shaving heads

[fennerford]

im going to shave the heads on my 305. they are the stock heads, so i was wondering if anyone had input about how much to shave off or the measurements on valve clearensce.

[Scotch]

Originally Posted by fennerford

im going to shave the heads on my 305. they are the stock heads, so i was wondering if anyone had input about how much to shave off or the measurements on valve clearensce.

Wow - lots of 305 heads were thinwall castings with relatively thin decks. I'd be careful how much I shaved off..there ain't much to start with!

I'd go to an aftermarket head with a better chamber, better port design, screw-in studs, bigger valves, and a thicker deck...but it ain't my money either.

~SP~

[fennerford]

im on a low budget and i can mill the heads and we got a cnc in the high school shop class. i figured if i could squeeze some horsepower out unil i could afford nice heads id tryso still wondering dimensions.

[Scotch]

Well if it's a school project, how about we make a lesson out of it?

Calculate your compression ratio, and adjust the size of your combustion chamber to acheive about 9.75:1.

Check piston-to-valve clearance and make sure you have at least .080-inch at the closest point.

You'll probably end up shaving like .060-inch off the head - make sure it's perfectly level front-to-back, side-to-side, and corner-to-corner or else your combustion chambers will de different sizes. While you've got it all apart, clean up the chambers and the port roofs. You really can't hurt anything by doing so.

Have fun, take your time, and don't expect a huge power increase. You might notice a difference, but if these are the only mods you're making, consider it more of a learning experience than a horsepower quest.

Good Luck with it!

~SP~

[fennerford]

how do i measure piston to valve clearence?

[Scotch]

Put a thin layer of clay into the valve reliefs of one piston. Install a single head with and old head gasket, torque it down, then install one intake and one exhaust lifter, pushrod, and rocker, on the cylinder with the clay. Turn the engine over two revolutions, then disassemble it again. Measure the thickness of the clay at the deepest point of the valve impression into the clay - this will be the minimum piston-to-valve clearance.

.080 is the minimum I'd feel safe with, although I've run as little as .060 before and not had problems. If you see silver through the clay, that means the valve is actually hitting the piston, and you need to machine the valve reliefs in the piston a little bit deeper to get adequate clearance.

~Scotch~

[fennerford]

thanks thats what i was reading about last nite

[fennerford]

.249 inches for the valve to pistin clearence. What do u think for shaving it off?

[Scotch]

Now you have to do the homework to calculate your compression ratio. This is basically a comparison of the amount of space in the cylinder/combustion chamber when the piston is at it's lowest point (bottom-dead-center, or BDC) and when the piston is at it's highest point (top-dead-center, or TDC).

It's usually easiest to calculate this separately, meaning the volume of the cylinder can be determined alone, then calculate the volume of the compressed head gasket, combustion chamber in the head, and any dish or valve relief in the piston.

Once you've got all of your dimensions, calculate the ratio of the volume at BDC vs. TDC and this will be your compression ratio. You can then determine what the TDC number should be to acquire your target compression ratio. The TDC target number will be smaller, which means you'll want to take some of the volume out of your combustion chamber.

Every slice you take off the bottom of your cylinder head makes your combustion chamber smaller. Typically, I measure the volume after every .010-inch slice removed from the head. Because the shape of the combustion chamber is tapered, you'll remove a little less from the chamber every time. Once you've reached your target chamber volume, you'll know your target compression ratio has been reached. You've got pleny of clearance to the piston, and I'd guess you'll remove .040-.060 or so from the head to reach your target number, which should be about 9.6:1 for pump gas.

Once you've got the heads shaved, use a file to cut the sharp edge all the way around each combustion chamber, bolt hole, and water passage on the bottom of the head. Sharp edges are where cracks begin. You don't have to massage these edges round (doing so may affect their ability to seal) but just take the sharp edge off.

Clean the head completely before reassembly, as the shaving process creates plenty of shavings and grit, which will find their way all over an into your heads. After a good cleaning, reassemble the heads and they should be ready to reinstall.

Once the heads are reinstalled, you'll need to reset the valve clearances. If you're running hydraulic lifters, this is as easy as adding one half turn past 'snug' with snug being the point where the pushrod no longer turns freely.

I hope this helps...

~SP~

[fennerford]

how would u recomend measureing the cylinders and cylinder head? ive seen it done with liquid? is that what i should do?