I keep reading and hearing stuff from time to time that makes my hair stand on end, so I thought I would give you an alternative view, and hopefully help lift a little fog.
I have a saying: "If you assume the obvious, you are probably wrong." Be careful when someone tells you something that makes a lot of sense, appeals to what you already want to believe and costs you money. For example, "My golf game is not very good because I have cheap golf clubs", or, "He is going faster because he has a better airplane than I do". Lack of practice might be the real reason, but you can spend a lot of money to find out.
Measuring top dead center (TDC) so that you can get head clearance is a pain. Someone published info last month on a tool to help get it right. The tool probably works well, but a better and more accurate way costs nothing. I can go into a long explanation about why conventional methods are not accurate, but the bottom line is that under pressure your rod compresses somewhat, so that you will have greater head clearance during running than you measure. This is ok, however, because you are looking for consistency, not absolute accuracy. If you measure head clearance the same way every time, and get the same answer on the same engine, then you have a good system. Methods that clamp the cylinder so that you can get a tight piston to TDC also compress the rod, but by a varying amount depending upon tightness of the piston and upon the engine temperature. If you are curious, take a tight engine and put the piston at TDC, then remove the head. After you have measured the TDC dimension, then move over to the outside of the sleeve and measure the distance from the top down to the case. Rock the shaft slightly, and note that the sleeve moves up and down slightly (usually .001" to .003"). This is the amount your rod compressed. If you do not know how much your rod compressed during measurement, then you will get bad data.
Now, forget all the obvious things people tell you about rod clearance and bearing play, etc. These tend to work in the same direction and therefore cancel out in the calculations. All you need to know is the stroke of the engine, which you can easily measure, or get from the manufacturer. Once you measure it, then it takes about 15 seconds to get TDC. Measure bottom dead center, BDC, by pushing the piston all the way down. You don’t even need to rock the shaft because it goes down easily. Take that number, (say, .896) and subtract the stroke, (say .708) and you get TDC, (say .188). This number is accurate, consistent, and the number you should use.
All that’s left is making sure you have the stroke correct. Find an old piston, rod, and sleeve combination, sandpaper or turn down the piston so that it will go easily up and down in the sleeve, and measure top and bottom dead center directly. The difference in those two dimensions is the stroke, which is usually about .708 for a .40. Another way is to run the engine, get it hot and kill it immediately. Quickly remove the plug and put the engine at TDC while the engine is still hot and loose. After the engine cools, remove the head and measure the TDC dimension, then rotate the engine to BDC and measure that dimension. Again, the difference is the stroke. The best way is the loose piston and sleeve. This is what I use.
Again, don’t listen to the obvious. Rod and length and clearance will not affect the stroke. You can use your loose piston and sleeve tool on all your engines to measure the stroke.
Two last things: Measure the same spot on your piston. They really aren‘t that flat after they have been run some. If you are using shims, it is best to measure the parts without the shims, then measure the shims with a micrometer and adjust accordingly. The stack height of shims is pretty hard to measure when they are piled on a cylinder, or head.
I have a long list of crap to trap. Perhaps later.
Dub
