Prop friction cold vs hot

[SwimmingDragonfly96]

For those of you who have checked this, is it harder to hand prop your hot engine vs hand propping your cold engine? I’ve gone down a crankcase fretting rabbit hole. My engine has 350 TTSN from the Lycoming factory. Runs great, doesn’t burn any oil, analysis at every oil change is clean, filter always spotless, but I do notice a little more resistance after flying. Fretting kind of scares me as there’s no definitive way to check for it unless I’m mistaken?

Thanks!

 

[Roadjunkie1]

I'll take a stab at this: Sounds like you have a pretty tight engine with NO oil burning (unusual!) and good oil analysis. Components of engines, especially the pistons and cylinders, have different expansion rates. A cold engine likely has different 'tolerances' than a hotter engine. In other words, pistons are going to expand into cylinders and rings are going to 'grab' a little tighter. Knowing this is in thousands of inches but still significant in our hard-working engines. Bottom line: I think that is fine and likely 'normal'. Seems like I remember SuzieQ's engine being that way for a while.

There: that should start the debate!!

 

[andoman]

I’m with Roadjunki. A hot engine is tighter than a cold one. Those tighter tolerances in the cylinders produce higher compression where I wager the majority of the increased resistance lies.
As for fretting: I’m sure someone that has experienced the issue will chime in with said experience.

 

[AN23]

The experts will chime in but I think you can relax. For the case halves to fret, something would have to loosen up (like the thru bolts). If that happens, you'll find a broken bolt and maybe a stud or two beside it broken. Ask me how I know!

 

[lr172]

the different feel in turning resistance could be a lot of things, but have good confidence that the differences are NOT attributed to case fretting. In a Tight engine, there is approx .003" of clearance between bearing shells and crank. When lubricated, this creates literally no resistance to turning. The clearance is dictated mostly by the bearing shell. While fretting could reduce the size of ID of the case web from material removal, it won't force the bearing shells to compress and reduce that .003 clearance, unless the wear is excessive AND someone re-torqued the through bolts and thereby crushing the bearing shells. A 1/2" bolt will stretch around .005" when tight, but my belief is that as the joining surfaces wear away, that residual stretch will not be enough to crush the bearing more than a thou or so, which is still plenty of clearance. Even if it gets tight, there are several thou of copper and tin on the bearing surface and it will wear down in minutes resulting in the needed clearance.

Larry

 

[jrs14855]

My 0 320 at 300 hours is still very tight when hot. It does have 10-1 pistons which have some effect.

 

[BobTurner]

I'll take a stab at this: Sounds like you have a pretty tight engine with NO oil burning (unusual!) and good oil analysis. Components of engines, especially the pistons and cylinders, have different expansion rates. A cold engine likely has different 'tolerances' than a hotter engine. In other words, pistons are going to expand into cylinders and rings are going to 'grab' a little tighter. Knowing this is in thousands of inches but still significant in our hard-working engines. Bottom line: I think that is fine and likely 'normal'. Seems like I remember SuzieQ's engine being that way for a while.

There: that should start the debate!!

+1 aluminum cylinders shrink a lot more than steel cylinder liners when cold. Not only more friction from the rings, but better air sealing (you’re always on a compression stroke somewhere.). I’d bet that if you pull a plug from each cylinder the ‘harder to turn hot’ issue goes away.