The International Cessna^® 170 Association

Hi everyone...i'm a fairly new member and 1954 C170B owner with A&P/IA ratings. Before removal and teardown of my C-145-2 engine i was concerned about not having the dampned crankshaft because of the 1/4 dia. hole in the flange. I had read all the messages here and read the overhaulhaul manual and i believed that it would not be dampned. However, my engine s/n did have a "D" in it and i knew it was the A/C's original engine with 1900TT and a major O/H @ 800hrs (TBO in 1958). Sure enough after removal it was dampned. I sent crankshaft and all other internal components to Aircraft Specialties. Crankshaft checked good , mains bearing journals were still high new tolerences however the rod journals were at minimums so i elected to grind them to .010. So in summary........ i would suspect that if your engine crank has the hole in flange but has the "D" in the engine S/N and has not been changed it will be dampned.

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from the recent thread Understanding Generator "Codes":

lowNslow wrote:You can tell if your crank is undamped by looking at the propeller flange on the crank. If it has a 1/4 in. hole drilled in the flange, it is undamped. The letter "D" in the C145 serial number is also supposed to indicate a damped crank, but with all the switching around of parts in these old engines the only way to tell for sure is to check for the hole.

this might be a good opportunity to comment again on this oft-repeated statement. Indeed the TCM Overhaul Manual says, "Early production C-145 crankshafts had no counterweights and were identified by a 1/4 in. diameter hole drilled between propeller bolt bushings through the propeller flange."

I've seen maybe a dozen C-145/O-300 cranks, some with flange holes, some without - but they've all been dampened. I've never seen a undampened crank for one of these engines. If somebody has ccme across one, I'd sure like to know what it looks like.

I have pics I took last year of 3 dampened cranks, 2 with a flange hole, 1 without. I'll post them here if someone's got a place to host them.

the parts book only shows one PN for the C145-2 and O-300A crank. One guess is that even the earliest ones had the blades to accept counterweights. The flange was drilled only if *not* installed (counterintuitive to me...). Of course, first time someone had the crank out they would've put counterweights on.

the point of this is just 'cause you have a ¼" hole in your prop flange, don't assume you have an undampened crank. The only real way to know is to have a look. Should be pretty obvious if you have a #1 or 2 cylinder off.

- paul

[...hoping the moderator can correct the spelling in the thread title...]

No Paul the undampened cranks do not have any flanges or blades to except counter weights. The cranks look just like any car engine crank would look except they have the prop flange at one end.

I have one in my engine and payed a big price to view it when I needed a rebuild. Hope I don't get the opportunity again anytime soon.

I don't recall if my crank has the 1/4" hole or not.

spiro wrote:I have pics I took last year of 3 dampened cranks, 2 with a flange hole, 1 without. I'll post them here if someone's got a place to host them.

Very interesting. I have a "D" in my serial number but have a 1/4" hole in my flange. I always assumed I had had the crank replaced with an undamped one at some point due to the 1/4" hole in the flange, but I have not checked to see. Thanks for the info.

UN-dampened crankshafts have a 1/4" hole drilled thru the prop flange between the prop bolt bushings. They have no provision for adding weights, and no longer used (although I have heard of them being improperly installed in C-125 engines to illegally increase compression.)
Engines which ORIGINALLY had dampened cranks have a "D" in their serial number on their dataplates.
Engines which may have a "D" in their serial numbers MAY HAVE HAD THEIR DAMPENED CRANKS REPLACED with UN-dampened cranks with the 1/4" hole drilled thru their prop shafts. (Not the best thing but not illegal, as long as other restrictions are taken under consideration such as generator amperage, etc. The dampened cranks are designed to relieve gear-teeth wear at the crank gear.)
Dampened cranks, which have had their counterweights removed may have been drilled "in the field" to denote them as undampened (legality of which is unknown without further info. I personally listend to an "engine" mechanic explain that he thought it OK to remove the weights if worn bushings could not be replaced with serviceable ones because...."this engine flew for years without them.")
Warning: C-125 crankshafts may be improperly installed into these engines, but their "throw" is shorter than C145/O300 engines and will not produce rated power.
(Does this thread help indicate the importance of proper records during engine rebuilds?)

gahorn wrote:UN-dampened crankshafts have a 1/4" hole drilled thru the prop flange between the prop bolt bushings.

and so do many dampened cranks. That was the point of my post.

gahorn wrote:Dampened cranks, which have had their counterweights removed may have been drilled "in the field" to denote them as undampened

that seems unlikely. Why would one remove counterweights? They're pretty easy to come by from reject cranks and are a big reason the C145/O300 is so smooth. Let me rephrase, *most* of the cranks I've seen have the ¼" hole in the flange, and they've all had counterweights on them.

