OS FS-52 Surpass Four Stroke Piston Orientation

[barrti]

I am rebuilding an OS FS-52 Surpass engine. Someone else has removed the piston so I was not able to mark it for orientation.

I notice that there is a small rectangular casting lump inside the piston, under the piston crown (See image below).

It seems like a deliberate feature, so I am guessing that it is there to add some strength to a hot part of the piston?

What is the correct orientation? Should the piston be oriented so that this small rectangular "lump" is under the exhaust valve or should it be 180 degrees away?


It would make the CG slightly off centre as well.

 

[barrti]

Greg from rcdieselrc (YT) confirmed that the piston casting is used in two and four stroke engines. The lump is used for the piston peg location in a two stroke engine. He said that there is no orientation.

I'll put it back with the lump under the exhaust valve in case it helps with heat transfer and hot strength.

 

[petertha]

Interesting. I don't know many of the OS engines intimately but I was curious about ringed 2S versions. Did he say which model?
The OS-56-alpha 4S doesn't look like that, its fully machined. I'm not doubting, just curious.

Looks like your FS-52 has a 23mm bore?
https://www.os-engines.co.jp/english/line_up/engine/air/single/catalog/34200.htmhttps://www.os-engines.co.jp/english/line_up/engine/air/single/manual/fs52s.pdf
55-AX-BE has matching 23mm bore
https://www.os-engines.co.jp/english/line_up/engine/air/aircraft/catalog/15620.htmlhttps://www.os-engines.co.jp/english/line_up/engine/air/aircraft/manual/55ax_be.pdf
Discontinued 50-SX has 22mm bore
https://www.os-engines.co.jp/english/line_up/engine/air/aircraft/catalog/15600.htm
Discontinued 50SX-H has 22mm bore
https://www.os-engines.co.jp/english/line_up/engine/heri/manual/50sx-hring.pdf

 

[barrti]

He didn't say which one. Yes mine ha a 23mm bore. My OS FS-40 has a similar piston too (with the lump).

Cheers

 

[kf2qd]

Is the lump there to make sure the rod stays on the crankshaft?

 

[barrti]

Nice idea but no, it clears the rod by quite a bit. I'm pretty convinced it is as rcdielrc says: it is there to use for a ring peg if the piston is used in a two cycle engine.

The rod big end is located by the back plate and the crankshaft web. This is the usual arrangement for these overhung crank designs.

There is quite a bit of room either side of the little end on the rod. In other words, there are several mm clearance between the rod and the piston.