I Have A 98 C5 Stock Shortblock And Am Adding 243's. The 65 Cc 243 Leave Me @ 10.5 How Much Should I Mill Off Them To Get Me To 11.1. I Was Told 7mil/cc. If So How Much To Get Me There And Has Anyone Had Problems With 11.1. I Live In Fla And Have Decent 93.....thanks Kevin

 

I’m far from an expert in these matters, but there are many factors to consider beyond how much to mill the heads. Off the top of my head I can come up with a few more things to consider.

How far are the pistons “in the hole”
What head gaskets will you use?
What cam will you use?

Generally I think you will want to take off between .035 and .040. .040 will put you at about 59cc and around 11.3:1 compression, which will be no problem on 93 octane with the right cam.

 

If you want to be precise you need to get some tools that will allow you to check the volume of the combustion chamber, the compressed head gasket and the piston at TDC. There are two methods that would work; the block with crankshaft and a piston/rod assembly installed will need to be on a stand with a swivel head that will allow you to reposition the deck for measuring with tools:

1) With the piston at TDC apply just enough grease around the top of the piston to seal the top ring (for increased accuracy cut a small piece of strong thin plastic that will fit down between the piston and the cylinder and scoop out the excess grease above the top ring. Next clean up your old head gasket and your head and assemble it to the block with just the bolts around the cylinder you are checking. You should be using an old gasked that has been crushed and about 25 lbs/ft torque should be sufficient for checking. Roll the block with head over on the stand until the sparkplug hole is at 12:00 o'clock. Get a glass burette graduated 0-100 CCs (milliliters) and fill to zero with green rubbing alcohol. Pour fluid in through the spark plug hole until it just reaches the bottom of the hole and write down the burrette reading. Your swept volume for 1 cylinder in a stock LS-1 is 708.644 CCs. Add the reading you wrote down from your burette pour to 708.644 and divide that total by the burette reading you wrote down. Example; let's say your reading was 75.0 (chamber + gasket + piston deck clearance volume):
708.644 + 75.0 = 783.644
783.644 / 75.0 = 10.44 to 1 compression ratio

2) Method number two is better suited for accuracy for checking your work while milling your head. You will need to get a square piece of plexiglass large enough to completely cover one combustion chamber. It will neet to be thick enough to ensure flatness; at least a half inch would work. Drill about a 5/16" access hole toward one side so that it will be just inside the edge of the combustion chamber when in place on the head deck. With a thin film of grease around the chamber you are checking put the plexiglass plate on the head deck over the chamber and position the hole just inside the chamber edge. You will fill from the burette through this hole; make sure the spark plug is installed. Pour carefully and fill to the hole being carefull about trapping an air bubble as you go; when you finish write this reading down. Next, prepare the piston as before at TDC sealed with grease around the top and with the plexiglass plate over the cylinder measure the volume of the piston at TDC and write this figure down. Finally, measure an old compressed head gasket to determine its volume. Add these three figures together to get the total TDC volume. If the opening in the gasket is round this volume can easily be computed using the diameter and thickness with the formula below. Using the math figures as before we can compute the compression ratio.

Formulas:
To convert cubic inches to cubic centimeters multiply by 16.387
Displacement = Bore size squared x Stroke x .7854

I haven't looked at an LS-1 chamber but a rough guess would be .006/.007 removed would equal about one CC

Regards, Greg

 

uuuuh 243's have 59cc chambers and will put your cr up there without milling.

More Than Zero