Pressure test basics + Change of rings

[champagne_charly]

Im' doubting myself... could somebody please confirm if my logic is correct?
I'm pressure testing a hs81 from the carb orifice. with piston at TDC . Am i right that by doing this at the carb, I'm testing the bottom end? Seals etc?
If I remove the sparkplug, should the rings hold back the pressure in the bottom end? As soon as I loosen the sparkplug, holding the flywheel. the pressure equalises rapidly. Am i right that the rings should isolate the chambers regardless of direction?

Also when changing rings on an older machine where the crosshatch on the bore is no longer visible, am i best to use a honing tool such as https://www.ebay.co.uk/itm/152534826619

 

[stihltech]

Stihl says TDC. This is adding the vacuum (always first) then pressure below the piston to fill the crankcase. It will get to the top of the piston.
Yes that is the glaze breaker I use. Just to clean and give a crosshatch for the rings to seat to.
Remember,not trying to oversize, just break the glaze.

 

[champagne_charly]

Stihl says TDC. This is adding the vacuum (always first) then pressure below the piston to fill the crankcase. It will get to the top of the piston.
Yes that is the glaze breaker I use. Just to clean and give a crosshatch for the rings to seat to.
Remember,not trying to oversize, just break the glaze.

Thanks!

When you say "It will get to the top of the piston." Am i right in understanding you mean it is normal for the pressure to equalise between top and bottom end when pressure is applied at the bottom. So the valves only seal one way? excuse my novice knowledge

 

[stihltech]

Try this, leave the sparkplug out with the piston at TDC and see what happens.

 

[champagne_charly]

Try this, leave the sparkplug out with the piston at TDC and see what happens.

That's exactly what I did. No pressure was held by the rings& piston. But rings look new. and there are no deep scores. nothing my nail can catch on.
I later tried a compression test with 2stroke down the plughole. got 125PSI. previous readings were 75psi.

 

[stihltech]

Rings seal better when running. Will not seal well enough to stop vacuum and pressure from going by on a test. Which is how it should work.

 

[Automender]

Rings cold have a gap when installed so when they get hot that gap closes up but never enough to hold a tester pressure. Even though the rings on two ring pistons are staggered the first ring will leak into the area between the rings and then the second ring will leak all through the end gaps.

 

[Automender]

End gap on piston rings new ring vs. worn ring

 

[champagne_charly]

End gap on piston rings new ring vs. worn ring
View attachment 1177764

That is quite a difference. thanks for that.