"CNC racing cylinder head" big joke

gary55

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Nov 27, 2012
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Some are 6cc some are 8cc, For the 8cc to work well you have to remove .030"+/- from the jug. This reduces the chamber volume about 1.2cc and makes the 8cc head a little bit higher compression than stock, and with the jug mod along with the better chamber design makes the engine run better. Not all bolt on mods are really just bolt ons.
View media item 60560
 


extremeodd

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Mar 30, 2019
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287
To measure the gasket thickness you need a caliper, digital calipers can be had from harbor freight for $12.99. They also are needed for checking your squish.
https://www.harborfreight.com/4-in-digital-caliper-63710.html

Your squish will easily tell you if a head is increasing compression (to some extent). All you need to do is pull the plug with the head/gasket installed, take a length of solder and insert it far enough that your at the outer edge of the piston (make sure your not in a transfer, intake or exhaust port as that will cut the solder) then slowly turn the motor over by turning the rear wheel. This will crush the solder, measure the thinnest point and this is your squish band.

A good range to be in for performance seems to be 0.7mm +/- 0.1mm. You don't want to go much below this as metal expands with heating and you need to ensure that you can accommodate it. This is why you can buy copper head gaskets in thicknesses like .2mm, .4mm, etc. Personally my build has a 0.75-0.8mm squish with a roughly .5mm head gasket and I want to try a .4mm to bring the squish down to the roughly .65mm range. This is with a CDH "high compression" head that seems to have the same amount of compression as a stock head.

Compare the squish of both heads, you'll probably find the head that has less power to have a larger squish.
 

gary55

Well-Known Member
Joined
Nov 27, 2012
Messages
3,942
To measure the gasket thickness you need a caliper, digital calipers can be had from harbor freight for $12.99. They also are needed for checking your squish.
https://www.harborfreight.com/4-in-digital-caliper-63710.html

Your squish will easily tell you if a head is increasing compression (to some extent). All you need to do is pull the plug with the head/gasket installed, take a length of solder and insert it far enough that your at the outer edge of the piston (make sure your not in a transfer, intake or exhaust port as that will cut the solder) then slowly turn the motor over by turning the rear wheel. This will crush the solder, measure the thinnest point and this is your squish band.

A good range to be in for performance seems to be 0.7mm +/- 0.1mm. You don't want to go much below this as metal expands with heating and you need to ensure that you can accommodate it. This is why you can buy copper head gaskets in thicknesses like .2mm, .4mm, etc. Personally my build has a 0.75-0.8mm squish with a roughly .5mm head gasket and I want to try a .4mm to bring the squish down to the roughly .65mm range. This is with a CDH "high compression" head that seems to have the same amount of compression as a stock head.

Compare the squish of both heads, you'll probably find the head that has less power to have a larger squish.
Not trying to be critical or a smart ass, but when you check the squish you have to run the solder parallel to the wrist pin. Anywhere else will give a false reading. It can be done through the plug hole, but needs to be bent to accomplish this.
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While the squish has a direct effect on the compression it does not tell you the effect the head itself is contributing one way or the other. The geometry/volume of the head's chamber is the only way to tell this, that and of course if nothing but the head has been changed a good ol compression test.
 

extremeodd

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Joined
Mar 30, 2019
Messages
287
Thank you for pointing that out, I didn't know it needed to be parallel to the wrist pin. Whenever I get my headgaskets I'll have to double check mine.

I just figured the squish might give a rough idea of the head's potential compression utilizing the least amount and simplest to use tools to try and make it easier on the guy. But I agree, a compression test would reveal way more than a squish test and is basically as easy. Just need an autozone rental compression tester.
 

gary55

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Joined
Nov 27, 2012
Messages
3,942
Thank you for pointing that out, I didn't know it needed to be parallel to the wrist pin. Whenever I get my headgaskets I'll have to double check mine.

I just figured the squish might give a rough idea of the head's potential compression utilizing the least amount and simplest to use tools to try and make it easier on the guy. But I agree, a compression test would reveal way more than a squish test and is basically as easy. Just need an autozone rental compression tester.
No prob. I didn't know either when I first heard about a squish test. The deal is the piston rocks back and forth perpendicular to the pin so parallel eliminates as much of the rocking as possible.
 

Spare_Parts

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Joined
Nov 29, 2017
Messages
2,024
Some are 6cc some are 8cc, For the 8cc to work well you have to remove .030"+/- from the jug. This reduces the chamber volume about 1.2cc and makes the 8cc head a little bit higher compression than stock, and with the jug mod along with the better chamber design makes the engine run better. Not all bolt on mods are really just bolt ons.
View media item 60560
What head is on the left?
 
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