When is a piston considered to worn to reuse?

[fidiro]

I just threw new rings on my TS360 and gave the OEM piston a shot. Tested comp before first attempt to start it up and came up with 135psi. So I went ahead and got it all together to fire it up. It ran good for a little while and idled fine for a few minutes and then shut down. I attempted to restart with no luck. End gap on rings came up at .012

Now I don't remember exact #'s of piston skirts but it was somewhere around 48.5 mm while the top end of piston around landings came up close to 49mm.

What is considered too worn of a piston to reuse? I ask because now piston locked while I attempted to restart so I'm gessing a ring got stuck on something and wonder if it's to do with worn skirts. At least it didn't lock while running on it's own. I haven't taken it apart again yet to see where it got hung on but I did pull plug and see that top of piston is probably just above exhaust hole. I'll pull muffler tomorrow to see if it caught there.

Also, I removed the head gasket that measured about .50mm and if I squshed the gasket some with caliper and it compresses to around .42 Instead of reusing a paper gasket I cut out a piece of aluminum house flashing and made a gasket to fit, the thickness of the flashing is .37 plus I added some gray sealant to both sides of new aluminum gasket and on bottom end so that added some height, if any. I don't have anything to check squish but I tried without any gasket first and I could hear the piston tap the top of cylinder, I tried this without the rings on just to see. With these two numbers being so close, is it still possible that the squish could be too tight?

 

[Fred482]

Most of my service/workshop manual's have a min/max spec for piston fit. They are all different, depending on bore size and engine type. I'd want to search out the correct spec for that particular engine, they're somewhat critical.

As for the piston contacting the head, that problem would not be good, if not corrected. the factory gaskets are designed to provide the proper crush, when tightened to the correct torque. I won't guess as to why yours is hitting, but I would keep checking further.

Just re-read the post, I see you were just checking fit without a gasket. Sorry, I'd guess that's correct, there's not much room at the top, the gasket is designed to create the necessary space.....

 

[bonden]

I have been doing a repair on a TS 460 the last week an the piston was worn out real bad,i got a new piston and rings fit it with new gaskets and rebuild the carb, piston was worn down about 0.7mm of the diameter at the skirt of the piston,it looks like it has been run with no oil in fuel,I think that you just have to replace that piston with a new one,the saw i got had very high squish so i dont think you get any problem there.

 

[fidiro]

I confused myself rereading what I wrote above.:msp_ohmy:

Yep, I tried the jug without any gasket at all and piston tapped the top some. Between the two gaskets the aluminum one is about .005 thinner.

I thought that if a piston is 49mm it would be exactly that all around or close to it.

I'll pull it apart tomorrow and see what I find about why it's stuck, and stuck good. I tried pushing down on piston through plug hole and no budging.

 

[fidiro]

Wow, that piston is bad. Mine is worn smooth no scoring anywhere.

Just realized that's your intake side that's all messed up. That's what happens when concrete saws suck in concrete dust because of filters. Is the other side the same?

 

[splitpost]

Now I don't remember exact #'s of piston skirts but it was somewhere around 48.5 mm while the top end of piston around landings came up close to 49mm.

What is considered too worn of a piston to reuse?

if your measurements are correct ,that piston would be rocking itself to death,thats .5mm/.020" clearance ,way too much

 

[leeha]

The top of the piston should not be
larger diameter than the skirt.



Lee

 

[fidiro]

I was in a hurry to get it back together and forgot to write down the exact measurements. I'll have it apart tomorrow again and give exact. I am new at all this stuff and although I can pull a saw apart and get it back together I've got lots more to learn with measurements and all techniques and this saw is a good hands on learning experience, or maybe my 029 was more hands on.

There was no scoring anywhere so I figured reringing would be good enough but I guess not when it gets that worn.

I noticed there was no brand on cylinder so I'm guessing it's got an aftermarket. It has some numbers etched on top but no brand anywhere. I guess I can't complain too much about it for what it cost me.

 

[Mad Professor]

The top of the piston should not be
larger diameter than the skirt.



Lee

Yes in most cases top is several 0.001" smaller than skirt.

Regardless that piston is toast.

I wonder if you hung a ring or broke off the skirt?

 

[Terry Syd]

I wonder if you put the piston in backwards and the ring stuck out in the exhaust port. Pulling the muffler would be a good idea.

The pistons are tapered from the top to allow for heat expansion. It is the skirt clearance that you need to measure.

Now here's a tuner's hint about that old piston. If it is not way out of spec or damaged - save it. You can take that old piston and modify it to try different port timings. If you take about half a millimeter off the edge of the piston at the exhaust port, you will find out how the engine runs with a couple of extra degrees of blowdown. You can also try raising the intake skirt a couple of degrees to see how a bit more intake duration makes the saw run.

You can keep trimming the piston until you realise you have gone too far - no biggie, now you know what durations NOT to use in your engine.

If you get an idea about some different timing figures that you would like to run, then you can port the jug to those specs with the new piston.

 

[fidiro]

Piston isn't backwards but it could very well be a broken skirt. I'm hoping it just hung a ring.

 

[bonden]

Wow, that piston is bad. Mine is worn smooth no scoring anywhere.

Just realized that's your intake side that's all messed up. That's what happens when concrete saws suck in concrete dust because of filters. Is the other side the same?

The piston look the same at the other side to.the inner airfilter was not there ,so this saw have been eating a lot of concrete dust.

 

[fidiro]

Finally took apart again. Top of piston measures 48.64 Bottom same as top and about the middle of the skirts 48.84

The reason it locked up was because it tried to digest a circlip actually a G-Clip. Luckily it happened when I was pulling and not while running. Still managed to gouge the jug some just below the top line of exhaust. Damaged one ring and finished off the piston for sure. I'll save the good ring if I ever need it for my MS390. I don't do much work with circlips but can say that I will only trust C-Clips. G or double ended ears don't sit well for me.

Time to order another piston I guess.

It has a Vertex piston, is this aftermarket or is it something stihl used before mahle?