084 rebuild question

[thompson1600]

I will start out by saying that this is the first 084 I have rebuilt.

I took the saw apart and replaced the crank bearings, checked it out internally and put the case back together. Centered the crankshaft on the case and the shaft spun freely. Put in new oil seals and the shaft still spun real smooth all the way around. Put a new piston/rings and the cylinder back on. Pressure/Vacuum tested out good (once I took the decomp valve off and put in the plug). Put the flywheel on and went to spin the crankshaft. Crankshaft spun smoothly but when it hit TDC it had a "bind" or slight (very slight but noticeable) hard spot to spin. It was like the magnets of the flywheel were "sticking" to the ignition module, but the coil/ignition module is not on the saw.

My question is, during the entire stroke of piston, should it be completely smooth or is there that transition spot from up to down stroke. It wasn't like it couldn't be turned just spun smoothly and then bang, a little glitch and spun smoothly again. This happened each time I turned the crankshaft.

At first I thought I had forgot to put in the needle cage by the wrist pin. But the piston does not stop moving it continues to move in the cylinder all the way through the stroke.  Or, are the new piston rings just that snug at the top?

Is this normal and I am just noticing it because it's a lot bigger saw?

thanks

Tom

[VA-Sawyer]

Could there have been a ridge at the top of the cylinder?

[Minnesota_boy]

If the spark plug is out, it should turn freely throught the revolution.  Check for a piece of carbon stuck to the inside of the cylinder.

[Tony_T]

Checking for a ridge in the cylinder is a good idea, but on a stihl if you've got one the cylinder is shot, as they have a chrome bore (honing is a no no). 

Another thing to check is to be sure the gap on the rings are lined up with the locating pins in the piston and that the rings are not catching in one of the ports.  The edges of the ports need to have a slight chamfer too to keep the rings from catching. 

If the binding is at/near TDC a ring catching a port is not likely as you will be past the top of the exhaust port.

One other thing comes to mind is the base gasket, if too thin may allow the piston to touch top of cylinder.  A trail assy with a bit of clay on top the piston followed by turning a revolution, disassy, and inspecting the clay will tell you if you have clearance. 

On the same thought as above if there are more than one version of piston/cyl assy you can get a clearance problem from mismatched parts.  Are using genuine Stihl parts or aftermarket?

[thompson1600]

I have to check the squish clearance.  That is a good thought.  I am using OEM Stihl parts.  I put a new piston in, and changed out the bearings.  Everything spun free and good until I put the piston/cylinder back on.  Still spins free just has that one rough spot at the transition from up stroke to down.

Tom

[StihlDoc]

It is normal to have a very slight "catch" at TDC on a complete rebuild. It is just the transition of friction from the new parts. If it is more than a slight stop of the piston at TDC then you have some issues that need to be investigated, but if it is just very slight and hardly noticeable then go ahead and run the machine. If using OEM parts, I doubt that there are any clearance or tolerance issues if you are confident that everything has been assembled properly.

[thompson1600]

Thanks Stihldoc.  It is a slight "glitch" or "hitch" at the top only.  Like I said it's almost like when you spin the flywheel and the magnets go past the coil/ignition.  It's just a momentary glitch at the transition point from up to down in the stroke.

Tom