New top end no compression?

[Jbgabriel]

Well my buddy bought a basket case 85 rm 125 with a rebuilt engine. When we picked it up the motor felt like it had good compression and i looked in the exhaust port and i could see the new piston and rings. The guy said the dealer did all the engine work(new top end and crank bearings).
Today when we try to fire it up it feels like it has no compression. I have a bunch of bikes in my garage and they all have more compression than this bike. I dont have a guage so I dont know the exact number but it was real low.
So I tore down the motor and to my surprise the motor actually did have a brand new wiseco piston and rings. One of the head studs is pretty badly stripped but the rest were pretty tight. The bore looks fresh and I even did a real light hone just for insurance. Im going to finish putting it back together tomorrow.

So what do you guys think? Was it a bad head gasket or that stud? Or are the rings not sealing for some reason? Ive been doing this for awhile and this one has me stumped.

 

[Jbgabriel]

Well after further review it would apear that you need all head studs to hold the head effectively. LOL I can see where the oil and fuel are pasing the head gasket and its right where the head stud that doesnt work is.

I need to fix the stripped out stud hole in the cylinder. Whats the best way togo about this? Tap to the next size up or helicoil? I know the best alternative would be new cylinder but this bike is just a beater.

 

[Coop]

I used a helicoil before and it worked ok. It was on an old beater so I didn't really care if it worked and it was more of an experiment, lol.

 

[DougRoost]

Yes, helicoils are a decent choice here and have the advantage of keeping all the stud sizes the same for more even torquing, maintenance, etc.

 

[Jbgabriel]

Alright hopefully i can get this thing back together tomorrow. I guess the helicoil is the path i will take.

Does anyone know the thread of the cylinder studs? When i look on bike bandit all it says is 8x35. Does that mean its an M8 thread? Is that the size helicoil I should get? Thanks

 

[armyvet2]

double check your bore as well,i have seen some messed up bore jobs in my day.Not just any yo yo can bore a cylinder correctly.

 

[Jbgabriel]

Well after fixing that head stud and throwing it all together again the bike still feels to me like it has low compression. Keep in mind that I dont have a compression gauge so I dont know the exact number. But like I said before compared to all the other bikes in my garage this one feels real weak. It WILL blow my finger off of the plug hole but I cant get it to start. So I tore it down for the 4th time and brought the whole top end to a honda dealer. The head mechanic said that everything looked fine. So we are all stumped. I have resurfaced both the head and the cylinder. I have no idea whats going on. Maybe its because this bike doesnt have powervalves so the exhaust port is higher and its just the way it is. Any ideas? The only other thing I can think of trying is pull starting it and maybe the rings will seat and compression will go up?

 

[KTM Mike]

Double check ring end gap. Measure up piston diameter and actual cylinder - make sure all is OK there. Then run down to an auto parts store, and buy a compression guage - you can get in expensive ones for around $30. You might even find one at Walmart. It will be money well spent - if you keep tearing this thing apart, you will be buying gaskets etc and easily spend that amount of money anyhow.

 

[Jbgabriel]

I have triple checked end gap as well as bore size and piston diameter. It all specs out. I guess im going to have go buy a tester....

 

[lou2008]

Just a shot in the dark here but is the engine breathing properly?
If it can't suck air it would be hard to make compression.
Just a thought.

 

[whenfoxforks-ruled]

Is the bore plated or a liner? Does any part, like the head surface or ports in the cylinder, look as if they have been "modified" Are the head and exhaust real shiny?

 

[Jbgabriel]

It is a cast iron bore and it looks fine and stock. Im pretty sure im having a head gasket leak somehow still. I really dont see how it can be s i resurfaced both the head and the cylinder. Im gonna get another new head gasket and get some of that copper spray and see if finally i can get this thing together.