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1st Porting Attempt, an MS200T Clone

BuffaloBill

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I don’t have a real need for more chainsaws but enjoy engines in general and wanted this experience. I’d watched a few eBay auctions and Facebook marketplace adds for a genuine Stihl 200T, but they‘re asking too much IMO. So when I saw a “sale” price on an orange parts kit I ordered one.

I have no prior porting experience, but have enjoyed reading all the information shared here and on YouTube and wanted to give it a go. I do have a lathe & mill and liked the idea of raising compression by lowering the cylinder and cutting squish to some minimal dimension. Will these low cost components stand up to high compression, etc., I don’t know….. I’d come across a few of you experienced builders who shared numbers, and set out to replicate port timings that Mastermind shared in a post a number of years ago (102, 126, 81.X).

Stock numbers w/o base gasket measured 98, 130.5/133, 77.5, squish .036. I took .036 off the cylinder base and landed on Mastermind’s timing numbers. I then made a squish cutting tool and sanding drum as Treemonkey showed in a video (thanks Treemonkey, I’m about 60miles south west of you). And even though I was cautioned to not create a ridge that can falsely show a big squish reading, I let it happen. I hadn’t ground enough relief at the end of my tool bit. By the time I corrected that and got the ridge cut out, I was right back at the stock .036” squish.…..

image.jpeg


So I took another .019” off the base, knowing the intake duration would end up greater than the target, but I figured I had really nothing to loose and experience to gain. Not wanting to recall the math needed to calculate port height vs degrees, I made a simple model in Solidworks using the stroke and rod length. From this model I determined the primary and secondary transfers should be raised .089 and .079 respectively, to land on 126/129 degrees.

The open transfer design allowed me to grab the cylinder upside down in the mill vice and accurately cut them “deeper” (and keep them flat) by those dimensions, using a 1/4” carbide end mill. I attempted to blend the remaining step into the corners of the transfer ports, making a small radius there with an 1/8” dremel bit (I hated to show pic of this work!). I raised the exhaust a bit and slightly widened the exhaust and intake to end up with these timing numbers:

EX 102
pri 126
sec 130
IN 82.5

I, more carefully this time, cut the squish to .019”. The muffler outlet I made with 3/8” OD tubing. I used three of these outlets to land on a combined area I’m hoping will run well. I have not yet measured compression, but am guessing it’s fairly high. Piston is a Hyway pop-up. I have no bar yet so only ran it w/o load this morning. The throttle response seems good and it is “very angry” in nature. Am looking forward to getting it in some wood.

Possible concerns:
Will the cheap crank ass’y and bearings take it?
After removing .055” from the cylinder base, the flange thickness is only .155”. Will it fracture there?
Is the resulting 165 degees intake duration likely to cause poor performance and/or waste fuel?
Is the width of the squish band too great (or the dome volume too small)?
Will I be sucked into yet another hobby where no number of chainsaws will ever be enough?

Thanks for reading!

image.jpegimage.jpeg
 
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PA Dan

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I’d lose the popup personally. It already looks like it will have enough compression based on the chamber size.

Did you pull a compression reading yet? My bet is north of 250, maybe even north of 275.
Mine was at 190 right after building. It had .030 out of the band and about .023/.024 off the base and .016 squish. Haven't checked it recently and I bet I have 10 or 12 tanks now.
 

BuffaloBill

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Let's hear it running and see your throttle response! Can't wait to see how it cuts once you get the bar and chain on it.
I attempted to shoot a very short video hoping the file size will allow attachment. But the 2 second vid didn’t show in the list of attachable items. So I tried out uploading to YouTube. It appears to have worked👍. I’d say this saw sounds just like the other 200T vids I see on youtube(?). I couldn’t move my throttle finger any faster than that. But it seemed like the saw was responding as quickly as I could muster.

Here s the link:
 
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BuffaloBill

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I’d lose the popup personally. It already looks like it will have enough compression based on the chamber size.

