How do you check squish?

Steelie90 said:

064 on the bench, cut the chamber and then base to get my desired squish. Landed on .023 PTO .0235 flywheel, l checked 1 side at a time.

I seen a post recently where squish was checked using 2 small pieces greased to the piston. I liked the idea, eliminate “rock” in the piston and bearing tolerances. So I thought I would check that way also. After doing that it came out .017 both sides.

Pretty surprised by the difference.
This is with new crank bearings and wrist pin bearing installed. This piston does have skirt wear and cylinder is well used, which I’m sure could contribute to the vast difference in readings. But I was still surprised. Makes me concerned about a few saw I built with squish at .020 checked 1 side at a time.

How do you do it? I’m thinking I will change my way from here forward. Few build pics. Forgot to take some of squish process.
View attachment 418812View attachment 418813View attachment 418814

Click to expand...

Ketchup said:

If you’re checking squish one spot at a time then the best place is PTO or FW sides. But it’s always more accurate to use four points. PTO, FW, Intake, Exhaust. The bad news is that with the rings installed is even more accurate.

My usual process is to check the PTO side before disassembly. I usually check it a few times in the machining process using just the PTO and FW. Before final assembly I test all four points (without the rings).
Make sure you use the right piston and base gasket for all measurements.

If I’m just checking squish, I just check the PTO side with the saw assembled. It’s not quite as accurate, but I don’t need it to be.

Click to expand...

Steelie90 said:

Aw yes, rings. Good point!

Four point check before final, I like it. Definitely be adding this to my process.

When you’re just checking a single location for reference Ketchup, what do you like to see?

Have you done enough to say 25thou in one spot usually equates to .020 in four point test. Or are there too many variables to that? Just curious.

Click to expand...

huskihl said:

I always check on the flywheel side but I make sure to roll the piston over top dead center several times to make sure it’s not give me a false reading. I honestly don’t see how you could be off .006”. Even if the piston had .003” skirt clearance and the height of the piston is the same as the bore dimension, you could still only have .003” error if you checked it over the intake or exhaust. Something in the wrist pin, rod, or bearing has to be flexing if you’re getting that much differential. Or maybe the squish band is tapered and you read the dimensions in two different spots

Click to expand...

Steelie90 said:

Admittedly I did not roll over several time during first checks. Could have contributed to discrepancies. I wish I rechecked before I recut. I bought home my good mic to check.

Tonight I cut .007 from band, and did a 4 point check with .050 solder, all I currently have on hand.
.050 to big?

.0242 intake .0240 Exh .0221 FW .0220 PTO.
Not sure on .002 difference?? Thoughts?

Doing a single point check with several rolls
FW .0277
PTO .0270

Checking skirt clearance, .003 does fit in the lower jug but tight. .0025 fits with a drag. Too loose?

Rod bearing no noticeable radial play, pin bearing new and feels good in bore, pin is used till new one arrives at dealer. Very snug pin fit in piston bosses.
View attachment 418875View attachment 418876View attachment 418877View attachment 418878View attachment 418879

Click to expand...

drf256 said:

Your skirt clearance should be ok, but it’s getting close.

Using .050 solder is just gonna be super inaccurate IMHO. I use 3 different sizes for checking squish. It’s surprising how much one can be fooled by any sized solder. If I have to twist solder to get a reading before a base cut, I almost always take 5-10 less off the base than its says I should. Generally it’s safer that way. Then I take my Final Cut after figuring out where I am with different solder as I get closer to goal.

Click to expand...

Ketchup said:

Definitely get smaller solder. I like 0.032”. I can’t remember the chemistry off hand but it’s softer than a lot of solder. Just a personal thing, but if all four points are stuck in the chamber I feel like I had to use too much force to turn it over.

Click to expand...

drf256 said:

Order .6mm off of Amazon. That’s what I use. I leave it on the roll, stick it through the plug hole bent or straight through the decomp port, and get my reading. Then, just cut off crushed part w a razor blade and roll it back up. I’ll probably never get though the roll in my lifetime.

Click to expand...