1/5 Chenery Gnome Rotary: Piston fit with cylinder

[albd5a]

This is a general question; but before I fabricate nine pistons to the plans dimensions: how should the fit within the cylinder walls be - both with the rings and without rings in place? (Photos show ‘slave’ fixture for aluminum piston fabrication).

 

[Jasonb]

With 1thou clearance the piston should slide freely within the cylinder, you will feel resistance when the rings are fitted particularly when new before they bed in.

 

[petertha]

Not sure if it is directly applicable to your build but Terry typically includes a nice dimensional sketch of his pistons within the bores and he directly relates them to shop made piston rings using Trimble method originally published in SIC magazine. Attaching sample pics here, but worth reading the build details. For example link to his Offy engine (post 453). Now people make rings using various methods other Trimble's method, which is a whole subject unto itself. Do a forum search & you will get LOTS to read. I'm just trying to say designing the piston will typically include considerations to intended compression rings above & beyond annular clearance.

https://www.homemodelenginemachinist.com/threads/270-offy.31486/page-23