Whats up with my compression?

I would go back and torque all the cylinder and head nuts. Could be loose and sucking in some air. Also make sure you have good pipe to cylinder connection. As well as checking your exhaust manifold.

Not sure what it's supposed to be when new, compression that is, do you know? Was it 145-155 before you tore it down? Did you measure the cylinder, I do and compare all reading to specifications. I hone mine with a ball hone with 220 aluminum oxide grit but only one or two passes, does not remove any material just deglazes. Some guys use scotchbrite green pads. The 400 grit could have removed some plate if you really worked it hard, the carborundum in it is close to the hardness of diamond. May want to measure the bore? Did you over gap the rings? I use a file but am real careful not to go nuts setting the ring gap. I use aftermarket pistons and go by their specifications for their rings. The gasket set you used, what was the base gasket thickness? If it were a thicker than stock gasket it could have effectively heightened your jug thereby decreasing compression. Same with the head gasket. Maybe you can check with the supplier of the gaskets to see if they are thicker than stock (if that's what you had before the top-end job. I'd like to hear what you find out, eh?

Digital calipers are great but you cant measure deep enough in the bore. You need a bore gauge (expensive) or at least inside micrometers. You could use snap gauges and outside mikes but it's tricky to get good readings unless your'e a machinist. The ring gap is basic. Go to Eric Gorrs website (you'll notice a link on this page to the left). He has some good pages about top-end jobs. Ring gap is explained. The basic principle is that new rings should be inserted in the cylinder and checked with feeler gauges. Usually you insert the piston and slide the rings down inside and use the top of the piston to align the rings straight. Insert the feeler gauge in the ring gap to see what it is. I think for most of my bikes it's been between .012 and .014. Again, use the specifications for the rings that came with the piston. Usually the rings have a information paper that comes with them that expalins this also. I use Wiesco stuff, they have their own specs included. The ball hone is also explained in the Gorr website, I believe he suggests 10 percent under the bore size (don't quote me). If you have a clymer it should explain about ring gap also. Still wanna know what factory compression normally is for that bike? What piston/ring combination did you use (what brand)?

Compression pressure can be tricky. Temperature, how you do it each time, condition of all parts affects the outcome. Did you read the Gorr website, tech articles? The one under top end. There is alot of good stuff explained. The rings can be a problem if the end gap is too tight. That's why you check them and may have to file a little bit. They may have been spot on - who knows. I personally don't like the stock honda pistons and rings. I would use wiesco. Did you check your reeds? Did you replace your reed gasket? Reeds can affect compression also. Don't get too caught up in this since you have changed things and the engine is worn a bit. Go back to the basics and make sure you covered all of them. Isn't it winter there? Or are you riding in the snow....Goog luck.

The lower compression (your worry) shouldn't be an issue. Most definetly check your reeds when you do the top end. I have the boyeson power reeds in one of my Crs and they help real well with low end but flutter on top. Maybe if you rip hard you frayed or lifted them. You can check them with a feeler gauge or by eye for lift. They should set flat. new gaskets are always a must - I think. FYI, I heard from other sources the stock honda stuff is not as tight and may cause a little piston noise. That said I went to wiesco for my needs, howvere, there are other aftermarket pistons/rings out there. You should ask others in this site for their opinion. A dude named Marcus Gunby is on top of the CRs, he usually lurks in the 2 stroke threads. Anyway, good luck and keep in touch.

A thicker base gasket will move the head AWAY from the piston, (Think of your fist moving in a tube. The air is squished smaller and smaller as your fist moves closed and closer to the top, more PSI. Got it?? Like a pump up water gun.
Rings have to be a complete circle, if there is a gap, the squished air pushed through the gap and you get lower PSI.
Got it??
If you think of what's happening when the piston comes up to fire you can see it's pretty simple, the rings seal the piston, which squished the air/fuel charge, then spark/fire, explosion, the air heats up and exspands, the PSI moves the piston and wham it starts all over again.
Before you freak, try this;
Take the spark plug out, squirt some oil in the cylinder and run a compression test, (Full throttle, spark plug in the wire, against frame so you don't short out your ignition).
The oil will seal the rings/cylinder wall for a short time.
What was the first, "hit"?? then the final??
If it goes up your rings aren't seatin the cylinder.
Could be a bad cylinder, (although I doubt it), ring gap too large, ring gap lined up instead of at 10 and 2 oclock.
When you honed your cylinder did you clean out the hone waste???? Did you dry hone it or wet??
I use a flexable stone hone, 400 grit like 3 seconds just to break the glaze, lube everything with kero or deisel fuel, then dip the whole cylinder in solvent to clean it, then brake clean sprayed all over, then blow it dry and wipe it with a paper towel. The cross hatching should be like 60 degrees.
When ever I pop the top off, I check the reeds. For the $2.00 for the gasket it's worth it, your there, make the job complete.
I also take silcone and run a bead across where the reed gasket meets the head and reed block. Ever look at that area when it dusty, a oily film on it?? Fuel seeping out. Silcone seals it up tight.
Try the oil and let us know.
Good luck and take NOTHING for granted on this! :)
Joe :)
I go slow cuz I payed good money to race and I want to enjoy every minute of it! :)

The gasket on the cylinder side will cause a loss so will lifted or frayed reeds. Not sure what the 00 CR125 has for a seal i on the cyl side of the reeds though. Remember that when the reeds intake they draw air in and seal off the cylinder after they close. Don't worry about the wiesco pistons, they are good to go. The CR cyliders (to my knowledge) don't have the designation. The YZ's do (a-b-c-d) but the wiesco is one-size-fits-all anyway. Go back to the basics (Gorrs website and search in this site for top end rebuilds, there's just too much to list to do). I DO NOT recommend a expandable hone (the 3 stone type) you can break the ports in 2-strokes. Stick with the method you used or a ball hone.

The rings go on one way. The little pin in the ring groove of the piston keeps them from rotating in the bore. The gap must be located there. Usually any markings on rings would face up. Yes the cylinder should be washed in wam soapy water, rinsed, and dryed prior to assembly. Have you read the different threads on top-end rebuilds yet? They offer up most of this information. I would not worry about compression right now just the basic stuff. If it's that cold you may want to tear down the top end and check out what's up. You may have prematurely worn down the rings by leaving grit in the cylinder from the wet sanding.

Your right about the wear from the grit.
My son did his first top end rebuild on his KTM a few weeks ago.
He installed a new piston, poped the rings on and slide the cylinder on.
I quized him about the ring location, (the pin the holds it in place, the way the ring is insatlled, (up or down)).
He gave that basic teenager look. AAAAAHHHHHHH??????
When we poped the cylinder off, sure enough the ring was in the wrong location, he said it went together easy, (The cylinder over the piston/ring).
I thought you'd break the ring or not be able to get it to go in the cylinder, but I guess it can!
The ring gap was good, piston to cylinder gap good, so I guess it can go together wrong.
I'm thinking the grit took the rings out. I hope the cylinders ok.
I've been using my 3 stone hone now, since "83???, I've yet to ruin a port, (maybe the edges of the stone are rounded so they don't hit the ports)????
I hade some ball hones, but guys borrow then and they come back-somethimes.
I'd try the oil in the cylinder. Do a test the compression, (even cold) run a compression test, add the oil, do it again, and see if it goes up), if not then rip her down.
You might be able to see if the reeds are bad, take the carb off, have someone kick 'er over and with your hand where the carb goes into the intake. You might be able to feel a, pulse of air being pushed out, (but I kinda doubt it, if it was that bad it would be hard to start and run like ****.
Let us know.
Joe