Measuring Squish After Popup

[bacon K5]

Tried searching, but didnt find an answer yet. I bought a project 394 that will either become a ported 394 or a ported 395 big bore depending on the condition of the current jug (just got delivered today).

Anyway, after doing a popup piston, what is the proper way to measure squish? Would it still be the solder method, but measured at the pinch point indicated with the red arrow below?

Is there any standard amount of machining to do on the piston? I think I have all the timing numbers figured out, but not sure for the piston top. Planning a torquey work saw vs a race saw like some of the others around here.

 

[tacomatrd98]

The piston is to be cut at the same or slightly lesser angle than the squish band. if the band is flat. the ring around the edge of the piston should be flat as well. If it were cut as your picture shows it would not direct the gases out fo the squish band and into the chamber properly. Ideally, you want the squish to be the same across the width of the band or slightly tighter at the side closest to the cylinder wall.

And yes, just use solder to recheck it and measure the thinnest part.

 

[blsnelling]

I cut it flat and put a 45° on the edge of the pop-up. Squish should always be measured at the very edge of the piston against the cylinder wall. That's where it should be the tightest.

However, I don't recommend using a pop-up on a 395. If you do, compression will be well over 200 PSI. If you simply set the squish to .020", you'll be at about 190 PSI, right where I like it. A .035"pop-up will put you at about 215 PSI.

If you do decide to do a pop-up, don't make the crown any thinner than .100", as measured from the top of the piston edge to the top of the first ring groove.