# Experimenting With A 2stroke Trimmer Piston



## petcnc

My 29.9cc 2-Cycle Gas-Powered Straight-Shaft String Trimmer with Split Shaft died on me a year ago (full story HERE)



Cause of death was a broken securing pin




That found its way between piston and cylinder and distroyed the piston.




Instead of throwing the trimmer away (and miss all the fun) I decided to keep it and experiment with a piston replacement project.
As the cylinder was usable (with scratches though) and the old piston was beyond repair I tried to find a replacement piston.

*Original piston measurements:*

Diameter 36.36mm
Height 28mm (top to bottom skirt)
Top to wrist pin center 16mm
8mm wrist pin

Nearest piston I found at e-bay was:
A “37MM Piston Kit With Ring For STIHL 017 MS170”

37mm diameter.
30 mm height.
Unknown Top to wrist pin center distance (I estimated from the photo that it was close).
8mm wrist pin.
Cost $6 including postage from Hong Kong.

So I bought it to have a go at trying to make it fit.




When the piston arrived and I measured it was close to what I needed!

Top to wrist pin center distance 16.5mm (need to be 16mm).
Diameter 37 mm (need to be 36.36mm).
The piston will need a bit machined off the top of it and all around.

The problem I had was *how I could hold it on the lathe to do the machining*.
A quick look at the Internet revealed some diy very sophisticated devices to hold  a piston from the wrist pin to the chuck.







Nice but far too complicated! 
Not to mention that I do not trust the chuck for such a  work...
I prefer the faceplate not the chuck for a work like that.
I thought that I can use the “hold from the wrist pin” principle but in a simple way.

A piece of 10mm pipe threaded M8 at one end, made flat at the other end and drilled 8mm for the wrist pin will work just fine to hold the piston securely on the faceplate.




To machine all around the piston to bottom without scratching the faceplate an aluminum washer will give the necessary clearance.
So all parts needed were available and ready to be placed on the lathe.




The rod will go through the spindle, washers and nut at the back...




the aluminum washer and the piston at the faceplate.




To minimize the play of the pipe on the wrist pin I added two extra washers to the pin.




The completed setting looks like this.




The hardest part was to make the piston running true.

I hand-tighten the nut, then using a soft-blow hammer I taped the piston with it till it was running true and then fully tighten the piston on the plate (at the following photo is the old piston I used to try the setting).





I skimmed 0.50 mm off the top of the piston and 0.64mm off around it.
I know that pistons are not supposed to be cylindrical but I could not make any accurate measurements to the original one to tranfer it to the new piston.





I also shortened the rings (using my dremel)  according the size of the original ones



...and put the piston-rings assembly back to the cylinder.
Here the Piston is at BDC.
You can see the top of it exposed through the exhaust  port.





Some piston mod will be in order after testing that the engine works....




Similarly when the piston is at TDC the intake port is not fully opened.




The original piston was shorter at the intake side (Lower left at the following pic).




So after testing the engine some fine tuning will be in order.

Thanks for reading

Petros

To be continued...


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## Ulma Doctor

i like your thinking!


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## Chris Hamel

I really liked your solution for how hold the piston in the lathe.  Looking forward to hearing if it actually runs when you are done.


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## ndnchf

Very interesting project - please keep posting your progress!


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## cascao

"Similarly when the piston is at TDC the intake port is not fully opened."
You have to use your dremel here and match piston skirt with intake port.


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## petcnc

cascao said:


> "Similarly when the piston is at TDC the intake port is not fully opened."
> You have to use your dremel here and match piston skirt with intake port.



Well... I was thinking of something more sophisticated than that. To use the mini mill for instance and make a cleaner finish.

What bothers me is not the skirt but the holes I must drill aside the pin of the new piston!
As you can see at the following photo the old piston has 2 squqre openings next to the pin. If I do the same to the new one I think it will be too weak to support itself and run.


This one of the reasons I have this project in a stand-by mode until I fell confident on that matter to put it on the mill and make a series of holes on the piston.

I am all ears if anyone has any suggestions on that.

Thanks

Petros


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## ndnchf

I don't know anything about 2 strokes.  What is the purpose of the 2 square openings?  Do you need them or could they be simpler round holes instead?


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## cascao

The square opening around pin bearing? You don't need mill it.


