Rebuilding 327 Heads
Article by Mark Trotta
Small-block Chevy engines are known for faster-than-usual valve-guide wear, so it's essential to measure the valves and guides when rebuilding 327 heads. If measurements are found to be within tolerance after disassembly, all that's required is a thorough cleaning and reassembly. If measurements prove to be out of tolerance, servicing the guides and valves is best left to a qualified machine shop.
Tools needed for rebuilding 327 heads include a valve spring compressor, thread chaser, gasket scraper, and micrometer. Supplies include engine oil, new seals, and a few small containers for parts.
If you'll be doing any cylinder head work, a good quality valve-spring compressor is essential. Once the cylinder heads are off the engine, this tool is needed for removal of valves, locks, retainers, and springs. They are hand-operated with an extendable arm. Most will work on a variety of cylinder head designs, so if other engine rebuilds are in your plans, it's a good investment.
Remove Valves and Springs
After compressing the springs with the valve-spring compressor tool, carefully remove the two small keepers with either a small flat-blade or needle-nose pliers. Release the tool slowly, then remove the metal shield and valve spring. If the head has been rebuilt before, there may be a shim underneath the spring. Remove this also. Remove the o-ring from the groove and slide the valve out. It's a good idea to keep each valve, spring, and shield together.
From the factory, stock cast-iron Chevy heads had cast-iron valve-guides, which were made by boring a hole through a boss cast into the head. Refinishing the guide is done either by knurling, or pressing in a new guide of iron or bronze alloy.
The machine work on these heads was fresh. The valves and seats were cut and the valve guides were knurled. Regardless, I still checked valve to guide clearance. I measured both top and bottom of the valves with a micrometer.
The knurling process is done by passing a tool down the guide, which rolls a spiral of material into the spaces of the tool. The guide is then refinished with a reamer to proper size for the valve stem to be used.
Another method for checking guide wear is to hang the valve out the guide just an inch or so (with the head laying sideways), then rocking the valve side to side.
From the mid-fifties thru the mid-eighties, small-block Chevy's used an o-ring combined with the metal shield over the spring. When the engine is fresh, this works very well, but as it ages the stem to guide clearance increases and allows oil consumption. GM used this design until emission concerns came along.
Some who don't understand this design will use the o-ring but discard the metal shield as unneeded weight. The shield was intended to direct oil onto the valve spring and away from the stem. Taking away the shield renders the o-ring useless.
Note: when reinstalling, do not put the o-ring on the valve before the metal shield. This will push the o-ring down the stem of the valve and do nothing to help oil control.
After 50k miles, SBC valve-springs will begin to lose pressure. If yours are worn slightly, a spacer washer (shim) can be placed under the spring to compensate for the loss of strength.
Shims should never be used to increase pressure, as they may bind. Heavy-duty springs should be used only if recommended for the cam you're using.
After lapping the valves, I found they were airtight.
Paint Cylinder Heads
I cleaned and scuffed the heads, cleaned them again with brake parts cleaner (leaves no residue), taped them up, and shot a few coats of Chevy Engine Orange (no primer needed). That's a set of old valve covers I use when painting SBC heads.
Read: Spray Paint Engine Block And Parts
Cylinder head installation will require RTV sealant, engine oil, and a torque wrench. Thoroughly clean all head-bolt threads. I cleaned mine a wire wheel. Also, chase the threads in the block.
Some techs like to put a small dab of Permatex Ultra Black on all head bolt threads (on the bottom 1/4"), and a smaller dab of engine lube assembly underneath the top bolt-heads. The sealant takes up the remainder of the space between the threads to prevent any leaks. Lubricating underneath the bolt-head gives more consistent torquing.
SBC Head Bolts
Re-use or replace? Although the head bolts on this 1966 327 are nearly 50 years-old, I re-used them. I can't think of the last time I heard of a SBC head bolt failure.
Make sure both surfaces are clean before setting the head back onto the engine. Your block should have two dowel pins for each cylinder head to align onto. These Felpro head gaskets are installed dry. Last step is to torque the bolts in steps. First, I hand-snugged them with a 3/8" ratchet, then torqued to 40, 60, and 60 again.