Stihl 038 avs to mag conversion plus a few mods

[Canadian farm boy]

Let me first start by saying that I'm new to the forum. I have read almost every thread I could find that deals with the stihl 038 saws.

I work as a professional heavy truck mechanic and have done a lot of different work on a lot of different types of engines. I've done some port and polish work before on car engines. I've never really done much work on small 2 stroke engines.

Now to my build plans.

I recently acquired a stihl 038 avs. electronic. The saw was built in Brasil and originally purchase in 1993. The saw didn't run when I got it. I found that the piston and cylinder was shot. I've done my research and decided to install a mag p&c. I'm aware of the necessary modifications that need to be done to the cylinder mounting holes.
The muffler that came with the saw is a dual port and is in excellent shape. I plan to do a muffler mod as soon as I get my new p&c. I plan to measure the exhaust port on the cylinder and mod the muffler outlets to total 70-80% of the port outlet area. Gasket match muffler to the cylinder.
I've read that a lot of guys will leave the cylinder base gasket out as a means of increasing the squish. I too was planning to not use the base gasket as long as I have enough clearance between the piston and the top of the cylinder(0.020" minimum I believe)
I want to clean up the casting flaws in the intake port and the transfer tubes, leaving the surfaces slightly rough to assist with fuel and air flow into the crank case and into the cylinder. I may try and open up the intake runner but haven't decided for sure yet.
I plan to open the exhaust runner between the muffler and the cylinder port opening as much as I can and polish it being careful not to change the opening height. I was planning to widen the exhaust port opening as much as the piston will allow for. Leaving 0.100" of piston skirt width on either side of the port for a good seal.
I plan to run a 7 tooth sprocket and a 28" bar with 3/8 chain. Would an 8 tooth sprocket be better?
I know this saw is known for its torque and is one of the best saws stihl ever built (IMO). My 038 will be a working saw that I plan to keep for along time. Ultimately I want this saw to preform as well or better then a stock 044 or 046.

Please let me know your thoughts. I want to learn from the best so please be honest and don't hold back, if I'm out to lunch on my build plans please tell me.

 

[cgraham1]

Have you read this one...

http://www.arboristsite.com/community/threads/wanna-talk-about-port-shapes.260764/

 

[Mad Professor]

I run a 8-pin with 20" bars and use a 7-pin for longer ones. Type of wood and rakers will also have an effect.

For the muffler I open things up as big as possible while still retaining the screens.

 

[Canadian farm boy]

Have you read this one...

http://www.arboristsite.com/community/threads/wanna-talk-about-port-shapes.260764/

cgraham1

I just finished reading through the thread you suggested. Awesome read and great pics too.
Thanks. It definitely cleared up a few things for me. I'm still kinda wondering what I should do about the shape of intake runner. Seems to me that its gonna end up being the biggest restriction. Maybe I'm wrong, IDK.
I think I'm gonna stick with running the 7 tooth sprocket. I can easily change to an 8 later if I feel the saw could use it.

Have you read this one...

http://www.arboristsite.com/community/threads/wanna-talk-about-port-shapes.260764/

 

[Canadian farm boy]

I run a 8-pin with 20" bars and use a 7-pin for longer ones. Type of wood and rakers will also have an effect.

For the muffler I open things up as big as possible while still retaining the screens.

I agree. The bigger bar with the 7 tooth would probably be best. Keep the saw in the rpms where it makes the best torque. Keep the chain sharp and don't worry about the chain speed. Im usually cutting on sugar maple, ash, elm and the occasional walnut. I try and keep my rakers .025" lower then the cutters

 

[Rockjock]

I agree. The bigger bar with the 7 tooth would probably be best. Keep the saw in the rpms where it makes the best torque. Keep the chain sharp and don't worry about the chain speed. Im usually cutting on sugar maple, ash, elm and the occasional walnut. I try and keep my rakers .025" lower then the cutters

Another Canadian... where you at? Sooner or later we need a Canadian GTG

 

[Canadian farm boy]

I live in sothern Ontario, just outside of the Kitchener Waterloo area. We get very unpredictable winters around here. Sometimes it gets to -25C plus wind. I heat with wood. Usually go through about 8-11 bush chord a winter.

Another Canadian... where you at? Sooner or later we need a Canadian GTG

 

[Rockjock]

I live in sothern Ontario, just outside of the Kitchener Waterloo area. We get very unpredictable winters around here. Sometimes it gets to -25C plus wind. I heat with wood. Usually go through about 8-11 bush chord a winter.

Last winter was brutal -30C and I am out doing ice storm clean up near Lake Ontario so i know your pain! You are not far from Dieselshawn then! I am sure he will tell you of the awesomeness of the 009L and the 017.. it is up to mythical proportions now !

 

[Canadian farm boy]

So I got my new p&c and did some measurements on the new piston. Comparing the old piston from the super to the piston in my new mag kit. The mag piston from the center of the wrist pin to the bottom of the piston skirt on both the intake and exhaust sides is .030" longer. Is this normal? The ports in the new mag cylinder are very close to being in the same locations as the old super cylinder. Should I have the piston skirts shortened or leave them alone? Won't the longer piston skirts change the port timing?

