The 61 is a solid performer. I think it is the best overall of the three common St models (45, 61, 90), all of which seem to share the same carb. The carb appears to have been designed for this motor, which makes it easier to tune than the other two models. I'm not looking at their site right now, but as I recall they actually make two motors of this displacement, right? One is a ringed motor, which is what I have. Clearances for the ringed motors are set looser than the unringed versions, because the ring is not a total circle but has an end gap. This allows the ring to expand as it heats to create the final tight seal. Versions without the ring are set tighter and actually loosen with heat to create the final dimension maintained during running. Break-in of my 90 has been fussy, but I suspect that has more to do with the compromised carb situation than the ring, as my 61 is also a ringed motor.