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## petcnc

ndnchf said:


> I don't know anything about 2 strokes.  What is the purpose of the 2 square openings?  Do you need them or could they be simpler round holes instead?



I'm not sure I fully understood the purpose of the openings on the piston.
They are called "Piston Ports" and their purpose is to increase the efficiency and the power of the engine by leting the fuel mixture to circulate during the engine cycle.
The following sketch gives an idea of the working principle of the piston ports (marked as 58) during the cycle of a 2 cycle engine.


You can read the explanation of that here

Its far too complicate issue for me though


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## cascao

IF your engine do admission through cylinder, than piston will have this openings.
IF your engine do admission direct to carter, (or very low in cylinder) then piston will NOT have this openings.


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## petcnc

cascao said:


> The square opening around pin bearing? You don't need mill it.



You mean not to mill it but to drill? Or not to bother at all?


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## cascao

Or not to bother at all. It's there due cast process in piston fabrication.


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## cascao

You removed material from piston diameter. Check if you need to remove material from rings groove too. (rings radial thickness need be smaller than groove depth)


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## petcnc

cascao said:


> You removed material from piston diameter. Check if you need to remove material from rings groove too. (rings radial thickness need be smaller than groove depth)



Rings fit ok I think the initial depth was more than needed.
On the piston port issue I think its part of the design for the particular engine to run with these ports. as there is a provision to the cylinder for fuel mixture circulation through these ports as you can see in the next photo of the cylinder.


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## ndnchf

Very interesting - I'm learning a lot, thanks.


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## cascao

When piston goes up. there is vaccum on crankcase and fresh mixture air+combfrom carburetor goes there.
At same time other mixture from previous cycle is burned.
Than piston goes down. mixture goes by this shown ports (transfer ports) to upper part of engine at same time when burned old mixture goes out by exhaust port. Its always about transfer por angles and exaust pipe to clean cylinder from burned gases and not lose fresh mixture.
When piston goes up, it closes the transfer and exhaust ports, compress the mixture and explodes again.

Its {WATCH YOUR LANGUAGE.**G**} simple,free revving and powerful engine. Its one of these things that everyone someday would be given opportunity to race a motorcycle with


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## petcnc




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## Silverbullet

Doctor Frankenstein , if the monster lives it won't know how to run. I've done lots of rebuilds using odd parts from different manufacturers. For fifty years small engines have been my way to earn spending money. There shouldn't be any problem with your machine handling the piston your making . When you install it use plenty of two cycle oil in the rebuild on the piston and cylinder. Some pistons only mount one way ,, toward exhaust ,,. Hope your carbs adjustable . Sealed ones with your rebuild may not run right , never know till it's running.  Good luck be careful pushing piston and rings in cylinder they snap easy. I know many times over.


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## petcnc

Silverbullet said:


> Doctor Frankenstein , if the monster lives it won't know how to run. I've done lots of rebuilds using odd parts from different manufacturers....



He he he I like it! I never thought myself as Dr Frankenstein.

Well I see the crankshaft of the engine on the neck! The piston must be behind the intake port (nose)


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## crazypj

Bit of an old thread but I didn't want to start a new one if I didn't need to.
The slots in side of piston are very unusual, normally it's to lighten piston as the transfer ports are fed from the bottom and discharge over the top. It may be to limit crankcase compression and max rpm possible? (plus much lower power output?)
 For a single piston modification the faceplate method works well but if you have a set (usually 4 or more) making the piston holder with locating diameter is much faster to set up (I'm doing at least 8 and possibly 12 Honda CB750 motorcycle pistons to fit into CB550 giving a 591cc motor) There is only one critical dimension and that is the locating spigot, the rest can be roughed out.
 There is a machined section inside skirt plus all are machined  to a fixed inside diameter (evenwhen  oversized on outside diameter)I will be doing multiple operations including cutting new valve pockets so te holder is 'double ended' the opposite end will be machined to fit piston crown. I'll post more pics or start a new thread if there is interest?
 Couple of pictures


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## rzbill

crazypj said:


> Bit of an old thread but I didn't want to start a new one if I didn't need to.
> The slots in side of piston are very unusual, normally it's to lighten piston as the transfer ports are fed from the bottom and discharge over the top.



+1.


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## cascao

Two old South Bend books with some good attachments. Maybe it can inspire someone.


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## petcnc

cascao said:


> Two old South Bend books with some good attachments. Maybe it can inspire someone.


