340 build
What I dont get at this point is why do I need to lay the metal across the bore or across the top of the piston and measure how far up off the deck the metal is pushed, why cant I just zero out the piston at the top of the deck by fastening down a piece of straight metal across the top of the deck and across the top of the piston like I have done in previous pictures and then remove the metal and then set my dial indicator on the deck with the plunger sitting in the center of the piston and then measure how far further up the piston comes out of the hole?
No reason that you cannot do it this way with pistons that pop above the deck.
The whole point here is to:
- Measure the compression height of the same piston/rod assembly in each hole to find out the total variations compression heights due to the deck variations AND the crank variations.
- Then, put all of the piston/rod assemblies 1 at a time into the same hole, and measure the compression height variations. This 2nd step allows you to ID the variation in length due to JUST the variations in rods and pistons.
- Once you have both sets of data, then:
-- You marry up the shortest piston/rod assembly(lowest compression height found in step 2) to the hole that gave the tallest compression height in step 1.
-- And then the tallest piston/rod assembly can be married to the hole that gave the lowest compression height in step 1.
--Then you try to match the intermediate compression height piston/rod assemblies into the middle height holes.
This is all to to try the get the compression height variations minimized. If the total compression height variations are down to .005" versus a .010" overall compression height variation, then you have decreased the compression ratio variation by about .1 point. You won't feel this in engine operation or performance directly, BUT it allows you set the quench gap more closely and accurately IF you have closed chamber heads, and you want to optimize quench and its improvements in detonation resistance.
If you have an engine with open chamber heads, you won't have a quench gap, so this procedure is a bit of a waste of time for open chamber heads IMHO.
AND, if you have an engine with stock type pistons, with notches on the piston tops that point towards the front of the engine, you are not supposed to swap pistons side to side in the engine. These pistons have what is called 'pin offset', where the pin is not centered between the thrust faces of the pistons. This pin offset is for smoother engine operation, and the offset goes one way on the passenger side, and the other way on the driver's side. Pistons like the TRW/SpeedPro forged units that are in the 340 discussed here don't have offset. Most performance aftermarket pistons don't have any pin offset either.