How much scoring on a piston is too much?

[bama]

I am working on a 451 Jonsered that I bent the crank on when I put the case together. I will be ready to do it right when it is time to put it together after I get the crank straightened out.

Here is my problem: When I took the jug off, the piston had two scratches, one on each side of the exhaust port and one of those scratches was even in the ring(the ring still moves freely in the groove). The cylinder has a little material on it that it didn't have when I put it together.

My question is since they are hard to find, can I buff out/smooth out the scratches and re-use the piston with a new ring. I only have maybe 10 minutes on the saw and yes, I will pressure test it this time. It didn't sound lean on low or high speed and I only tried it in about 5 or 6 cuts. The rest was idle time.

I am wondering if I missed a fleck of something in assembly or if the bent crank helped cause the scratching, also.

I have not had this happen to the bigger saws I have rebuilt(621's and 920), so I want to know what I did wrong so it doesn't happen again. I have learned some expensive and time wasting lessons on this smaller saw.

 

[Farley9n]

As long as the ring land is good that's an easy save. Check your wrist pin clips if you don't already know what caused the two scores.....Bob

 

[bama]

Do I buff it out with fine sandpaper? The clips were good and in place. I used some gasket sealer on the base gasket. Maybe some of that got in there? I will pressure/vac test to see if I was leaking and was too lean. I didn't have one built until after I ran this saw.

Do you put gasket sealer down when you put the base gasket on? I cut my own gaskets so I thought I should to prevent leaks. Maybe I don't need to put anything other than the new paper on it?

 

[Justsaws]

If I have a gasket I usually do not use sealer. Make sure that there is nothing in the bottom of the crankcase(wood chip, metal flake, gasket sealer, etc.). Blow it out with compressed air and rinse it with mix. Also check all the edges of the piston and cylinder for burrs and hang-ups. As to how score is to much hard to say without seeing it. It is amazing how much damage can be done and still have a running saw. Performance will suffer but it might run. I use 1000 grit silicon carbide paper, scotch brite pads, buffing wheels, hones, etc. Remember you don't want to significantly change the shape of the piston or cylinder you just want to remove any burr that sticks out past normal profile. Also you might replace you piston bearing. good luck. How do you like the 920?

 

[bama]

Thanks, Justsaws, for the info. I will try the re-re-build with no gasket sealer. I am pretty good at cutting gaskets, so I guess I should trust my work more.

I love my 920! I have used my dads since he quit using it in 1993(bought new in 82). The module went bad, but other than that, it has been a great runner. I just rebuilt another one and the compression is so strong that I can't "one hand" it like my 920. I guess I might need rings for mine one of these days. It does get a little heave after using it all day, so I have collected some smaller saws for smaller wood/bucking.

 

[havvey]

scores

fine sandpaper on the scores!!:jawdrop:

Thanks, Justsaws, for the info. I will try the re-re-build with no gasket material. I am pretty good at cutting gaskets, so I guess I should trust my work more.

I love my 920! I have used my dads since he quit using it in 1993(bought new in 82). The module went bad, but other than that, it has been a great runner. I just rebuilt another one and the compression is so strong that I can't "one hand" it like my 920. I guess I might need rings for mine one of these days. It does get a little heave after using it all day, so I have collected some smaller saws for smaller wood/bucking.

 

[bama]

I figured out where the scoring came from. When I used the gasket sealer, a little bit squeezed out under the cylinder and into the crankcase. It was in the crankcase of the 451 when I split it. That is what it has to be. I was pretty careful about cleaning the crankcase, piston, and cylinder before assembly.

Live and learn!

 

[Oddvaark]

ikin relate to yur 920 Jonse startin hard . I pull da sprkplug and chokr 10 pulls and den put daplugin and hope fer a quik start . What kinda brainf**t di Jonse hav whin dey firgot da dicomp buttin .. 910s wir da bes ba far. yuh, ba far

 

[Oddvaark]

1 of mi 920s has scoring from a bad filtr sistim . less yu chaing er cleen da fltr evir taink shez gonna scor . bad injinearing , reel bad

 

[04ultra]

ikin relate to yur 920 Jonse startin hard . I pull da sprkplug and chokr 10 pulls and den put daplugin and hope fer a quik start . What kinda brainf**t di Jonse hav whin dey firgot da dicomp buttin .. 910s wir da bes ba far. yuh, ba far

English please...LOL

 

[Marco]

A few scratches shouldn't hurt anything, try the piston without rings in the bore to see if it will move freely. Turn it over and check that the piston skirt is not worn or "heat shrunk". Reed valve engines you can get away with quite abit. On one that is piston ported you have to pay very close attention that the skirt is not worn down or you can have decent compression and it won't run. Some models fair better than others. If you sand don't try to remove all the scratches just sand to remove any high spots, keep in mind that a high spot in one part probably corresponds with a low spot in another. Amazing what will run, even more amazing what won't.

 

[LarryTheCableGuy]

English please...LOL

They are just about ready to pass a law about that.

.

 

[bama]

I will start working on my 451 this week. I got the sandpaper and will do like March said. I think I need a new ring since the scratch goes into it. At least I know what did it so this time it won't happen when I run it!

I appreciate all the help on what to do to clean up my mistake.

One thing I should mention is that it was running fine. Had I not needed to straighten the crank, I would have never known that there were scratches in the piston(well, I think I would eventually learned when it quit running).

Thanks, guys!

 

[hornett22]

a gasket should not need sealer.

i do not understand why everyone buys into some of this marketing.i cringe when i open the hood on a car and see sealer near the waterpump or thermostat housing.

 

[Freakingstang]

English please...LOL

"ikin relate to yur 920 Jonse startin hard . I pull da sprkplug and chokr 10 pulls and den put daplugin and hope fer a quik start . What kinda brainf**t di Jonse hav whin dey firgot da dicomp buttin .. 910s wir da bes ba far. yuh, ba far"


I think he meant:

"I can relate to your 920 Jonsey red starting hard. I pull the sparkplug and choke her for 10 pulls and then put the plug back in for a quick start. What kind of moron did Jonsey red have when they forgot the Decompression button? The 910's were the best by far. Yeah, by far."