O-300 Bent Cranks

[KS170A]

I visited a local repair station that specializes in reciprocating engine component inspection and repair to discuss their services applicable to the TCM O-300A engine. I received the nickel tour of their facility, which was intereting enough. One thing the host told me was that they find that (in his words) they find about 90% of O-300 cranks are bent. He further commented that most of those can be brought into limits via grinding. My question to those of you who have been involved in an overhaul, does this sound right? I found it surprising that so many would be bent.

[T. C. Downey]

One thing the host told me was that they find that (in his words) they find about 90% of O-300 cranks are bent. He further commented that most of those can be brought into limits via grinding. My question to those of you who have been involved in an overhaul, does this sound right? I found it surprising that so many would be bent.

No that is not the normal.

What I see most is corrosion at the prop flange and a groove at the case nose seal, and minor scratches at the bearing journals.

When supported at the first and last main bearing journal, we are allowed a runout the the center main. we should never sacrifice journal material to correct this. hop that do this is the major contributor to us having no cranks

[blueldr]

I've been fooling around with these things for a good many years and this is the first time I have ever heard of a "bent " crank out of an active engine.
It seems to me that a bent crank would have to constantly flex while running, and I can't immagine one running very long under those conditions. I can't immagine that any crankshaft could have any flexability at all.

[marathonrunner]

TCM cranks are also fully nitrated which does not usually give them a lot of flexibility and if bent they usually crack and eventually fail. Lycoming uses copper to flash the areas during the nitriding process which does allow them to bend and be rebent or did until a recent bulletin did away with that. I have re bent several Lycomings after a prop strike and dialed them and did a mag inspection and no cracks. Continentals are usually not so lucky.

[Sixracer]

I owned my own crank grinder for many years. I used it in my race Engine business. yes, you can grind a certain amount of bend out of a crank and you can correct the index & correct the stroke by grinding, but..... you can't grind metal into an area where it isn't and is supposed to be. I'm saying there are limitations to what you can do by grinding. Most cranks that I found bent beyond limits had been abused in some manner. I'm like the other poster that said he had never seen a bent crank out of a running engine. I will qualify that statement by adding the word Good...I've never seen a good bent crank come out of a running engine. If it checks with too much bend you had better look real close for a flaw that is a crack or a weakness in its structural make up. Then there is abuse. Detonation, broken piston or broken rod can all twist up a crank.
That my friend is not normal. A crank doesn't just bend beyond limits in normal operation. So I will have to say you have had a sales pitch put on you and you will most likely have to spend some pretty big bucks at that crank shop. Check around and check any shops customer list. The customers should be the shops best advertisement.

[KS170A]

Thanks for the responses; they all seem to support the red flag that went up when he told me that. I think I'll double-check to make sure I understood the gentleman as this is a widely-advertised repair station and been in business over 30 years. I doubt you stay in business that long by failing cranks at a higher rate for reasons that don't pass the sniff test.

That said, anyone here have suggestions on a shop that would do the MPI, grinding, reconditioning, etc, of the O-300 innards? I plan to do the disassembly, reassembly, and installation myself but lack the tooling and expertise to do those details.

[T. C. Downey]

Aircraft Specialities Tulsa OK.

http://www.aircraft-specialties.com/engine-parts/

[Bruce Fenstermacher]

Aircraft Specialities Tulsa OK.

http://www.aircraft-specialties.com/engine-parts/

Tom and I think alike on this.

[GeorgeH]

I have the C-145 engine that was originally in my plane sitting in the hanger. It has 6000 TT with 4000 hrs since the crank was turned .10 under.When we changed it out it had 2400 SMOH and was running good except for a bad cylinder. Any ideas on the odds of the crank still being useable? Thanks!

[T. C. Downey]

I have the C-145 engine that was originally in my plane sitting in the hanger. It has 6000 TT with 4000 hrs since the crank was turned .10 under.When we changed it out it had 2400 SMOH and was running good except for a bad cylinder. Any ideas on the odds of the crank still being useable? Thanks!

pull it, measure it then you will know.