September 4, 2023 - Engine Update - This update is long overdue.
After I got the engine back from the machine shop I mocked up the first cylinder piston because I had the machine shop zero deck the block and I wanted to take measurements to get an idea of what I had for valve clearance. I got a degree wheel and rigged up a pointer. Installed the camshaft and a lifter. I used a sharpie marker as a pushrod to measure the lift of the camshaft and bought a dial gauge bridge to measure the drop of the piston (Picture #1 minus the dial gauge bridge). I played around measuring a bit until I was comfortable with what I was doing and then took measurements every 3 degrees out to 18 degrees. I also took measurements going back from 18 degrees to TDC to make sure the measurements were repeatable. They all were to within .001.
Once the measurements were taken, I graphed piston drop vs valve lift (pushrod lift times 1.5) so I could see what was going on. Graph is shown in Picture #2. So obviously, at the start the valve is opening faster than the piston is dropping. By three degrees it has opened about .01 more than the piston has dropped. By 6 degrees, it's catch up rate is slowing and it's opened about .014 more than the piston has dropped. This number stays the same to 9 degrees and by just after 12 degrees the piston has dropped as much as the valve has opened. After 12 degrees the piston drop rate just keeps climbing over the valve open rate. I included a line in the graph for a linear valve open rate just for comparison and the valve is opening pretty close to linear through 18 degrees. The camshaft spec has the lift starting at 1 degree before TDC, but I did not notice any measurement change between 1 degree before TDC and TDC.
So, now I know that out of whatever clearance I end up with, .014 is how much the valve is catching up to the piston in the first few degrees. After research on the forums, I would still need clearance for thermal growth and movement of the crankshaft. (Interestingly enough, I work with natural gas compressors. The big Ariel frames (7800 HP) allow up to a 10 rpm variance in their torsional analysis calculations, which means that the compressor frame crankshaft can twist enough to differ by 10 rpm from the driven end to the free end.)
In taking all these measurements, I had found out that the first cylinder piston was above the deck by .012. So I mocked up the remaining pistons, and sure enough, all pistons were above the deck at TDC and the first cylinder piston was the least above the deck. So I went and measured six ways from Sunday on all cylinders to see how bad the problem was, including measuring actual stroke lengths on all cylinders and how far each valve was recessed into the head. All measurements are in Picture #3 for interests sake.
So, in the end, I have the pistons back at the machine shop to get them shaved back down to zero deck. Machinist isn't sure what happened, but is doing the work for free. Think he had outsourced it. I'll re-mock up the engine and measure everything again. I think I'll end up with a total clearance of approximately .065 at TDC for the smallest valve recess value, which will translate to approximately .05 of clearance at the point the valve is closest to the piston. This is .01 less than the minimum recommended by those smarter than me in the forums, so I'll consider fly cutting the last .01 into the piston.
I've found this bit working with the engine very insightful and satisfying learning. Having a ton of fun with it!
