Stihl 039 squish

[ale6894]

So, I got me a good ole 039 off fleabay, piston and cylinder and everything looks good. Put new impulse/fuel lines on it. New skf bearings and seals along with a cross popup piston and caber rings. My issue is I'm only getting 148 psi and the squish is 87 thousandths!!! It has about 3 tanks through it. I measured the new and old bearings and they're are identical in diameter as well as the oem piston and the popup piston, all same measurements. Put just enough sealant on the pan, not too thick. I'm stumped as to why I'm getting that large of a squish. Anybody have any ideas? It runs good but I'm trying to make it run better. Anybody know How much taller a ts400 piston is than the oem?

 

[L34]

What is the measurement of the OEM piston from top of pin bore to top of piston?

 

[SteveSr]

So, I got me a good ole 039 off fleabay, piston and cylinder and everything looks good. Put new impulse/fuel lines on it. New skf bearings and seals along with a cross popup piston and caber rings. My issue is I'm only getting 148 psi and the squish is 87 thousandths!!! It has about 3 tanks through it. I measured the new and old bearings and they're are identical in diameter as well as the oem piston and the popup piston, all same measurements. Put just enough sealant on the pan, not too thick. I'm stumped as to why I'm getting that large of a squish. Anybody have any ideas? It runs good but I'm trying to make it run better. Anybody know How much taller a ts400 piston is than the oem?

Is this an AM or OEM piston and cylinder?

 

[ale6894]

Ok so the caliper I had was wonky somehow, (rookie move not checking it), got a better one. Squish is measuring in at 30/31 thousandths, I double and tripled measured. Looks good. The wrist pin bore to top of pistons measurements were the same for both Pistons as well, I checked just to be safe. That is 0.6120. Everything is oem except the bearings, seals and the piston. OEM bearings and crank etc. still wondering why I started out with 150 psi with the pop up and it has went down about 147 after 3 tanks of fuel. The ring gap was also about 6 thousandths and has went up to around 8/9.

 

[rupedoggy]

I am only flying by the seat of my pants here so if you have better info spill it.
The 039 is a large homeowner saw. The homeowner saws are made to please homeowners, not professionals. It is, as you know, a clam shell design. The Stihl engineers wanted to be sure the homeowner could crank and start the saw. A compression pressure of 180 would make people complain and, who knows maybe the pollution would be more also. Since the assembly line process has various tolerances when a saw is built the squish is usually generous. Stihl does not want a homeowner complaining that the saw hurts him arm. So the squish is made for low compression and the ignition timing also. Ergo the saw is not very powerful nor is it conducive to easy hop-up methods. If you want to do what it seems like, I say get a pro model.
To address your loss from 150 to 147. Well that relates to a closer fit for a factory piston (yes even a worn one) Again the aftermarket piston has to fit all kinds and sizes of cylinders and not get stuck or rub so hard as to melt some aluminum off the side. The piston fit has more to do with compression than the rings in a two stroke.

 

[Lee192233]

My MS460 probably took 5 or so tanks for the Caber rings to seat. I wish I checked the compression right after the rebuild compared to now. I have about 20 tanks through it now and it sure feels like lots more than right after I put it together.

Run 2 or 3 tanks through it and check again. You may be surprised.

 

[Jasent]

Yup still seating. Run her rich till she wants more air

 

[ale6894]

I am only flying by the seat of my pants here so if you have better info spill it.
The 039 is a large homeowner saw. The homeowner saws are made to please homeowners, not professionals. It is, as you know, a clam shell design. The Stihl engineers wanted to be sure the homeowner could crank and start the saw. A compression pressure of 180 would make people complain and, who knows maybe the pollution would be more also. Since the assembly line process has various tolerances when a saw is built the squish is usually generous. Stihl does not want a homeowner complaining that the saw hurts him arm. So the squish is made for low compression and the ignition timing also. Ergo the saw is not very powerful nor is it conducive to easy hop-up methods. If you want to do what it seems like, I say get a pro model.
To address your loss from 150 to 147. Well that relates to a closer fit for a factory piston (yes even a worn one) Again the aftermarket piston has to fit all kinds and sizes of cylinders and not get stuck or rub so hard as to melt some aluminum off the side. The piston fit has more to do with compression than the rings in a two stroke.

Well the psi reading are all from the pop up piston. I know these saws usually have about 150 with a stock piston, at least that's what I read, I could be wrong. The guy sold it because he claims is had a scored piston (it didn't) and the compression was only 105 tested at the dealer? No idea. I got it and everything was fine with it except for the crank seals and all the other rubber. The bearings were a little gritty so I went ahead and replaced them. I myself made the mistake also of not testing compression before tear down. I will run a few more tanks through here and see what she does. I've had only husky before this saw so it's a little adventure and experiment to see what I could do with it. Think I'll look for an 044 next.