Experimenting With A 2stroke Trimmer Piston

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.
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+1.
 
Two old South Bend books with some good attachments. Maybe it can inspire someone.
 

Attachments

  • south bend Auto Mechanics Service Book - No 66.pdf
    14.6 MB · Views: 24
  • south bend How to Finish Pistons - Bulletin No 9.pdf
    2.2 MB · Views: 15
larry4406 said:
What is/was the outcome of the original poster?
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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.
 
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?
 
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
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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.
 
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.
 
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/
 
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.
Click to expand...

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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