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  #1  
Old 04-11-2021, 11:09 AM
Jslow2 Jslow2 is offline
 
Join Date: Dec 2020
Location: Cement City
Posts: 58
Default crankshaft pitting

how bad is this?



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  #2  
Old 04-11-2021, 01:53 PM
theduff theduff is offline
 
Join Date: Jan 2012
Location: Fallbrook Calif.
Posts: 332
Default Pitted Thrust Face

Pretty bad. Definitely not airworthy in the Certified world. That is the thrust surface for a tractor configured engine, the prop is always pulling the crank against that surface. Having said that I have seen guys run pitted cranks like that in experimental’s. Likely hood of failure is very low in my opinion but it’s not what I would recommend. You can do some minimal grinding in this area depending on your thrust clearance. Wouldn’t hurt to have AEA out of Dallas take a look and see what they can do for you.
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Last edited by theduff : 04-11-2021 at 02:05 PM. Reason: Additional Info
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  #3  
Old 04-11-2021, 07:08 PM
salty salty is offline
 
Join Date: Nov 2020
Location: FL
Posts: 104
Default

Pretty interesting to think about how that little surface is what pulls your entire plane around everywhere it goes.
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  #4  
Old 04-12-2021, 07:07 AM
Joe Jet Joe Jet is offline
 
Join Date: Jun 2014
Location: Seattle, Wa.
Posts: 13
Default

Is that a hollow crank? (Lyc SB 505B)?
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  #5  
Old 04-12-2021, 07:43 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Default

Quote:
Originally Posted by theduff View Post
Pretty bad. Definitely not airworthy in the Certified world. That is the thrust surface for a tractor configured engine, the prop is always pulling the crank against that surface. Having said that I have seen guys run pitted cranks like that in experimental’s. Likely hood of failure is very low in my opinion but it’s not what I would recommend. You can do some minimal grinding in this area depending on your thrust clearance. Wouldn’t hurt to have AEA out of Dallas take a look and see what they can do for you.
+1

As long as the pitting doesn't extend into the radius area, it seems the likelihood of catastrophic failure is pretty low. I believe it is a low stress area. That said, I don't think I would return it to service as is. I vaguely remember the thrust clearance range as around 20-25 thou or so and it's likely you're already in the middle of the range. Based upon the pics, those pits seem too deep to machine down and meet clearance specs.

One option is to have ASA machine it down and give you a new spec. Then see if Divco will build up the matting area on the case and machine back to a custom spec that gives your crank the proper thrust clearance. There is no thrust bearing on the case. It just rides against a machined part of the case. This would make your parts a one off that wouldn't work in any other combination, but worth considering given the crazy cost of crankshafts. I suspect that neither shop will machine to any tolerances other than whats in the Lyc manual though without a lot of pushing.
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Last edited by lr172 : 04-12-2021 at 07:52 AM.
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  #6  
Old 04-12-2021, 07:22 PM
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emsvitil emsvitil is offline
 
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Location: SoCal
Posts: 496
Default

Can it be ground out and down,

Built back up with some welding.

Then machined to correct spec?
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  #7  
Old 04-12-2021, 08:39 PM
georgedouglas georgedouglas is offline
 
Join Date: Jan 2015
Location: florida/tennessee
Posts: 37
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Guys, in my opinion this crank is not even suitable for an airboat. Why would anyone even think about using a crankshaft this bad in anything that leaves the ground. I know what new crankshafts cost, as I had to buy a new one last year for $4900.00. george
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  #8  
Old 04-13-2021, 02:06 AM
KayS KayS is offline
 
Join Date: Jul 2009
Location: lake constance
Posts: 358
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i wonder how many of us are flying around with similar crankshaft pitting, not knowing there is any.

besides splitting the crankcase... how to detect that? could surface pitting be seen as debis in the filter?
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  #9  
Old 04-13-2021, 06:09 AM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 5,891
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Quote:
Originally Posted by KayS View Post
i wonder how many of us are flying around with similar crankshaft pitting, not knowing there is any.

besides splitting the crankcase... how to detect that? could surface pitting be seen as debis in the filter?
Since it is corrosion, I suspect oil samples would have shown some thrust bearing materials from erosion after the corrosion events. Also if it is significant wear, the axial movement of the crank (dial indicator) would measure the increased clearance.

One might also expect small shiny particles in the filter, but only an analysis would link it to a bearing. I don't know what the construction of the thrust bearings are, but suspect, lead/tin overlay, copper barrier layer and aluminum body and possibly a steel shell.

It would be odd to see corrosion isolated to just the thrust surfaces, I would expect to see evidence widely in the crankcase.

IMO, it is worth sending the crank for inspection as a minimum as pitting can greatly reduce fatigue life, but certainly not use it as-is. Why? Not because I know, but because I don't know.
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Last edited by BillL : 04-13-2021 at 07:01 AM.
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  #10  
Old 04-13-2021, 06:40 AM
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rocketbob rocketbob is offline
 
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Location: 8I3
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There's a lot of going off in the weeds here. To answer the question "how bad is this", one has to first measure.

The only way to accurately measure this is on a granite surface plate using a shim and a height gage.

Pitting is usually a few tenths deep but just keep in mind what you see now may not be what gets revealed after grinding.

In this case the minimum dimension for the thrust face to the rear crankshaft gear face is 13.962".

Knowing how much "meat" is left will give you an idea if the thrust face can be cleaned up.

Also consider if there is any thrust face wear in the crankcase and how grinding the thrust face on the crank will affect the slinger clearance.
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Last edited by rocketbob : 04-13-2021 at 06:44 AM.
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