Very interesting read! 
Thank you
Petros


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## larry4406

What is/was the outcome of the original poster?


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## petcnc

larry4406 said:


> What is/was the outcome of the original poster?


A running engine but without the power of the original configuration!
It needs fine tuning to achieve the performance it was designed to have.
It is in the "to do" list at the moment as winter prohibits work outside to test it.


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## crazypj

Just went back to pictures, the lower edge of transfer port on piston is much lower on original piston (below gudgeon pin) That may be affecting crankcase compression? If it's running though it's a good sign as carburettor relies on crankcase pressure variations to work. The spark arrestor on exhaust outlet isn't partially blocked?


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## Hit-N-Miss Tom

crazypj said:


> Bit of an old thread but I didn't want to start a new one if I didn't need to.
> The slots in side of piston are very unusual, normally it's to lighten piston as the transfer ports are fed from the bottom and discharge over the top. It may be to limit crankcase compression and max rpm possible? (plus much lower power output?)
> For a single piston modification the faceplate method works well but if you have a set (usually 4 or more) making the piston holder with locating diameter is much faster to set up (I'm doing at least 8 and possibly 12 Honda CB750 motorcycle pistons to fit into CB550 giving a 591cc motor) There is only one critical dimension and that is the locating spigot, the rest can be roughed out.
> There is a machined section inside skirt plus all are machined  to a fixed inside diameter (evenwhen  oversized on outside diameter)I will be doing multiple operations including cutting new valve pockets so te holder is 'double ended' the opposite end will be machined to fit piston crown. I'll post more pics or start a new thread if there is interest?
> Couple of pictures




I like your idea of a fixture, perhaps even if it is for only 1 piston. In this case it may really be a benefit as he may be making more than one piston due to future run-in testing changes.


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## markba633csi

Petros: Those two square openings in the piston- could they be for improving lubrication of wrist pin (piston pin) by helping circulation of
fuel/oil mixture in that area
AND/OR
could they be integral to the intake or exhaust scavenging process? Most 2 cycle (2 stroke) engine pistons I have seen do not have such
openings so I'm puzzled also- is there a needle roller bearing in the connecting rod small end? Or simple bush bearing?
Mark S.


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## crazypj

If doing more than one piston or if your planning on more than one in future it's worth making a fixture. Even if you still prefer faceplate mounting, a fixture will be more substantial to beat into true running. You can see the Stihl piston is machined inside the skirt. If you have a local  RC model club, check if they use Stihl engines in anything( although I've only seen them in RC boats.) They always  break stuff  and pay too much to get repairs or replacements, worked with a guy who started a semi-full time business tuning weed-whacker motors for racing RC boats (traded as 'Toxic Marine')
Wow, just looked him up. Name has changed but it seems he's doing well enough to sponsor people
http://trmpower.com/


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## petcnc

markba633csi said:


> Petros: Those two square openings in the piston- could they be for improving lubrication of wrist pin (piston pin) by helping circulation of
> fuel/oil mixture in that area
> AND/OR
> could they be integral to the intake or exhaust scavenging process? Most 2 cycle (2 stroke) engine pistons I have seen do not have such
> openings so I'm puzzled also- is there a needle roller bearing in the connecting rod small end? Or simple bush bearing?
> Mark S.



Mark there is a needle roller bearing in the small end as you suspected. It seems to me that the openings may serve lubrication purposes as well as better fuel/air distribution.
The engine as it is now it takes many pullings of the starter rope to come to life and it does not sound healthy. I will try to fine tune the carburator to see if it gets better but I'm a bit frustrated after all this effort to be in square 1 again.
At the moment I'm not playing with it as I learn a cad (Eagle) for making PCBs (printed circuit boards) on my cnc mini mill.
Thanks for pointing the bearing lubrication issue!
Petros


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## markba633csi

Petros: 2 cycle engines are the product of many years of engineering and improvement. You can be fairly certain that the shape and size of the piston and ports has been carefully designed to produce maximum power and any changes in the configuration will affect the performance.  Although I don't understand exactly why the piston is shaped as it is, I would wager that is where you need to copy the original as close as possible
Mark


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## petcnc

markba633csi said:


> Petros: 2 cycle engines are the product of many years of engineering and improvement. You can be fairly certain that the shape and size of the piston and ports has been carefully designed to produce maximum power and any changes in the configuration will affect the performance.  Although I don't understand exactly why the piston is shaped as it is, I would wager that is where you need to copy the original as close as possible
> Mark



Words of wisdom!
Thank you Mark. I adopt this way of seen things... but on the other hand... if we do everything "by the book" we will miss all the joy hehehe

Petros


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## markba633csi

The development of 2 cycle engines is quite interesting, much of the pioneering work was done in Japan in the 60s
In my younger days I had two Kawasakis: a 250cc twin with rotary valves and a 500cc H1 triple cylinder.  Crazy fast bikes.
It's a wonder I'm still alive.
Mark S.