Did you pull a compression reading yet? My bet is north of 250, maybe even north of 275.

I could use suggestions on how to best measure compression when the combustion chamber is so tiny; what model gauge to buy, or what to build, etc.

I’ve got an old automotive style compression tester that has a hose between gauge and plug fitting. It does have a schrader valve at the “end”, but it’s positioned in from the end far enough that that it effectively adds a bunch of volume to the combustion chamber. And these kind take so many pulls to pump up all that hose volume.
IMG_2367.jpeg

What I typically use on larger engines is this rubber tipped version, jamming it against the spark plug opening. This is sometimes difficult to do without a helper. With this saw’s tiny combustion chamber, I would think even the volume normally occupied by the spark plug, that isn’t being filled by this type of compression tester, would introduce quite an error making it read low.

I tried by my self, to wedge the saw with this gauge in the plug hole, against my workbench leg, and pull the recoil while holding the saw against the floor. I got a reading of about 180psi but could hear some leaking going on. Maybe I can talk my wife into giving me a hand with this. She may not be strong enough……..
IMG_2366.jpeg
A few years ago I played around with spark plug bases, epoxying valve stems in them to position low spring force schrader valves near the bottom of the plug. That part worked out OK but I didn’t really finish making a decent way to plumb to a pressure gauge. Then I began thinking that maybe the schrader valve (and the error they introduce) could be avoided by using a pressure transducer.

I had lying around from an old 1990’s work project, some Honeywell signal-conditioned and temperature-compensated pressure transducers. So I made up a spark plug base with a small diameter piece of tubing epoxied down its center, that the transducer plugs in to. The idea being to keep that internal volume very small. I tried it out on an 076 Super that I’d just gone through, using an oscilloscope to capture the voltage waveform.
IMG_1502.jpegIMG_1504.jpegIMG_1503.jpeg
This transducer has been discontinued by the mfg and I haven’t been able to find a complete datasheet. What I had found at that time, was a spec sheet listing the nominal gain & offset, and calibration tolerance, but didn’t include parameters related to response. From memory, I believe the frequency response was like 100Hz. So I don’t know how much overshoot (or undershoot) error the voltage waveform may be showing. But with the relatively slow cranking speed as the recoil is pulled, I’d expect this error to be minimal.

It outputs (nominally) 16mV/psi, offset 0.5v at 0psi. The peak shown on the oscilloscope of 3.48v equates to about 186psi. The sensor’s datasheet gain & offset tolerance suggests about a +/- 4psi possible error at that operating region. At that time the 076 had only about 1 1/2 tanks fuel on new Caber rings.

So this morning I was going to measure the 200T’s compression with this setup but found the plug base is too large. I can screw it in to the saw but there’s not enough clearance to get a wrench in there to tighten it. I had also made a smaller plug base with a schrader valve and a hose, but this hose doesn’t fit the transducer spigot. So I’ll have to do some more improvising to make things work.
 

BuffaloBill

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Here’s a pic of the smaller plug base and hose.

If there was a 0-300psi version of these digital tire pressure gauges, I would attach one to this green hose. I have a couple of these tire gauges and have been happy with them.

IMG_2364.jpegIMG_2365.jpeg

Update: I found this on Amazon so ordered one. https://www.amazon.com/gp/product/B098757WS4/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&th=1

So will see next week how this works out.

About how many psi error do the specialty schrader valves introduce? I should try to measure what pressure it takes for it to open, but what low pressure gauge would I use? Would a water column work? 5psi is 138.5” water (google search).
 
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PA Dan

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Try just tightening up to the O ring by hand. That should be enough to make a seal.
 

BuffaloBill

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Try just tightening up to the O ring by hand. That should be enough to make a seal.
Yes, I should try that. Presently that plug base still has the metal compression washer on it. But I could remove it and turn a slight recess in the base to help capture an o-ring.
 

legdelimber

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BuffaloBill, have you seen the web site called hackaday.com ?
Sometimes some folks are commenting on there who have some deep background in industrial/commercial stuff.
Maybe someone has seen the device or has some old data sheets or something?
 

