binding on piston stroke?

[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 ignition module was not on.

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.

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

thanks

Tom

 

[Simonizer]

I think if you did everything correctly, it's the magnets attracting the iron of the coil. Remove the coil and try it to ease your conscience. Re-install coil with a 0.010" air gap after your test. Cheers.

 

[thompson1600]

The coil/ignition module was not put on the saw yet, just the flywheel. I think I did everything right, same as I usually do and haven't had problems. But you know when you look at something for too long you don't see the obvious.

Thanks

Tom

 

[Crofter]

Are you perhaps noticing the ring and piston drag and feeling it go slack and then pick up again after the few degree lull at tdc. I would like to be sure that the squish is ok too. The magnets will give a little tug as they pass the armature of the coil. How much trouble to remove flywheel to eliminate this.

 

[thompson1600]

That's a possibility on the drag. That was my other thought, but never noticed it on the smaller saws I've rebuilt. Just wondering if the bigger the saw the more noticable the transition from up to down stroke with new rings may be. Not hard to pull the flywheel, takes about a minute, but it's hard to turn the crankshaft over without the flywheel on. The saw is just the case with cylinder and the flywheel on, at this point.

Thanks

Tom

 

[Simonizer]

The coil/ignition module was not put on the saw yet, just the flywheel.

Thanks

Tom

Is the cylinder base gasket installed? Pull the plug out, bend a piece of 0.032" solder and slide it carefully against the cylinder wall on the left or right side. Turn the flywheel slowly by hand past TDC once and remove the solder. This will act like Plastigage. Mic the solder to see how much piston to cylinder clearance you have. Should be 0.020" or more for a stock saw.
Cheers.

 

[thompson1600]

I'll try that, thanks for that tip.

Yes, the base gasket is installed.

Tom

 

[Locoweed]

I don't know if this is applicable or not.

Replacing rings without removing the little ridge at the top of the cylinder when working on a car will cause the new square ring to hit the rounded top of the worn area of the cylinder.

I don't know if the same holds true for chainsaw cylinders.

 

[Lakeside53]

I don't know if this is applicable or not.

Replacing rings without removing the little ridge at the top of the cylinder when working on a car will cause the new square ring to hit the rounded top of the worn area of the cylinder.

I don't know if the same holds true for chainsaw cylinders.

There is no ridge... unless you have bad cylinder - the Cylinder is Nikersil lined.

Tom - The action should be smooth all the way - you may have a slight difference in friction near the reversal of top and bottom of the stroke, but it doesn't feel like the magnets pulling - much more subtle. I'd sure look closely at it before firing up.

 

[spacemule]

I think if you did everything correctly, it's the magnets attracting the iron of the coil.

Wow! You really are smart. Let me paraphrase: "If you fixed the saw, nothing is wrong."

 

[Ax-man]

I'll take a stab at this, for better or worse, did the the connecting rod have some play in it down by the rod bearings either up or down or side to side?? how about a slightly worn wrist pin ??? did the pin come out freely or did you have to press it out??

It is not all that unusal for an engine either 2 or 4 stroke to have that little lull at TDC after they have had some hours put on them. If the connnecting rod is a little worn causing some end play with the new rings that haven't seated themselves in yet, might be causing that little bind or flat spot at TDC.

Larry

 

[lesorubcheek]

Did you de-carbon the top of the cylinder? If not, you could have a buildup that may be causing obstruction at the top. I'd bet you've cleaned it up though, but wanted to cover the bases. Only other thought is grabbing bearings at the wrist pin since the rotational direction of the rod relative to the wrist pin will change as TDC is passed.

Dan

 

[Railomatic]

Hello

As a small engine expert for many years now I have come across this senario a couple of times before.
What I have encountered was, the new piston and rings were not matched to the old bore, I always try the new rings seperately inside the bore towards the top of the barrel, check if there is a gap between the open ends of the ring, if not the engine could sieze once it gets hot.

Also the new piston should be fitted into the bore without the rings to check if there are any tight spots, again the piston could sieze once the engine is hot, you never know what you are getting especially if the piston is an after market make and not a makers genuine part.

If the two test prove OK, then I would check the squidge band, this is done by using some Plastigauge, which many of the tuning guys use, it is placed on top of the piston, and when the piston reaches the top of the bore it squashes the gauge, which can then be measured using the supplied paper gauge.

Failing this, it could be as one other mentioned, and be the magnets holding things back or maybe with the new bearings you have fitted in there, the magnets are a tad too close to the flywheel, this would give you this type of phenomenon turn crank without spark plug fitted.

One thing to remember when fitting a new piston and rings to an old barrel or vise versa is, the old barrel will be shiny or glazed, polished from use, so the barrel surface should be de-glazed before fitting the new parts, or the new rings will not bed in properly, a gentle wipe over with some fine 600 wet and dry should be enough to take off the shine.

All the best

Davy.

 

[thompson1600]

Thanks for all the advice.

Larry - there was no play in the crankshaft, it was good.

Dan - I didn't de-carbon the top, it looked pretty clean.

I think I will have to take the jug off again and check it out. It's a brand new OEM piston going into the existing jug.

Any other suggetions are welcome.

Thanks

Tom

 

[Lakeside53]

so, what happened?

 

[thompson1600]

Sorry, forgot to post an update....

My mistake. I thought I had cleaned out the cylinder good. It turned out to be a very small piece of metal stuck to the top of the cylinder that was just barely touching the top of the piston. When I took the cylinder off to re-investigate I noticed a small little mark on top of the piston and then looked on the cylinder top and found it.

Tom