Head Squish Article Questions

[FFRDave]

I read the tech article on head squish. I am seriously about to go home and take that measurement.

I noticed that the solder strand was placed on the piston going from left to right with a little hump extending forward in the middle.

It was suggested that this measurement should be done a couple times.

Shouldnt I take a measurement on the forward and back of the piston as well ? Why or why not ??

With regard to taking more than one measurement, I am concerned about the repeated assembly and disassembly of my head from the cylinder. Would I really need to fully torque the head bolts just for this measurement ? In this way, I think I might save some compression on my head gasket (o-rings). I could also simply order a new set of head gasket o-rings, but dont know if its necessary. Your thoughts ??

[RBD]

Hi FRRDave,

It was suggested that this measurement should be done a couple times.

Shouldn't I take a measurement on the forward and back of the piston as well ? Why or why not ??

It is a very good idea to check this measurement three times. This way I can get a good average of the existing squish clearance.

No, do not measurement from front to back. The reason being that the crown of the piston can rock front to back on the piston pin pivot. This is why it is very important to lay the solder dead center across the piston and parallel to the piston pin (left to right).

The reason for bending the solder to a camel hump shape is to help stabilize the solder and keep it from moving on the piston as the ends rub against the cylinder wall as the piston moves up and down when rotating the crank back and forth at TDC.

It is a good idea to put a dab of grease or Vaseline on the hump shape to help hold the solder in place on the piston.

With regard to taking more than one measurement, I am concerned about the repeated assembly and disassembly of my head from the cylinder. Would I really need to fully torque the head bolts just for this measurement ? In this way, I think I might save some compression on my head gasket (o-rings). I could also simply order a new set of head gasket o-rings, but don't know if its necessary. Your thoughts ??

No Dave, you do not need to torque the head each time, just make sure they are snug and tight by hand.

Also remove the o-rings from the head before performing the tests as it will not matter with solder test.

Thank you,
Ron

[FFRDave]

Great.

I will check the compression a little later tonight, and do the Squish measurement.

You have an E-Mail RBD.

[FFRDave]

I have an old compression tester. Its a Craftsman and has been unused in a zip-lok bag for the past 7 years.

My tester indicated 140 PSI.

I prepared by fully warming my engine and then removing the fuel tank and installing the tester into the plug hole. I opened the therottle and kicked several times. (I did not put any oil into the sparkplug hole)

Its very warm and humid today, and I am about 500 or 600 ft. above sea-level.

My bike is a brand new 2005 Gas gas EC 250 with less than 40 hours.

[jeffd]

I called Les and asked him for input on this... Even with the low hours on your bike, your rings might be worn if you were "sucking dirt". Start by getting a good baseline reading without putting a light oil in the cylinder (Ron recommended wd-40). Hold the throttle wide open and kick it as hard as you can repeatedly and note what the guage peaks out at.

Then spray some wd-40 down the spark plug hole and repeat the process. If the reading goes up measurably - it would indicate worn rings, so plan on replacing them while the top end is apart (and check the condition of piston & cylinder, etc.). Note this higher compression reading and send it along with your cylinder to Ron.

Les also said two different guys will get two different compression readings and that it was best to get a baseline when the top end is fresh and then monitor compression as the engine wears. A 10% drop in compression from the 'new' value would be a good indication to freshen the top end, etc.

Ron might have something to add here....

jeff

[FFRDave]

I went and bought a brand new compression tester. It wasnt a Snap-On or MAC Tools brand, but my readings improved drasticaly.

It was 180 to 185 without a squirt of oil and 190 MAX with a squirt of oil as measured by the new compression tester. These tests were done with the engine somewhat cooled down. I had it all heated up when I tested with the first (older) tester. Because I had taken the tank off, and did not warm the engine before my second set of tests (a half hour to 45 minits after the first set) the compression testing was done on a somewhat cooler motor.

The bike runs crisp and fast with no detectable power-loss problems.