 

[Moparmyway]

You need to check those timing numbers before you attempt to modify anything. You will need a degree wheel to do so.
Dont forget to also check the squish on the Mag setup and post the numbers you get

 

[Canadian farm boy]

So just to update everyone. I now have my 038 back together.
Here is a description of what I did.
- I installed the 038 mag piston and cylinder without the base gasket. The base gasket measured .015 thick. The squish without the gasket measured .027. Since I had more then .020 clearance I figured I was safe.
-I lowered the bottom edge of the transfer tube inlets to .125 of the the bottom of the cylinder and blended them in. Removed the casting flaws from the transfer tubes and sanded them down with 60 grit sandpaper as to leave them slightly rough.
- I ground in a bridge port on both transfer tubes. They measured .375 wide and .125 deep.
-squared up the intake port and removed the casting flaws. Chamfered the upper and lower edges. Sanded it down with 80 grit sandpaper as well. I didn't raise or lower the intake port at all. I only slightly widened it by approx. .010 on both sides.
- squared up the exhaust port opening. I didn't make the corners quite as tight as I did on the intake port. I didn't raise or lower the port only Widened the exhaust port .020 on both sides. Still have .100 of piston skirt on either side of the port. Chamfered the upper and lower edges. Gasket matched the exhaust port outlet and blended. Polished the whole inside of the exhaust runner.
- I gasket matched the muffler and drilled out as many of the holes in the baffle as I could. Drilled extra holes in the baffle as well.
- I calculated the exhaust port outlet area and added a third port to the muffle. The combination of the original muffler port areas and the new port area add up to be 85% of the exhaust port outlet.
- I measured the compression of the new mag piston and cylinder without the base gasket and got 185.
-reset the carb back to factory settings and started the saw. It fired and ran on only the fifth pull. I allowed the saw to warm up and adjusted the idle settings.
- I tuned the high side in a cut. (24" ash log) Kept making the saw richer while cutting till I heard it start to 4 stroke and then leaned it back to just where it ran smooth while cutting. When the saw is pinned with no load, it still 4 strokes but as soon as it is loaded it smooths right out. I was running a new 28" bar and a new 3/8 FC chain.

The saw is very responsive and has very little trouble with the 28" b&c. I can lean right on it and it just keeps going.
I'm pretty impressed how well it works considering this is the first saw I have ever replaced a P&C on yet alone also doing the mods. Maybe I got lucky maybe not.

 

[Canadian farm boy]

As the new P&C break in, should I expect to have to retune the carb periodically?
Will the compression change as the rings break in? Increase or lower?

 

[Rockjock]

As the new P&C break in, should I expect to have to retune the carb periodically?
Will the compression change as the rings break in? Increase or lower?

You may have to tweak it a bit from time to time but you did good!

 

[Moparmyway]

Can you post any pics of the cylinder ??
Usually the top of the exhaust port is kept oblong shaped to help ring longevity, as a square top will ask the rings to compress/expand instantly whereas the slightly round port will allow the rings to compress/expand at a slower rate.

The piston skirt opens/closes the intake, so a chamfer isn't as critical as it is with the exhaust. A square intake is good, a square exhaust isn't.

Pull the internal baffle of the muffler entirely ........... its not needed and just blocks the spent gasses from getting out quicker. Its OK to go past 85%, and you wont loose any performance until you hit over 120%. A 100% opening will be louder, and it will have slightly more power.

Would love to see the bridge ports, did you direct them towards the intake or the exhaust ?

Compression will increase over the next 10 tanks, not by much, maybe 5 psi or so ......... what mix ratio are you running ?

 

[Canadian farm boy]

Can you post any pics of the cylinder ??
Usually the top of the exhaust port is kept oblong shaped to help ring longevity, as a square top will ask the rings to compress/expand instantly whereas the slightly round port will allow the rings to compress/expand at a slower rate.

The piston skirt opens/closes the intake, so a chamfer isn't as critical as it is with the exhaust. A square intake is good, a square exhaust isn't.

Pull the internal baffle of the muffler entirely ........... its not needed and just blocks the spent gasses from getting out quicker. Its OK to go past 85%, and you wont loose any performance until you hit over 120%. A 100% opening will be louder, and it will have slightly more power.

Would love to see the bridge ports, did you direct them towards the intake or the exhaust ?

Compression will increase over the next 10 tanks, not by much, maybe 5 psi or so ......... what mix ratio are you running ?

Unforunatly I didn't take any pictures of the cylinder mods that I did.
The exhaust port is heavily chamfered along the top edge to assist with the rings.
I'm running a fuel mixture of 50:1. Huskvarna synthetic 2 stroke oil and 91 octane gas.
I might try opening the muffler up some more. The saw is already really loud.
http://www.arboristsite.com/community/threads/wanna-talk-about-port-shapes.260764/
My bridge ports, intake and exhaust port look almost the exact same as the ones shown in this thread. My bridge ports are angled towards the intake port. Figured that if I could force more fuel/air mix towards the intake side and sparkplug that it would help to push all the exhaust out of the cylinder and help with the ingnition of the new fuel charge in the cylinder.

 

[Moparmyway]

Great !! Using that thread as your guide is the perfect way to do your research before your grinding. Lets see a video of the saw in some big wood !!!

Also, just throwing it out - the OP of the thread you linked suggests 32:1 mix ratio, as do I.
50:1 is pretty thin for the extra rpm that saw is now turning.