Thread: Assembled Squish
11-04-2007, 11:53 AM #1
This is a piece I received from Randy at Watcon. I'll leave it here a few days and then copy it to the tech section. Great explanation here.
Assembled Squish check..
You need some "solder" that is approx .060" -.070" thick, you will gently place it across the top of the piston and install the dome with gasket. Roll it up to TDC, and rock it back & forth a few times (by hand) to compress the solder. Take off the dome and carefully remove and measure the thicknesses of the solder that laid on the outer edges of the piston. You will be measuring at the outer perimeter of the bore… (Where the solder is thinnest). This is your "Assembled squish clearance".
Don't use the electric starter!
Don't use thick solder, as it won't compress fully. If the solder is too small, and it doesn't get compressed at least .005" go back to home depot for thicker solder.
Use calipers, and measure carefully. It's normal to get a variance in the readings of a few thousandths from the piston slightly "rocking in the bore". Just write down the numbers and average them out...
11-04-2007, 04:01 PM #2
11-04-2007, 09:40 PM #3
- Join Date
- Oct 2006
- Cleveland OH
Does anybody know what the limits are? I always read to check your squish, but what's good?
11-05-2007, 12:36 AM #4
.055 is safe on pump gas for a fuji, .045 on a domestic. the gas nowadays can be pretty shiddy.
11-05-2007, 12:55 PM #5
It's also best to place the solder directly above the wrist pin to minimize the piston rocking in the bore. As for the limits of clearance...totally depends on what the compression ratio is, peak rpm, pipes, combustion chamber shape, bore size, squish band width, etc.
11-06-2007, 12:44 PM #6
Not to be difficult but I have some thoughts on this and want to know how far off my thinking might be.
Wouldn't you rather check the squish as the engine sees the piston coming to TDC?? In otherwords, with the rocking motion or natural movement of the piston to TDC?
Kind of like the guy who makes all his WOT runs in a straight line and then wonders why it fails on a long left bender.
Or kind of like the guys running and testing the engines in controlled situations or on a dyno and then wondering why it failed in "real world" use.
It seems to me I wouln't want it "perfect" while testing it but rather with the movement so it can possibly see more squish as it rocks back and forth??
Or does it simply not matter?? I mean how much rocking is actually going on?
11-07-2007, 09:46 AM #7
Al, you're never difficult
When the piston is up to operating temperature, it fits the bore much tighter, so the rocking motion is very limited, if non-existant. When the piston rocks when checking squish, it gives a looser reading than if it is checked over the wrist pin, so you may end up with a squish clearance being too tight on a warmed up motor. This may be splitting hairs, but any reputable engine builder I've ever dealt with specifically says, "check it above the wrist pin", so I do.
11-07-2007, 10:55 AM #8
Although if you measure both sides and take the average it may be the same as a measurement up the center.
O.K...........I've got an engine I'm going to test tonight.
11-07-2007, 11:13 AM #9
As others have said, place the solder centered and parallel to the piston pin as possible, this will keep squish measurement deviation from piston rock to a minimum.
11-08-2007, 11:05 AM #10
I personally agree with Al.. But I think the test will be inconclusive..
It really doesn't matter how you perform the test, but that you adhere to the same methods every time.. Just because everybody has done it the same way for the last 20 years, doesn't mean it's the best method.. Many of the engine building techniques are passed down from other Motorsports, automotive, etc and are just accepted as the way to go... This isn't always so in our industry with slightly different parameters
How did your test results go?
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