038 mild port job

Al Smith

Mac Daddy
Mar 6, 2005
Northern Ohio
The 038 Mag I did was rather radical.To duplicate it a person would have to have access to a machine shop. This one is so simple a person with average mechanical abilitys could do it with a couple of files and a piece of sand paper .

The patient is a good running 038 Stihl that needed a new handle ,I said to myself,self since it's already half torn down why not to a little pepper upper to it ,so did .

The port timing was not changed the compression was not raised.Only made to "breathe better by increasing the area of the exhaust port a tad bit .

The first few pics show the tear down.Always do everthing possible to keep any dirt out of the engine .


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  • #2
the ports

Changing the actual port timing in a two cycle is about the same as using a special grind camshaft in a 4 cycle.
You can for example us bigger valves in a 4 cycle to get more power without doing a thing to the camshaft .This is basically what this simple port job is .

The exhaust was widdened about 20 thou on each side of the existing opening and "belled" out as the exhaust came towards the front .It just evacuates the exhaust faster making for a better transfer of the next incomming charge .

I used a Dremel but seriously you can do it with a file.Fact is I used a file for some of it .Oh,I know what some of them say about this,ignore them because it works .

After the final shaping of the exhaust it was polished smooth using an abrasive called Cratex,does a nice job .


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  • #3
The intake

The intake timing was not changed in any way but the whole thing was polished using a Cratex tool .

The second picture shows slidding the cylinder back in place. Some people are under the impression you need a ring compresser for this .Not so,the cylinder has a champher and slides right on with a little nudge here and there.About a two minute job .


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  • #5
The muffler

Last but by no means least.You can gain more power by a little muffler work than just about any thing else .

A pressure can muffler is nothing more than a pressure regulating device.They add the baffles to make it less noisey .Removing same does make it louder but not like a go-kart running a megaphone pipe or anything .Unless you get the bright idea of hanging a cute little pipe out of them,then they get noisy .

Noise is a form of energy.That putt that comes out of the saw is just what's left of the expanding gasses having shoved down the piston on the power stroke .The way a baffle works is to disperse that sound. That putt putt rattles around off those baffles and the sound energy collides with one another and effectively sort of cancels each out,to a point .

While it's doing that it also hinders the flow of the exhaust gasses to a great deal .

I just gut the whole thing leaving just the outer shell or can to still work as a regulating device.

Back togther and sounding sweet.In the wood tommorrow for a test drive .

Of ,that last picture is Momma's new ride


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  • #7
Did you run it?
Yea buddy.Took about ten pulls from a dry tank,ran good but not touched the wood yet,too dark at 8 pm .No great visions of grandure here like some at other places .

I tweeked a 72 cc that will keep up with or snuff a stock 80 cc .If I can get this 61 cc to hang with a 70 I'll be tickled pink . Ah,simple things for simple minds .;)
Nice post Al :)
can you get a better pic of the rotary file you use in the Dremel ?
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  • #9
I have a thread together about the tools used for this stuff .

I laid the saw to the oak,not too shabby,needs the carb opened up a tad but I can't find my skinny minnie screw driver.:what:

It's a runner but it will never be a super star like the 038 Mag I did.This thing is 10 cc's less,stock compression and port timing .Still not too bad for an old farm boy .;)

Hmm, I sure hope my dear kind friend in Atlanta sees this .He kinda called me a dumb a$$ about this stuff.Wonder what he thinks know ,Ha.:lol:
These are some good threads you have going Al.

Couple questions

I noticed you didn't do anythng to the intake port, other than polish it. I always thought or read that and from little porting I have done that if you alter the exhaust the same should be done to the intake port to keep the same amount of air entering the cylinder to be the same as what is existing out the exhaust. In other words if you widen the exhaust you should widen the intake the same amount. Does it even matter one way or the other because the amount of air volume is determined by what amount the crankcase can take in and hold.

I like your explaination about the muffler and how it works, much easier than some of those books I have been reading on the subject.

Now I will raise another question that might contriversial.

Ever since I have found out about modding muffler the norm seems to be to gut the muffler totally. I question this practice myself . I understand the why's of doing this for better flow and more power. Mufflers in most of today's saws seem to construsted not only to reduce noise but conduct heat away from the port and cylinder and tranfer it to the outer portion of the muffler. Most of the newer Sthils have a heat shield behind the muffle which to me acts like the exhaust manifold on a car engine. This is just my opinion and oberservation is that by gutting the muffler it loses it ability to conduct heat and weakens the muffler's internal structure. When i mod a muffler I try to improve the flow by either opening the baffles more or cuting out just small pieces in the direction of the exhaust flow.

This may seem to be on the conservative side but I also feel that gutting a muffler you affect the timing of the scavaging side of the cycle a little. This is just my own opinion is all. I don't have the answers because when it comes to modding saws I am just a shade tree mechanic but I am always open to a good disscussion and to learn more.
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  • #11
I did widden the intake on the 038 Mag a tad bit ,not much .The Av was just buffed a bit ,most likely only enlarged less than 5 thou .

I only took the exhaust on the AV out about 20 thou on each side,the Mag was about 40 each .

I kind of guessed at what size extra hole to drill in the Av cover figuring it 10 or so cc's smaller than the Mag which has a factory dual port .I made a little shroud for it also like Mag has ..

I layed the Av to the oak today and tweeked the carb.It's going to be just dandy,lot's more ooomph .

Now the muffler,good question .The 038 Series does have shielding on the cover though .I suppose it disperses heat .

I always figured a can muffler is just a pressure regulating device and the baffles mess it up ricocheting sound waves every way from Sunday,hither and yon .It certainly seems better and really isn't any louder than my old Macs for that matter .No wonder old loggers went deaf .:O
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  • #12
Another thought

In the case of weather to enlarge intake ports or by how much if inceased hinges on several factors .

One I will address is that a wide or perhaps higher port will evacuate the exhaust gasses faster .This in turn should cause a greater pressure differential accross the exhaust to transfer port area .This should effectively draw the crankcase to a lesser pressure there for making a more negative situation on the next intake duration .

How much,I have not one clue and would only speculate .I don't do too bad at this stuff but I'm not one that has all the facts and figures,only part of the big picture and I kind of fly by the seat of my pants in so many words .

Now,in the case of wide or high exhaust ,depends on the application.Generally speaking the movement of piston to crankshaft rotation is greater at around 90 degrees after top dead center and becomes lesser the closer you get to top or bottom dead center .There for a sliightly longer power stroke or holding the burn a tad longer should produce a tad more grunt.Lesser would produce more RPM's .Both have to be done though with some forethought on the matter . You could make a bull of a saw by getting to around 115 -120 degrees after TDC but chances are it would be slow .On the other hand it could be chopped off at 91 and run like a scalded ape but not have enough grunt to power a pizz ant's motorscooter .

For what it's worth most Stihls are around 100 degrees of burn ,more or less .The 038 Mag I did is setting at 102 .