The sparkplug electrode was a medium brown colar, but was dry when I pulled it. The threaded part was moist and black.

Ill go get some solder wire tonight after work and get my squish measurement.

[RBD]

FFRDave

I have an old compression tester. Its a Craftsman and has been unused in a zip-lok bag for the past 7 years.
My tester indicated 140 PSI.

I went and bought a brand new compression tester.
It was 180 to 185 without a squirt of oil and 190 MAX with a squirt of oil as measured by the new compression tester.

This is an example of why I say "use a good know and good quality compression tester". The average compression tester is only as accurate as the quality of it and how it is used.

One of the biggest problem with compression gauges is age, which can effect the rack and pinion of the gauge it self. Also the condition of the hose and sealing o-ring that seals the adaptor to the spark plug hole can become a problem, along with the check valve (they are the same thing that is used in wheel valve stems) getting dirty and leaking (these can be removed and cleaned or replaced).

Compression testing is subjective and can vary between users and equipment, but it can also be a good base line to evaluate things.

Again I can not stress enough, the quality of the gauge and how it is maintained and used. Compression tests need to be repeated several times to assure that you are doing it properly and getting consistent readings. It is hard to do compression tests on kick start engines as it depends on how long and hard you kick.

Just remember that compression testing can be a good tool if preformed properly and using a good known compression gauge.

Ron

[FFRDave]

A point proven by a real world case.

I knew from the start that my tester was 'suspect', but tried it anyways. If my reading had been 180 or 195 I would have taken that as a reasonable test result. My test came in lower than expected (140). A compression tester (I would expect) will fail low. If its broken, its not gonna read higher.

When my reading came in too low, I got another tester. I spent about $50.00 and got a reading that was more in line with what I expected. Im not gonna shell out hundreds of dollars for an instrument I will only use occasionaly. My $50.00 tester gave me the performance I needed, and I am happy with that.

I did a total of six checks. Three without an oil squirt, and three with a squirt of oil.

[FFRDave]

Today I removed the head and measured my squish band. Here are some of my observations:

The coolant was a clear fluid rather than the green stuff I am used to. Will the Green anti-freeze hurt anything ?

The O-Ring head gaskets are very small. When I placed them back inI had to use a dab of grease in the groove to help hold them. The smaller one was most difficult, but finaly stuck after I used the grease trick on it. I hope this wont do any damage. I did wipe the center of the water-jacket with my finger. Each bolt was torqued to 13 NM and then each was torqued to the required 25NM. I made a new gasket for the thermostat housing, and torqued it to 5 NM and then 10 NM as required.

The Squishband test went OK, if not a tad rough. I used .125" or 1/8" resin-core solder. It took a couple tries to get it to the correct width across the middle of the piston. Then, when I tried to compress it, I had to use my foot on the kicker. Then, when the kicker finaly squashed the solder the kick starter lever went down before I could get off of it, resulting in the piston rising and falling through several (2 or 3) complete revolutions. My wires did not stay in place during this process. I hope I didnt get it caught in the exhaust or intake ports. I dont need any chunks of solder rattling around in there I measured each, and they are very close in width.

I will attach a couple photos with my numbers written on them.

[Eric K]

Originally Posted by FFRDave

The Squishband test went OK, if not a tad rough. I used .0125" or 1/8" resin-core solder. It took a couple tries to get it to the correct width across the middle of the piston. Then, when I tried to compress it, I had to use my foot on the kicker. Then, when the kicker finaly squashed the solder the kick starter lever went down before I could get off of it, resulting in the piston rising and falling through several (2 or 3) complete revolutions. My wires did not stay in place during this process. I hope I didnt get it caught in the exhaust or intake ports. I dont need any chunks of solder rattling around in there I measured each, and they are very close in width.

Kicking the enigne over is likely to give you questionable squish measurements since you in effect hammered the solder instead of slowly squishing it.

It works far better if you slowly rotate the flywheel by hand.

You don't need the o-rings in to do the squish check.