[Grady Clay]

Guys,

The critical issue in fitting non-standard configurations is the piston-to-head clearance (deck and squish) and the piston-to-valve clearance.

If you have an absolutely stock engine and need to replace the pistons and cylinders with stock OEM parts, that is a bolt-on operation. As soon as you stray from that, you need to start paying attention to the clearances. The higher compression and more radical engine, the more careful attention must be paid. This may (in extreme cases) include building the engine, running it, disassembly, measuring, adjusting the clearances, running again, etc.

Number one rule; the parts (piston-to-head & piston-to-valve) can never, ever, ever have contact. Close is OK but no touching is allowed.

The reasons are:
There must be a “squish” area around the perimeter of the piston-to-head that is very close. This forces the combustion gasses into the actual highest compression area of the combustion chamber at TDC compression. The ideal would be to have zero clearance in the squish area but mechanical realities prevail. There is the mechanical stretching of components (crank, rods, piston, cylinder, head, case), differential thermal issues (the hotter parts expand more), the transient issues (some parts heat up sooner than others), and wear. This is absolutely a NO CONTACT sport.
Some say to lower compression ratio, just add spacers under the cylinder. NO, never ever reduce deck/squish. This provides an area (used to be proper squish) where detonation can occur, and just in the wrong place. At TDC compression the unburned mixture in that area won’t burn or will be extinguished, it’s too little mixture and too much relatively cool aluminum surface. It will be prone to detonation! Of course this is the area just above the top ring land. Guess what, this is like a hammer hitting the top of the piston and can collapse the crown of the piston around the top (compression) ring. As soon as the ring isn’t free to move, compression is lost and much worse things follow. Most have seen pictures of detonation holes in the center of pistons. That is from extreme detonation. The loss of squish allows insidious detonation that is usually not audible. The squish (deck) around the perimeter should be the closest clearance, clearance should be the same or slightly increase toward the center. Never should there be “pockets” of increased squish between the perimeter and center.

So, how do you measure these clearances?
To start with, I like putty – plain old modeling clay. Add a few extra shims under the cylinder, and carefully put putty on either the head or the piston for deck/squish, your choice. Have the surface exceedingly clean so the putty will stick. Give the opposing surface a light coat of Pam or silicone spray. Don’t put much more putty than the clearance you are expecting. Too little, you can repeat the measurement – too much and it won’t turn over. Put it in at least four (preferably more) locations around the perimeter of the piston. Use some old rings on the piston (piston must have rings for this). Torque the head(s) in place (you don’t need a head gasket), and carefully turn the engine over just past TDC – just once, not stirred.
I use an X-acto knife to cut the putty and measure the thickness.
When you know the approximant clearance and know there is appropriate clearance (after machining?), then remove the additional cylinder shims. Repeat the process. At this stage, use rosin core solder of the appropriate diameter. Remember the symmetrical spacing around the piston because the piston will rock in the bore.

Only after you have the deck/squish exactly where you want it do you light into the piston-to-valve clearance. You will need to assemble each bank with heads torqued in place and the cam housing. You can use just the outer valve springs, but you must time the cams. Do the cam timing very carefully so you don’t contact the putty while the cams are not in perfect time. I like using a solid tensioner for this. Just use putty as solder can bend a valve. On most engines (particularly radical cams) the valve dynamically chases the piston or visa versa. Pistons that have full valve relief are safer for the valve than pistons with only a slight relief cut-out. That is so when there is valve-to-piston contact, the piston closes the valve and doesn’t bend it.

OK, back to the overall picture. Having close clearances (deck and squish) are not necessarily only for high compression, it is for proper combustion. This is as true on street cars (maybe more) as it is on the best race cars.

Above all: NO CONTACT, just close.

Best,
Grady