I own 3 OS 61 SF engines, two ringed and one ABC
No problem with the linner at all ,on the ABC engine ,which i attribute to the fuel i use ,Morgan Omega 15% ,and never run the engine lean
i was only flying for a short time,when this series of engines was release by OS (early 80's )
I got one early on ,so i have been enjoying this engine for many years
A long stroke engine ,not an RPM engine ,so more torque
I cannot say enough about how much i enjoy running this series OS 61
Engine is supper user friendly and puts out real good prop turning power
I run all my glo engines on the same fuel and a K&B 1L glo plug
I run a 12/6 apc @ 12,600 RPMs ( max on ground ) on my 60 Sig Super Cahos
This is one of those glo engines that is --just better than it is supposed to be

I have a 91 FX in my 60 Ultra Sport
That 91 is in a 60 size case ,so its light ,a strong choice for many 60 size performance airframes

Both the 60 Super Cahos and the 60 Ultra Sport --fly on rails , at a bit over 1/2 trottle ( at a fast crusie speed ) ,and have unlimited vertical ,with the other 1/2 throttle

Pictures of both the above planes are in my -Gallery , to the left of this post

Enjoy