I called him this morning and told him to bore it to .001, and he said okay. Hondaman wrote me a very lengthy response as to why he suggests the clearance that he does in his book. If it's not posted anywhere already, it really should be! Here it is:
"First thought: find another machinist. There are NO motorcycle engines out there with more than 0.0018" new clearance, never have been, except the Harleys. Their engines are made very differently (and their reliability shows it...).
Every engine I've seen bored to those kinds of tolerances burn oil almost from the start. They also suffer from piston slap and overheating, because the pistons cannot transfer their crown heat into the cylinder bores (through the oil on their skirts) unless they are close-coupled. The worst situations are the ones that get 836cc pistons and 0.002" clearance: they burn oil from Day one.
The metalurgical "skinny" here: the aluminum on the outside of the cast-steel liners expands faster than the steel as it heats up. (This is why there are little O-rings around their bottom ends, keeps oil from getting up in the gap and make the liners slippery, and/or leak out at the head). As the engine heats more, the steel "catches up" about 5 minutes later and closes the gaps. The pistons heat first on their crowns, which is why the crown area is so much smaller than the rest of the piston, allowing it to expand and tightly seal the rings against the bores AFTER the steel is hot. The sides of these pistons are more than 0.010" smaller than the slipper skirts, and the material they are made from distorts out the sides more because there is more cross-section where the wristpin resides. After about 10 minutes of run time, the whole piston is much more round than when it is cold as the result of all this shape-shifting, which is one of the reasons why the bikes are pretty cold-blooded for the first 5-10 minutes.
Once the heating is complete, the skirts run about 0.0004" from the bores (racers in the 1960s used "hot clearance" numbers like this to determine their racing conditions, so they sometimes show up in the odd manual). This provides a nearly perfect oil stripper film on the skirts, helping the oil rings out by not sending quite so much oil up their way. Oil only needs to be 0.0001" thick to do its job when clean: if you let it get dirty enough it will scratch the pistons on these skirts which can raise edges that cut through the film and drag. This "lube breakdown" is what causes piston skirt seizures. On severely overheated engines, the pistons heat up faster than the bores can expand away from them: this often happens on air-cooled engines that get pushed hard when cold, or have excessive carbon buildup. This causes the scratches to appear, which leads to later damage or seizure.
Conversely, when the gap between piston and wall is too large, the oil cannot form a film, because there is not enough of it available in this area. If there were, the oil rings would be flooded and much of it would get into the combustion chamber. This is far worse than too tight in many respects, and more seizures occur this way: the pistons score up near the oil ring in this scenario, not on the skirts. This is the classic 836cc "did it my way" failure...
If I had $10 for every mis-bored Honda engine I have seen at the hands of "knowing" machinists (which immediately needed to be bored again), I would not have bothered to write that book...
Here's an example for you: I bored mine at 55,000 miles, to 0.0006" to 0.0008" clearance for 0.25mm oversize pistons, just to make the bores perfectly round and recover the lost power. Over 85,000 miles later, the engine has perfect compression still, does not burn oil, and has no piston slap. I will know in a few weeks (and will be posting pictures) of the condition of the pistons when I pull them for my Z1 replacements (like yours), but I know they will be fine.
If all else fails, tell the guy to bore it to zeroto 0.0002" clearance and don't hone it. Then go to Checker Auto and get a 3" long, 3-stone hone for your drill. It takes about 10-15 minutes per cylinder to make it 0.0008" clearance (with light oiling like WD40), check it about every 5 minutes (and clean the stones then with kerosene or Coleman camp stove white gas) with a 0.001" feeler gage alongside the front of the piston on the exhaust side of the bore. That's how I have started doing them, as I have grown tired of arguing with people who don't have 40 years of background with these engines. :-)
HM"
