This is what it takes to fit the larger chamber "R" heads on a stock block bore.

 

Install heads on a bare block and you will see that the chambers over hang the block deck slightly.   You will notice in the picture below I have placed four red lines where you will need to mark the cylinder bore with a marker.

Here are the marks on the block where the chamber starts to overhang the cylinder bore.

Measure between chambers on the GN1-R heads. This will be how thick the block will have to be between the cylinders.

Here is the head gasket laid on the head. I'm using Fel-Pro 1007 gaskets that require receiver grooves.

Here is the gasket laid on the block. Mark the deck of the block with a marker as a cutting guideline. This marker just so happens to have a wide tip that gives a nice template to work with.

With the previous measurement taken from the heads (between the chambers), you can now compare this measurement to the block and with the marked lines.

To keep from grinding too low into the bore I installed a piston ring in the cylinder at the top.

Then I installed another piston ring on a piston and pushed it down into cylinder which also pushed the ring down. The ring on the piston will stop the piston from going into the bore while it pushes the other ring down. This is to give you a guideline of how low to cut without going into the top ring seat area.

Mark the area above the ring and do not grind below this line!

Start grinding at an angle to allow incoming air to have a smooth transition when entering the engine.

These are the finished cuts with the gasket installed on block.   Gasket installed on head. Compare this one with the one above.

Now install the heads on engine to check if the grinding clears chamber to allow smoother airflow.

Now you will have 500 more HP because there's no ledge for incoming air to get trapped on!  Well, not that much HP but this can't hurt IMO. Wasn't that EASY!!  A lot of people don't know this but even a STOCK cylinder head combustion chamber over hangs the deck surface too....hmmm....something to think about..... :~)

 

NOTE: The "R" heads are for a 4.1L engine with a larger bore diameter and weren't purposefully built for the 3.8 liters smaller bore. You probably already know this if you actually have a set...which also means you should already know what and how to make them work on a 3.8.

I bought these because my plans are to buy a TA aluminum block in the future.