Experimenting With A 2stroke Trimmer Piston

petcnc

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My 29.9cc 2-Cycle Gas-Powered Straight-Shaft String Trimmer with Split Shaft died on me a year ago (full story HERE)
Leroy BC930N.jpg

Cause of death was a broken securing pin

P5250774s.jpg

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

P5260789ss.jpg

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.

New piston.jpg

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.

CHUCK piston.jpg

CHUCK piston1.jpg

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.

IMG_20160929_232542308ss.jpg

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.

IMG_20160929_232542308s.jpg

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

PA021410s.jpg

the aluminum washer and the piston at the faceplate.

IMG_20160929_231413997s.jpg

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

IMG_20160929_231422127s.jpg

The completed setting looks like this.

PA021409s.jpg

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).

PA021415s.jpg


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.

Pistons.jpg


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

Rings1.jpg
...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.

PA021419s.jpg


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

piston cutout.jpg

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

PA021420s.jpg

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

P8301347s.jpg

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

Thanks for reading

Petros

To be continued...
 
Last edited:
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.
 
Very interesting project - please keep posting your progress!
 
"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.
 
"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.
IMG_20160929_231609943s.jpg
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
 
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?
 
The square opening around pin bearing? You don't need mill it.
 
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.
US3905340-4.png
You can read the explanation of that here

Its far too complicate issue for me though
 
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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