According to the shop there is no need of mineral oil for just one cylinder, so we are running on regular oil.
That is definitely wrong; you are risking glazing the cylinder and it may never settle down. You have to follow the Lyco procedure for bedding in the cylinder also. See here
Long cross-countries are the best medicine for break-in. The cylinders get consistent ring pressure and the process happens much more quickly.
Actually there is a defined procedure, involving specific power settings, and specific power changes.
Whether long constant-power flights are good or bad I don’t know; I have heard arguments both ways. But of course constant power is how we all fly normally – especially with a turbo engine where MP is maintained over altitude.
placido wrote:
there is no need of mineral oil for just one cylinder
I am not sure if this is true. https://www.lycoming.com/content/hard-facts-about-engine-break
The secret to break-in is this: allowing the rings to wear on the cylinder walls without getting them too hot. Long cross-countries are the best medicine for break-in. The cylinders get consistent ring pressure and the process happens much more quickly.
From https://generalaviationnews.com/2012/09/05/how-to-break-in-your-engine/
These same break-in procedures also apply if a cylinder replacement takes place during the life of the engine.
When I had one cylinder pulled, after >2000 hours, the shop specializing on engine overhauls, explicitly mentioned to me to use straight mineral oil during break-in, even with only one cylinder.
So the fuel seems more than enough. It is cylinder 5 that got exchanged. We will monitor if we get a gradual decrease of the CHT over the next 10 hours. According to the shop there is no need of mineral oil for just one cylinder, so we are running on regular oil.
How much hotter would a new cylinder run compared to the old ones. Is 30 to 35 degrees about right or is the difference too much?
A TB21 is 250HP also.
29 USG/hr is about right for a 285hp TCM IO520 full power full rich setting. Anything less will tend to increase the CHTs during initial climb.
That’s a TB21…
Assuming the fuel flow is accurate, 110 is 29 USG/hr which is way more than enough. In fact it is probably way too much for a 250HP IO540 and I wonder who set up that fuel servo.
I also thought about this as I recall having a FF of about 115 lt at WOT but now I see less than 110 but it gets slightly more after a few minutes.
Yes that is a good point. I don’t know about the TB21 but on the TB20 it is common to have the full rich fuel flow (RSA5 fuel servo) set too low; about 22 USG/hr. It should be set to 24-25 USG/hr. That makes a big difference to climb CHTs and has no impact on cruise because you lean to peak EGT anyway (most TB21s seem to be flown 50-100F ROP but that’s another discussion, with the TIT being limiting 
).
Maybe your full-rich fuel-flow needs to be a bit higher. Are you WOT, full RPM and full rich for initial climb?