Bruce, is it possible you have a C-125 crank in your motor? Did you confirm the stroke?

and George, really, could you correct the spelling of the thread title so it could be found in a search? Thx.

Paul
My crank was dimensionally checked for a c-145 and reground at overhaul. That and no indication from the logs that it would be anything other than a C-145 crank.

Come on, Paul... lemme buy a vowel... ...
Don't know why anyone would drill such a hole...but according to TCM, ONLY undampened cranks have the 1/4" hole.
As for smoothness.. I think that may be a subjective thing. I believe the main reason these engines are so smooth is due to their being six-cylinder. The counterweights were designed to get rid of vibration that is undetectable to the operator, but which affects gear teeth wear. Most reciprocating engines have flywheels, sometimes associated with dampeners, to smooth out the individual impulses of firing cylinders. While the propeller on an aircraft engine behaves much like a flywheel, ... it also behaves a lot like a tuning fork also depending upon a lot of factors such as material, length, hardness, etc. (One advantage of many wood prop installations is the tendency for wood to absorb those impulses which is nice for the engine.)
If one can imagine a piston slamming downward after the firing of the spark plug, one can imagine the impulse imparted to the crankshaft. Next comes a short period of rapid deceleration caused by the resistance of the next-in-the-firing-order piston as it travels up in the compression stroke. The result is a rapid vibration of push/resistance/deceleration/pause/push/resistance/deceleration/pause over and again, which equates to gear teeth slamming first one way during a firing stroke, then hesitating, while the driven gear over-rotates slamming it's teeth against the driving gear during a compression stroke,... then it does that again and again 3 times 2450 = 7350 times per minute at cruise. (This is why the larger mass of the 35 amp generator armature is more harmful to a non-dampened crank gear.)
The counterweights dampen that vibration... but it's not something noticeable in the cockpit.

george, I'm not trying to be argumentative. I'm well aware of what TCM's publication says, I quoted it in my first post. Just reporting what I and others have found in the field.

I don't know why; Was hoping someone else could shed some light. Bruce's reply indicates my guess was wrong. Your guess is that any hole in the flange of a dampened crank was drilled in the field?

Paul, I'm not questioning your word on what you say you have observed. I'm only pointing out how much in contrast that is to the mfr's specifications. How the crankshafts you observed got in that condition is anyone's guess.... but it is wrong according to TCM, and should not have been done by whoever did it.

(Does this thread help indicate the importance of proper records during engine rebuilds?)

The AI that did the overhaul on my O-300 only wrote "New Crankshaft" in the logbook without any supporting data or tags. Shortly after I bought the plane he had his certificate pulled by the FAA. This did not inspire my confidence in his work. So, on reading this thread, I had to check for the 1/4" dia. hole in the crank flange. Sure enough, the hole was there.
But, after removing the #1 cylinder to confirm my fears, it turns out that it is a dampened crankshaft.

n2582d wrote:The AI that did the overhaul on my O-300 only wrote "New Crankshaft" in the logbook without any supporting data or tags. Shortly after I bought the plane he had his certificate pulled by the FAA. This did not inspire my confidence in his work. So, on reading this thread, I had to check for the 1/4" dia. hole in the crank flange. Sure enough, the hole was there.
But, after removing the #1 cylinder to confirm my fears, it turns out that it is a dampened crankshaft.

Well, I finally had an opportunity to check my crankshaft when I had to remove a cylinder and sure enough it also has counter weights AND the 1/4" hole in the prop flange as well which according to TCM is indicative of a undampened crank.