[seattle smitty]

Clay the squish areas and see if they all conform and have about the same measured squish. If it varies, do what Mr. Weiss says, even them up. Then mill the cylinder deck surface as needed to get to .035-.040" squish. When you are claying, find out also how close the valves get to the piston; it's possible that after you tighten up the squish, the valves may get close enough to the pistons that you need to deepen the reliefs.

Groove the heads if you want, but you might alternatively try die-grinding fire-slots in the piston crowns, since pistons are cheaper to replace than heads if you don't like the results. Fire-slots were the earlier (30-40 years ago) incarnation of Singh's grooves, were tried by many people, and were eventually abandoned as ineffective. I don't think this entirely discredits either technique, but I believe the effects will prove to be useful only in very specific combinations of combustion chamber and piston shapes and conformity.

Chamfer or round off sharp edges on the piston crowns and then mirror-polish them.

Check the new volume at TDC, and calculate your new compression ratio, which should be a bit higher after tightening the squish, even though a groove or fire-slot will have some small extra volume. Maybe you'll be where you want to be. If the compression is now too high, take material out of the squishband where it shrouds the valves. But DO NOT install a thicker head gasket or otherwise increase the squish dimension. A tight squish (quench), just short of letting the piston contact the head at peak rpm, is your main anti-detonation protection here.

Some of us think that heat transfer from the heads and cylinders can be improved a little with coatings. The rest of this post is about putting a heat-dissipating (we hope) coating on your air-cooled engine, the motivation being to help the engine accept higher compression without detonation. Blast the exterior of these parts with salt, glass-bead, walnut shells, or some not-too-aggressive medium. Scrub the parts clean (I use TSP and hot water, an SOS-pad or Scotch-Brite, and rifle brushes). Dry the parts and quickly rub some WD-40 into the cylinder liners, but don't get anything oily on the aluminum.

Some guys spray stove-black on the cooling fins, but I like a product from KG Industries (formerly Kal-Gard, which was a major advertiser in Cycle magazine during the heyday of 2-stroke roadracing in the '70s) called Gun-Kote. Order the flat black variety, about $35 with shipping. Gun-Kote has the additional advantage of being a good barrier against corrosion. If you want technical or applications advice, phone KG, ask for Chris (who will be running the company in a few years), and tell him Smitty sent you. KG also sells a heat-cured moly-coating called Gear-Kote (marketed by Kal-Gard also as Piston-Kote) which I use on piston skirts, carb slide, rotary-valve cases, etc..

I use an old record player at 33 1/3 rpm for spraying round parts, and an old portable oven for heat-curing. Pre-heat the oven to 300F. Shake the Gun-Kote extremely well, and I personally always warm up the can in hot water first (this also works well for spray-paint cans). Right before you are going to applay the coating, do a last treatment of the aluminum parts by acid-etching them with mag-wheel cleaner for a minute or less. Wash off the acid, then heat the parts in the oven or with a torch until all moisture is boiled off or evaporated off the parts. Now let the parts cool . . . but not all the way; you want them between 100-125F when you spray the Gun-Kote. Load your detailing gun or air-brush with the Gun-Kote, and spray the warm parts enough to cover the metal but not a heavy build. When you have all the parts coated, let them air-dry for 20 minutes or so, then put them in your 300F oven. Heat-cure for 60 to 90 minutes.

These are the tricks I'd use if I wanted to build a little extra compression into that engine. Obviously, close monitoring of fuel mixture and spark timing are always primary, along with using the appropriate heat-range sparkplug.

(How ya doin', Pinhead!!)