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## petcnc

Although I'm not young, (57) I  have 3 Yamahas, a 2 stroke 50cc Bop II, a 110 cc (13HP) 2 stroke F1ZR and a 4 stroke single cylinder 660 cc XT to play with.
All my friends tell me that at my age "bones heal slowly" to stop riding and use my car... But I like riding, it makes me feel younger!
Petros


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## crazypj

You could try drilling round holes in sides to see if it imptroves starting/running. Round holes would obviously need to be smaller but would 'weaken' the piston less. Unfortunately I haven't messed with two stoke piston modifications since the early 1980's
Oh, the main two stroke development was done in East Germany, the 'Schnurle'(sp?) design only came to 'western' notice after a defection by MZ works rider around 1965. Suzuki were probably first to use it?


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## petcnc

crazypj said:


> You could try drilling round holes in sides to see if it imptroves starting/running. Round holes would obviously need to be smaller but would 'weaken' the piston less.


As these engines run in high rev speeds I suspect that the piston needs all the  material around wrist pins to have the strength needed to push and pull back itself thousant times a minute. Inertia is the enemy here.
To be honest I hesitate to drill or mill at this area. I would easily drill or mill enywhere else but there.
petros


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## crazypj

Pretty sure holes at sides of pin away from main boss will be fine. I've been modifying engines since around 1974


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## yendor

Actually 2 Strokes go way back before the 60's and Japan.
I had a Lawnboy mower that was my grand fathers'. 
I use to get $1.00 when I was 10 to cut his grass with it. (he wouldn't let you run the power mower until you were 10, if you were younger you used the push reel mowers - he had 2)
I'm 61 and the mower was also used by oldest brother who is 9 years older than me.
I'm only guessing but I would say this was an early to mid 50's model.

It had an open flywheel and you have to wrap the starter cord around an open pulley.
Once it started you tied the cord to the push handle. - No auto rewind that didn't come for many years.
Unfortunately the material around the coil give up and disintegrated into dust, and I was unable to find anything to replace it.


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## crazypj

I'm only 60   
'Playing with 'motybikes' at 18 monhts old  but was 11 when I got 'more involved' with them
Fully aware two stroke motors were around a long time, Scott started in 1908 and was made until 1978 (as SILK)
Weird engine, 'overhung' crank, (only one flywheel each side for twin cylinder)) deflector pistons, etc.
Piston 'strength' with holes drilled shouldn't be affected, drilling holes would lighten it ever so slightly and it's 'apparent weight' would be substantially lower at 7~10,000 rpm (inertia makes an 'ordinary'1lb  85~90mm piston weigh around 5~7 tons at 7~9,000 rpm TDC when it has to 'stop dead' then reverse direction. As long as you don't remove material  too close to pin things will be fine


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## crazypj

WOW,just looked Scott up on Wiki. Seems Alfred Scott invented and patented the loop scavenge system and MZ copied it


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## petcnc

crazypj said:


> Piston 'strength' with holes drilled shouldn't be affected, drilling holes would lighten it ever so slightly and it's 'apparent weight' would be substantially lower at 7~10,000 rpm (inertia makes an 'ordinary'1lb  85~90mm piston weigh around 5~7 tons at 7~9,000 rpm TDC when it has to 'stop dead' then reverse direction. As long as you don't remove material  too close to pin things will be fine



Thanks! I will try it and let you know of the results. At the moment the weather is not the best to work in the garage.
Petros


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## crazypj

LOL, I'm in central Florida and garage is downright chilly at present. almost 10:30am and only 46f, usually high 60's this time of year. Weatherman just commented, should be 70~71f expect high 58f. Compared to further north it's 'warm' but 20 below normal feels pretty chilly


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