Squish Testing to Confirm Piston Head Clearance

Steve wasn't able to come down over the weekend, but was obviously as anxious as I am to move forward so he spent the day driving down from the frozen tundra up north to spend the evenings with me this week working on the little race bike. It was really nice to have the extra set of hands and the experience of someone who has done this before. 

I've come to appreciate, as I've mentioned before, that this isn't as hard as it is stressful for me. This is all undiscovered country and it's nice to be able to occasionally get some confirmation from someone who has done this before that I'm doing this correctly. Otherwise it's just me, alone in the garage, talking to myself and the bike. (I guess it's OK as long as the bike doesn't answer back, right?)

Tonight's project is to conduct a squish test to make sure there is enough clearance where the piston pushes into the combustion chamber. If not, we could damage the head and the piston, bend the valves, or otherwise create a boat anchor of my NHRS go-fast parts.


Before we could get started, we needed to make sure the gasket surface was clean and flat and that we had removed as much of the carbon out of the combustion chamber as we could. This was one of those times it was nice to have the voice of experience whispering over my shoulder. When I had attempted to clean out the carbon before, I had been afraid of damaging something. Steve's response was, "Explosions are going off in there several times a second, I don't think you're going to damage anything with a wire cup brush on a Dremmel tool."

With that in mind, I dove back into cleaning the carbon.


Steve suggested we tape the gasket surface to encourage us to keep the wire brush away from where the gasket will be. Although I don't think the tape would do much to protect the surface, it was a good reminder to pay attention to where I was supposed to be cleaning.


We also needed to clean off the residue from the previous gasket to make sure there was a clean surface so the new gasket would seal. We started with a scraper to get most of the material off, but weren't satisfied that the surfaces around the openings were quite clean enough.


We took a thick piece of glass shelving for a flat surface and taped some wet/dry sandpaper to the glass so we could remove the remaining material and make sure we kept the gasket surface flat.


With the glass on the bench, we sanded (in a circular motion) until we felt like we had a clean surface to instal the head onto the new cylinder.


Now it was time for the squish test. To do the test we took several small pieces of solder and attached them to the top of the piston with a dab of grease. The idea is to measure how much the solder gets smashed when the piston goes to top-dead-center (TDC) and that will tell you what your squish measurement is.

The tolerances on these Harley engines aren't very tight, so we expected there to be a lot of play between optimal and what we have. NHRS tells us that optimal is about .030". The closer to optimal we are the more efficiently the fuel and air mixture will combust. Because we're not doing any head work this year, we just want to make sure we have clearance—so I'm ready to accept a squish of less than optimal. Next year, if we do new heads, I'll probably have the heads milled to help us get to the optimal squish clearance.


With the solder on top of the piston, we need to torque down the head as if this was the final installation. Otherwise the measurement will be off.

This is an area that I would discourage cheating. There is a specific sequence to tighten the fasteners and another sequence for torquing the head bolts. Going through the torque sequence I was surprised at just how much torque it required. I wouldn't have intuitively torqued the heads quite that much. NHRS offers a nice supplement to your shop manual for this. The Cometic gaskets I was using required a different torque sequence than the factory gaskets so had we followed the shop manual, we wouldn't have torqued them down enough.

We used the tightening sequence from the manual and followed NHRS's torque sequence recommendations.


With the heads torqued down properly, it was time to advance the piston to measure the squish.

With the bike in 5th gear and the wheel up off the ground on the track stand, I rotated the wheel to advance the piston through TDC. I did it a couple of times to make sure we went all the way through the piston stroke before we removed the head to make our measurements.

It might sound like a lot of extra work to torque down the heads, remove the heads, make a measurement, re-torque the heads for another measurement (we still need to confirm our valve clearance once we have the new cams installed), remove again, and so forth. Although I admit to feeling that way when I was first approaching this project, I now feel like the time we take measuring and confirming tolerances is well worth it. Plus it will give us insight as to where we need to go next.

Because I wasn't expecting tight tolerances, I wasn't disappointed when we didn't mash any of the solder. In other words, our squish is greater than the .62" of an inch (the width of the solder). We decided to accept that for now. If we do new heads next year, we might try to push the tolerances more toward optimal.


We're meeting up again later this evening to install the new cams so we can measure our valve clearances. Compared to what we've been doing, I think the cams should be pretty easy and straightforward.

Overall, this has been a lot less difficult than I expected. The parts fit together logically and as expected. That being said, I'm really happy to have someone who has done this a time or two looking over my shoulder to nudge me the right way if he sees me doing something incorrectly and it's nice to have someone actually answer when I ask for confirmation about what I'm doing.