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Author Topic: K&B 40 bearing replacement  (Read 1991 times)

Offline Ron Duly

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K&B 40 bearing replacement
« on: May 15, 2025, 08:24:21 PM »
I need to change the crankshaft in a K&B 40 8011 front intake engine.  This is the small front housing engine.  Got the roll pin and set screw out but the spool/prop driver won't budge.  Ideas?  Thanks
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Offline Brett Buck

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Re: K&B 40 bearing replacement
« Reply #1 on: May 15, 2025, 08:43:02 PM »
Quote from: Ron Duly on May 15, 2025, 08:24:21 PM
I need to change the crankshaft in a K&B 40 8011 front intake engine.  This is the small front housing engine.  Got the roll pin and set screw out but the spool/prop driver won't budge.  Ideas?  Thanks

    Use a battery terminal puller. Heating the thrust washer will also help.

      Brett

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Offline Dave Hull

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Re: K&B 40 bearing replacement
« Reply #2 on: May 15, 2025, 10:53:11 PM »
If you are installing a used crank, be sure that the drive hub and the crank are phased the same way they came apart or that the roll pin holes were drilled perfectly on-center. I have at least one that was not. If the pin was partially (or completely) sheared expect that the puller will be working hard to move the metal around to get things apart. The puller will not likely fit behind the drive hub; you most likely will have to grab the tapered face closer to the front.

The new replacement parts from Randy (drive hub and crank) need to be match-drilled for the roll pin.  Roll pin part number 204091.

Also, be sure that whatever prop stud or screw that you use NEVER contacts the roll pin accidentally. I lost a couple of races until I had a Doh! moment....
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Offline Motorman

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Re: K&B 40 bearing replacement
« Reply #3 on: May 16, 2025, 08:28:14 AM »
What happened to the crankshaft?  You could make a little slap hammer that screws onto the prop stud to pull it off. Sounds bad for the threads but it doesn't take much to pull it off. I made a special plate that clamps onto the drive hub so I can tap it off the shaft. Of course pressing it on and getting the hole to line up is a trick. Can we just say this is the worst pita drive washer ever invented.

 
Quote from: Dave Hull on May 15, 2025, 10:53:11 PM
Also, be sure that whatever prop stud or screw that you use NEVER contacts the roll pin accidentally. I lost a couple of races until I had a Doh! moment....

Do you mean the prop stud tightens down on the front of the shaft and shears the roll pin?

MM :)
« Last Edit: May 16, 2025, 05:23:40 PM by Motorman »
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Offline Dave Hull

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Re: K&B 40 bearing replacement
« Reply #4 on: May 16, 2025, 05:30:50 PM »
Yes. I described it wrong.

I found that if I used a straight stud and was not careful, the stud would keep going deeper into the drive hub with lots of prop changes and then contact the end of the shaft, putting the pin in shear, and damaging it. The drive hub connection on the 8011 was always a bit less than robust, so you didn't need more things to help it along towards failure. The easy solution was to get rid of the stud and spinner-nut and just use a hex bolt that you know wouldn't bottom out. If you are just sport flying probably not a big issue. With a racing plane and lots of different pitmen and prop changes, it was one more detail than you wanted to keep track of.
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Offline Steve Helmick

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Re: K&B 40 bearing replacement
« Reply #5 on: May 16, 2025, 06:30:15 PM »
Do you know when K&B added the split pin to the prop driver? I don't recall that my Series '61 .35's had the pinned propdriver.

They didn't need them, either, because the shaft always broke just behind the front bearing, so the prop, propdriver, stud and front bearing went sailing off into the boondocks. Sometimes the crank pin came out a bit during the shaft-run and ate the backplate. TERRIBLE design. I would suggest swapping the entire front out for the newer style...but heavier...one.   R%%%% Steve
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Offline Motorman

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Re: K&B 40 bearing replacement
« Reply #6 on: May 16, 2025, 09:08:16 PM »
Thanks Dave, I'll have to find the one engine I have with that front end and see what I've got in there. Maybe a stud with an unthreaded section would keep it from going back.

Another thing about that drive washer is having to bore out the back of all your props 3/8".

MM :) 
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Offline Dave Hull

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Re: K&B 40 bearing replacement
« Reply #7 on: May 16, 2025, 10:02:40 PM »
True. But boring an APC prop out to the molded counterbore is far better than running it on the "hand-drilled" 1/4" hole. You picks your poison, and enjoys those results....
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Offline BillLee

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Re: K&B 40 bearing replacement
« Reply #8 on: May 17, 2025, 06:38:39 AM »
On that style front end, the roll pin is ALWAYS under shear stress, irrespective of where the prop stud is located. Every cycle of the engine, the pulse of the power stroke creates shear in the pin. The pins is quite hard steel to resist that and you do not replace it with something softer, such as a bolt since that WILL shear off under power.  (Don't ask me how I know!)
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Offline Ron Duly

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Re: K&B 40 bearing replacement
« Reply #9 on: May 18, 2025, 09:34:47 PM »
And the winner is.... Brett!  I used a Monokote heat gun to warm the prop driver, a drift and a small hammer to "help" the crank out and it worked just fine.  Swapped the crank, reinstalled the roll pin and set screw and it is now back together.  Thanks to all for the helpful ideas. 
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