Heres a question ive pondered.  is there a set ratio to use when determining engine parts?

i mean should the piston length be determined by the diamerter of it, should the rod length be determined by its diameter?

Hi PD's & welcome Nicodemus38....you have asked some questions without defining what type engine it is...however as a rough thought for most engines...we could consider

1. piston length must be considered as relative to the bearing requirement [against buckling] of the piston.....therefore it is also dependent on cylinder stroke

2. rod length...[I assume you mean stroke] is relative to a number of design criteria, however the diameter of the piston rod is also relative to a strength requirement [power being transmitted] & against buckling forces

In time you could go to the PD's LINKS/Downloads section & view the mechanical manufacturing Drawings of some model steam engines....here you will see the dimensional relationship between bore to stroke + the height of the Piston ...etc - Derek