Clarification on 2t engine compression and when cylinder need replacement if scored

[Jonny Vee]

Hi Everyone.

I was given a non worker Stihl Edge trimmer and due of this cold weather, I decided to take it apart during the weekend.

Compression test was 75psi. I then decide to remove the cylinder to see inside the condition and found a nice score starting from the bottom to the below of the inlet port.

As I`m a newbie and self taught I still have no clear how the engine works and if the score below the inlet / outlet port compromise the compression.

I know this may sound a stupid question but can anyone clarify this for me?

This will help me understand if piston and ring would be enough to replace or you need to spend much more on the full engine kit.

Thanks

 

[Mottie]

Compression won’t take place until the piston is on its upward stroke and has gone past the higher exhaust port.

 

[conny]

Compression won’t take place until the piston is on its upward stroke and has gone past the higher exhaust port.

So are you saying to the OP not to worry about the score mark?

 

[Mottie]

So are you saying to the OP not to worry about the score mark?

Well, it won’t affect the compression although it could cause wear/damage to a new piston and rings. If it was mine, I wouldn’t bother with changing the piston if it wasn’t damaged but I’d try a set of rings first and if that didn’t work out, I'd go for a new barrel and piston.

If it’s not been run for sometime, 75 psi doesn't seem that bad to me. I’d start with the basics - clean out the carb, put some fresh fuel in, mixed to the correct ratio and a check for a good spark before stripping the engine down but the OP doesn’t say if that’s been tried yet.

 

[Jonny Vee]

Piston has few mark and rings have lost the elasticity. The tank was leaking and started to strip down to check where was the leak. once removed everything was no much left to remove few more screw and got carried out to check cylinder condition. I can replace ring and clean cylinder with some 240 and then 400 grit to make sure is smooth around the score if help. What else would you suggest?

 

[mointainwalker]

At the moment nothing. It isn't going to be this score stopping the engine starting and I greatly doubt it will have any effect when it does run. Do what Mottie said about carb and fuel.

 

[Burnerman]

You'll notice that the fuel / oil mixture on a two stroke is sucked into the crankcase first as the piston ascends........as it descends again the fuel charge is forced up the transfer ports until it's above the piston, awaiting combustion.
A tiny amount of the fuel charge is lost through the exhaust port overlap, but that's another story!
That's why you can get scoring in unusual places!
However - check the piston and ring area where the barrel scoring has occurred. If there is any damage here then replace the piston and rings (at least). See the pegs in the ring lands that prevent the piston rings from turning and snagging in the ports?
Don't get emery or any other medium on the barrel - it will remove the original hard lining (Stihl use a coating called Nickasil).
Your compression is reasonable - chances are it will improve when some oil gets in that area but don't add more oil to the fuel mix. 50:1 is just fine if the oil is high quality.
Enjoy!
John

 

[Jonny Vee]

You'll notice that the fuel / oil mixture on a two stroke is sucked into the crankcase first as the piston ascends........as it descends again the fuel charge is forced up the transfer ports until it's above the piston, awaiting combustion.
A tiny amount of the fuel charge is lost through the exhaust port overlap, but that's another story!
That's why you can get scoring in unusual places!
However - check the piston and ring area where the barrel scoring has occurred. If there is any damage here then replace the piston and rings (at least). See the pegs in the ring lands that prevent the piston rings from turning and snagging in the ports?
Don't get emery or any other medium on the barrel - it will remove the original hard lining (Stihl use a coating called Nickasil).
Your compression is reasonable - chances are it will improve when some oil gets in that area but don't add more oil to the fuel mix. 50:1 is just fine if the oil is high quality.
Enjoy!
John

Thanks John