BuffaloBill

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BuffaloBill, have you seen the web site called hackaday.com ?
Sometimes some folks are commenting on there who have some deep background in industrial/commercial stuff.
Maybe someone has seen the device or has some old data sheets or something?
I wasn’t aware of that site, will have to check that out, thanks. The datasheet I did find is this one:

I’m retired for a couple yrs now, but had designed circuits for industrial temperature controls as a career. I have a cousin who worked for Honeywell, who also retired recently, who might be able to have more info dug up.

I had posted a message a few years ago about this compression measurement topic on the other site, and a member there proposed a good method to test for overshoot. I could connect the transducer to my airline through a ball valve. Have the pressure set to say 175psi and quickly open the valve. Then check the resulting voltage waveform on the scope to see how it settles out, etc. I should do this sometime.
 
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legdelimber

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Been many years since I had my nose in a couple of books and then only while trying to cobble something together.
so..
Always bare in mind the old quote about possessing a smattering of ignorance about many things, with any of my comments on this.

That said, I wonder about tolerance to any stray bits of oil getting into the sensor.
I am assuming there's a gas tight diaphragm between outside atmosphere and the actual element?
Can't recall what piezos do under a static or "held" deflection or compression vs a strain gauge.
Are you looking for pressure pulses too, or just the pressure rise/offset vs resting state in open air.
Static or very slow pressure reading, I would think that the sensor/amplifier can perhaps cope with a little gunk.
But I have to wonder what oil/fuel or a slow varnishing of the sensor would do to pulse readings due to mass change of the sensor guts.

IF I'm just cluttering things here, someone say so and I'll chill out.
 

BuffaloBill

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I agree, the sensor may not be rated for gasoline or oil. If it were to fail at some point it wouldn’t surprise me. I’m just playing around!
 

BuffaloBill

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The digital pressure gauge and 1/8NPT hose barb fitting arrived yesterday. I first tried Teflon tape at the threaded joint but soapy water showed it was leaking there. Then I tried some of the Loctite 518 I’d used on the cylinder base. That seemed to eliminate that leak. Next I tried a new schrader valve. That eliminated all but a slow leak.

After pulling vigorously on the recoil a number of times, the gauge was showing 215psi. By the time I snapped the pic the pressure had bled off to what’s shown. I’m not certain if the remaining slow leak is at the valve or one of the hose barbs. I have not yet tried to measure the opening pressure of this schrader valve, but plan to do so.

I searched the web for specs on these specialty schrader valves but did not find a specified opening pressure. I thought I remember reading at one time, that the error they introduce was around 5psi. Do any of you know if that’s in the ballpark or not?

I’d like to replace the hose barb fitting at the spark plug base end, with some sort of pneumatic disconnect. So you could wrench in the plug base with a socket, then connect the hose.

IMG_2379.jpeg

Pics of the modified spark plug base are shown in post# 10. I did not run the saw to warm it before taking this reading. What is considered the better method….cold or hot?

Link to the gauge:

Link to an 1/8NPT female to 1/8” hose barb fitting:
 

legdelimber

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An old thread from over at Arboristsite about the Schrader cores and compression gauges.

These are the cores that I got from NAPA part# 700-9556 (quantity of 5)
Based on old memory....
Checking in an Echo blower, I got about 155~ish, with the lightweight spring (gauge rated) tester core.
I switched the Napa core out for one from a random wheelbarrow tube and I think that the wheelbarrow core tested at least 35~45 psi lower than the Napa one.
 

SCHallenger

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What's up buddy?
Nothing earth shattering, Dan. I'm going in for some injections & possibly laser ablation of some nerves in my ancient back. 47 years of practicing dentistry has taken it's toll.
 

Junk Meister

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Reading You guys posts gives me the Odd feeling of Eaves Dropping on a conversation spoken in HEBREW. I get the Gist but the digesting Tain't Happenin'.
 
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