Cheap CNC Head CC

[skywalker73]

I ran #8 plug this evening. It ran good. I only ran it for about 15 minutes, but gave it everything I had. The plug looked good without signs of detonation, and the head didn't feel quite as hot. I tried to run a compression check when it was still hot, but the wife and kid caused it to cool down completely. lol I did manage to get a cold reading of 105-110 psi. I squirted a little oil down the cyl and tried again. Still around 105 psi. I'm wondering how much the pressure will increase when its hot? I will try again when I have more time.

 

[skywalker73]

I got some time to work on my bike today. I did a hot compression test and it was only around 105. I put the stock slant head back on, and the compression was 110. The stock head actually ran a whole lot better. I then took the head gasket out, and the compression bumped up to 130psi. The stock head without the gasket ran really good. It also seemed to run cooler. That cheap CNC head is pretty much junk in my opinion, or I don't have the proper combination.

 

[Deleted member 12676]

squish is the space between the piston and the cylinder head when the piston is at the top.
.3 to .6mm is ideal. More than that lessens engine power because the fuel mixture in that space burns later. It should all be in the main combustion area to burn quickly.
to test it:
buy solder from Radio Shack. take off the head. position the piston so that when you push the bike it will rise. set it just a bit down.
put a length of solder on top of the piston from the left side to the right side. put the head back on and then push the bike. if you feel no resistance from the engine having to squish the solder then your squish clearance is ridiculously too much. try not to turn the engine over much past the TDC point. that gives less chance for the solder to fall into a port. take the head off and use a micrometer to measure the squashed thickness of the solder. that thickness is the squish clearance.

 

[Deleted member 12676]

when you buy a high compression head with squish it is essential to make sure the squish band is correct. Otherwise the engine will have less than optimal power. and in your case the power is even less than with the stock head because now your compression is too low.
measure the squish band. if it won't squish the solder then coil two lengths of solder together to make it twice as thick and then test with that. subtract .45mm from the measured squish clearance and that is the amount you have to take off of the mating surface of either the head or the cylinder.
it is just plain wrong that the sellers of these heads don't make this all clear. they know but they just won't tell. they know that sales will lessen when people understand that often it isn't as easy as just putting the new head on. For some engines it is as easy as that. not so with others.

 

[Deleted member 12676]

I see now that some sellers of high compression heads do say that if you have a GT-5 engine that you need to lather down the cylinder till it matches where the piston edge is at TDC. Is your engine a GT5?

 

[skywalker73]

Jaguar, Thanks for the tips. I don't know if its a GT5. I really have no clue what it is. It was sold as a 66cc. It does have 8mm studs and a 40mm exhaust. I eyeballed the piston at TDC, and it looked like piston edge was even with the top of the cyl.

 

[Deleted member 12676]

test it for squish clearance and if it is too much then plane down the head surface using sandpaper and thick glass (or take it to a machine shop)

 

[skywalker73]

I will give that a try Jaguar. What are your feelings about not using a head gasket?

 

[Purple Haze]

Fabian has a video posted on youtube where he explains the various heads available that he has used. A very informative video.

 

[Deleted member 12676]

for use without a head gasket the two surfaces have to be planed to perfection first. Then put some copper coat on the mating surfaces.
I always use a gasket, with my preference to fiber versus metal.