thinner base gasket? That is the cheapest and easiest way to move the piston closer to the head and increase compression. Measure the deck height and valve clearance first, of course. Base gaskets are available as thin as .005 for the 1100e

when you remove the base gasket you move the piston closer to the head. Which can be very bad if you don’t have the proper piston to head clearance. When you mill the head your not moving the pistons any closer to the head your making the combustion chamber smaller.
You base gasket thickness is determined by the piston to head clearance nothing else when building a preformance motor.
compression should only be increased by milling the head or using aftermarket pistons.

[rphillips]

That just don't sound right. If you remove the base gasket, you don't raise the piston at all, you lower the head toward the piston by lowering the cylinder block that has the head mounted to it, piston is at exact same place. Only way to raise the piston is stroker crankshaft, longer connecting rod, or taller piston. Either way the closer the piston gets to the head, the higher the compression will be. If I'm thinking right !

[Steve]

"Raise the piston" (by removing the base gasket) or "lower the head" (by milling it), you are reducing the combustion volume.

As he mentioned, though, milling the head only shrinks the volume of the combustion chamber, it does not reduce the piston-to-head clearance.

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[John Park]

"Raise the piston" (by removing the base gasket) or "lower the head" (by milling it), you are reducing the combustion volume.

As he mentioned, though, milling the head only shrinks the volume of the combustion chamber, it does not reduce the piston-to-head clearance.

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Good luck with that theory.

[rphillips]

Now I see, base gasket reduces deck height. Milling the head does not. Milling the head still does reduce the piston to valve clearance.

[metalfab]

That just don't sound right. If you remove the base gasket, you don't raise the piston at all, you lower the head toward the piston by lowering the cylinder block that has the head mounted to it, piston is at exact same place. Only way to raise the piston is stroker crankshaft, longer connecting rod, or taller piston. Either way the closer the piston gets to the head, the higher the compression will be. If I'm thinking right !

I am using .010" as a number not that they make an .010" gasket. Imagine if you have a .010" base gasket, and you put the cylinders on. The piston will be at a certain height in relation to the cylinder deck if you measured it. Now if you remove the .010" gasket and reinstall the cylinders, they will go on .010" further down because the gasket isn't there. If you measure the piston height in relation to the deck of the cylinders the piston would be .010" higher in the cylinder with an .010" gasket removed. You are not changing the stroke, just relocating the TDC height of the piston in relation to the cylinder deck height. You are effectively raising the piston in the bore by reducing the base gasket thickness.

"Raise the piston" (by removing the base gasket) or "lower the head" (by milling it), you are reducing the combustion volume.

As he mentioned, though, milling the head only shrinks the volume of the combustion chamber, it does not reduce the piston-to-head clearance.

.

Milling the head or using a thinner base or head gasket will reduce the combustion volume by; reducing the piston to head clearance by the amount of metal removed from the head, or by the thickness removed from the base or head gasket. Removing thickness of the base gasket places the piston slightly higher in the bore by the amount removed from the base gasket, thus reducing the piston to cylinder head clearance at TDC. Milling the head or using a thinner head gasket places the roof of the combustion chamber closer to the piston by the amount shaved from the head, or by any reduction of thickness removed from the head gasket thus reducing the piston to cylinder head clearance at TDC. Any of these mods alter cam timing but not a great deal.

[stetracer]

Now I see, base gasket reduces deck height. Milling the head does not. Milling the head still does reduce the piston to valve clearance.

unless you recut the valve deeper in the seats also.
I have a 1150 head with .105” milled off and the valves cut .040” deeper in the seats. This is on a street bike.

[stetracer]

Alway make sure you have the proper clearances no matter how you build it.

[metalfab]

unless you recut the valve deeper in the seats also.
I have a 1150 head with .105” milled off and the valves cut .040” deeper in the seats. This is on a street bike.

Seating the valves deeper does increase the valve to piston clearance. But what is important with seating the valve deeper into the head, whatever dimension you seat the valve deeper by. is removed from the height of the fully open valve in relation to the combustion chamber roof. In other words seating the valves .040" deeper gives you a fully open valve that is .040" closer to the roof of the chamber. The valve still travels the same distance, but it starts from behind in relation to how far it is away from the roof of the combustion chamber. Getting the party started at high RPM's can be effected by a valve not open as far into the combustion chamber. Only a dyno or flow bench can tell you how this effects your setup.