During my winter rebuild, I felt it was my duty to "help the economy" by purchasing a custom ground solid roller cam for the GT. Since my pistons were already .002 above deck, I fully expected that my new cam might have some piston-to-valve (PTV) clearance issues. The valve opening and closing events, when combined with the .655" valve lift, proved that I was indeed right. Thanks to a little advance research, I was able to implement a low buck approach in order to deal with this common cam swap problem.
A little technical information:
Basic PTV clearance consists of 2 different concepts. The most obvious is that the valve must have sufficient clearance when it is opened by the cam so that the valve does not hit the piston. The standard recommended allowance for the intake valve is .080” minimum clearance and for the exhaust valve the specification is wider at .100” minimum.
The second concept is that due to the piston rocking in the bore, a possible side-to-side valve movement in the guide, and normal expansion from heat, the notch in the piston must have .060” additional radial clearance beyond the diameter of the valve. As an example, a valve with a 2.08” head should have at least a 2.20” diameter pocket in the piston. (2.08 + (.060 x 2) = 2.20)
So I used my piston stop, a degree wheel, light checking springs, and a degree wheel to first check PTV clearance.
I found that the 2.08” intake valves had over .100” of PTV clearance, which was fine. The 1.60" exhaust valves measured only .080”, which is less than safe for a motor. I had no choice but to notch the pistons deeper and wider. On the net I found that it was possible to purchase (or even rent) a professional notching tool from Isky or buy one from Lindy. Instead I decided to take an low buck approach and simply make my own.
This notching tool consists of a cheap intake valve, a good adhesive, a locking collar, and a strip of cloth backed 50 grit belt sanding paper. Because I wanted to make sure that I had sufficient radial clearance, I ordered a 1.72” diameter (11/32” stem) Chevy intake valve. (1.60 + (.060 x 2) = 1.72)
For this tool, it doesn't matter if you use an intake or an exhaust valve--generally an intake valve will be cheaper. Total cost of the 1.72" valve was under $5 at Autozone. A huge 2.21” (11/32” stem) BB Chevy intake valve was less than $10 and would have been used if my 2.08” intake valves needed clearance:
A link to this Melling catalog is here: http://www.mellingengine.com/Portals/5/pdf/pdf_catalog/valve-progressive-size-chart.pdf
I found a dis-guarded sanding belt at a friend's woodshop and I already owned the Loctite industrial super glue. Quick dry 2 part epoxy is a suitable substitute.
I cleaned the head of the valve with lacquer thinner and then put an ample amount of the industrial super glue on it. I placed the valve face down on a strip of the sanding paper and then slid several sockets over the stem of the valve for additional weight. This assembly was left to dry overnight:
The next morning I took a pair of industrial shears and cut the excess sanding paper from around the valve face:
I installed the valve into the head (using a little lube on the stem) and temporarily retained it with a locking collar:
Note that the oversized head of the valve will always remain above the valve seat and it will never touch the seat.
Next I prepared the block. I rotated the crank until the piston was in the location where the valve was at its closest (in my case 15BTDC) and taped off that cylinder, leaving the subject valve relief untaped:
I installed an old head gasket on the block that was the same thickness as the one I was going to use when the block is final assembled. Then I installed the head on the block using 3 head bolts torqued to 50lbs.
I pulled the valve stem up as high as it would go, then I loosened the locking collar so that I could slide a .025” feeler gauge between it and the valve guide. This gave me a way to determine approximately how deep I had notched the piston:
Next I chucked up my cordless drill to the stem of the valve. I slowly spun the valve while adding moderate pressure to the drill. It took approximately 2 minutes to sand the piston away enough so that the locking collar would touch the top of the valve guide.
Before I removed the head, I used a shop vac to gather up as may loose filings that I could through the ports. A vacuum is a very important part of this process because it helps to remove the loose flakes:
I then removed the head and checked my work:
As you can see, not all of the filings can be removed through the ports with the shop vac. Again I used the shop vac and cleaned up the block and the head, followed by a clean rag. Next I reassembled the head with the checking springs and checked my work again by measuring PTV clearance. This time I found that the position of the locking collar did not allow for enough clearance, and I still needed another .010” to reach MY goal of having at least .105” exhaust PTV clearance. Again I reassembled the head with the cutting tool and lock collar. The head was reinstalled on the block and this time I used a .010 feeler gauge to set the desired depth:
I spun the valve with the drill until I had zero contact at the collar. The head was removed and the clearances checked. This time I was happier with the results. Here are before and after photos of the piston valve relief:
Satisfied with the clearance, I removed the tape-using the shop vac to catch any filings. I also took the time to rotate the piston down and then thoroughly wipe out the cylinder bore with a rag:
I moved on to the other 7 pistons and repeated the procedure. Once I had checked and clearanced all 8 pistons, I wanted to remove some of the sanding scratches that the sand paper left on the piston. The deck of the block was fully taped (as was the open areas of the block) and then I used a fine sanding drum on the dremel to smooth the deck and taper the sharp edges:
Here is the final product (which is a lot smoother in person than this closeup photo appears):
After I cleaned up the block, pistons and heads, I reassembled the head so that I could do a clearance mock up using clay. I put a little strip of clay on both valve pockets and then I installed the head on the block:
The valves were adjusted to zero clearance and then I rotated the crank 2 full revolutions. The head was removed and I laid the smashed putty on a flat surface. It was cut into strips to check the thickness of the impression and to verify the radial clearance:
It goes without saying that there are times in our hobby when you spend a few more bucks than what was anticipated in order to get the desired results. There are also times when a simple idea and a little research can reap these same results, without breaking the bank. I am happy to report that with a little patience and attention to detail this inexpensive procedure can get you out of a predicament, plus give you the satisfaction that you used old fashioned hot-rod ingenuity instead of modern technology in order to resolve an undesirable situation.