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'Home-made' replacement propshaft & bushing for 7.5 K&B lower.


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#1 CapeTown73

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Posted 07 August 2017 - 05:25 AM

I posted the information below on a Facebook page this morning, where a few OB boaters were talking about the well-known concerns of the poor durability and lifespan of the standard prop-shaft bushes in our 7.5 lowers. In response, I feel obliged to share my solution for this problem.

About three years ago I asked a good friend here to machine some longer phosphor-bronze bushes (of my own design and which fully replaces the steel K&B screw-in bush assembly), and to make up longer steel prop-shafts. (See my design sheet below from which my friend machined the new assemblies and a further photo lower down of the new assembly parts as I received them.) 

The new bush unit (as I receive it) is not yet ready for installation so I then proceed to machine a recess into the front of the new bush so that once the new bush is fully screwed-into the leg, the front lip (with square recess) of the new prop-shaft is in the same position as the original K&B prop-shaft, thereby ensuring that the standard length flex-shaft fits perfectly. I then machine a taper on the rear end of the new bush so that it ends up to match the size of the prop-drive-dog. The new unit is then ready for installation into the lower, and importantly, it now has an extended length of bearing-surface of around 25mm (1 inch). This is almost twice the length of the standard K&B unit and phosphor-bronze is tons more durable than the original soft Lead-Teflon bushes in the K&B assembly!

However, there is one further very important requirement to ensure the long life of the new bush, and that is to have good lubrication!!! So I made up a pressurised oiler system which ensures continuous oil (castor-oil) delivery to the full length of the new bearing surface. See the images below which show how this sealed/pressurised setup works. The main requirement is for the oil to remain under pressure all the way down the lower, past the flex-shaft, and along the prop-shaft with the only ‘escape’ being where the prop-shaft protrudes out of the rear of the bush against the prop-drive-dog. 

The end result in my boats has been excellent!!!
I installed two of these new units into the lowers on my son’s and my boat over two years ago and they have ‘lived’ through at least 20 to 25 race days, with each race day consisting of 4 heats of 2 minutes milling + 5 minutes of racing AND AFTER TWO YEARS, THERE IS ABSOLUTELY NO NOTICEABLE WEAR ON EITHER OF THE SETUPS!!!! And I run 7.5 Novarossi’s in both boats so we’re pushing respectable power and revs through both prop-shaft assemblies!!! 

So in my opinion, there is a good solution, but it does require a bit of additional work on your boat to make it happen! 
I hope this info is of value to you. (THERE ARE A TOTAL OF 5 RELATED IMAGES/PHOTOS BELOW!)
Kind regards,



Attached Files



#2 Carl Van Houten

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Posted 07 August 2017 - 06:30 AM

Winnie,

 

Looks like you have been busy. I have a question, if you had flats milled on each side of your bushing (like K&B does) does the bushing stay tight in the lower unit? I would think you have tried the bushings without using the epoxy to seal everything up in order to retain your lube? When you use it without the epoxy and just use a gravity-drip oiling system, how well does it hold up?

 

Thanks!

-Carl




Edited by Carl Van Houten, 07 August 2017 - 06:18 PM.


#3 Randell Doane

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Posted 07 August 2017 - 08:45 AM

That's pretty interesting, maybe you can start manufacturing and selling these!  Also Hyperformance has great 7.5 prop shafts as well as Nitro Watersports (I'm not sure if you can get those anymore though)



#4 CapeTown73

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Posted 07 August 2017 - 12:21 PM

Winnie,

 

Looks like you have been busy. I have a question, if you had flats milled on each side of your bushing (like K&B does) does the bushing stay tight in the lower unit? I would think you have tried the bushings without using the epoxy to seal everything up i order to retain your lube? When you use it without the epoxy and just use a gravity-drip oiling system, how well does it hold up?

 

Thanks!

-Carl

Hi Carl, I trust you and your family are well. Herewith my answers to your questions.

  1. I simply use pliers to hold and tighten the new bushing into the lower. However, the secret is to screw the bush into the lower before any oil or grease is present on the components. I use a drop of CA-glue on the thread of the bronze bush and have found it to hold everything in place a lot better than any thread-lock product.
  2. I have found that a 'gravity only' oiler setup mostly does not provide sufficient oil delivery to the prop-shaft/bushing surface. In fact you will often find that the water pressure around the PSA will then actually push water back up past the prop-shaft. I know that sounds strange but it happens and water obviously has no lubrication properties in such an instance. And without the epoxy most of the oil in a 'gravity oiler' setup will end up leaking out of the lower before it gets to the prop-shaft. So the fact that your gravity oiler is empty after a heat, does not mean that the oil got down to the prop-shaft. For the record, the flex-shafts in my boats have actually also been in the lowers now for more than two seasons!!!

    That's it for now,
    Regards, 


#5 LohringMiller

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Posted 07 August 2017 - 02:55 PM

Wennie

 

That's a great solution to the K&B bushing problems.  I once had a K&B lower fitted with a ball bearing that used a similar shaft oiler.  I ran pipe pressure into the lower through a  nipple with a small diameter hole.  Your pressurized setup is a better idea.

 

Lohring Miller

PS I'm looking forward to meeting you this summer.



#6 Carl Van Houten

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Posted 07 August 2017 - 06:09 PM

 

Winnie,

 

Looks like you have been busy. I have a question, if you had flats milled on each side of your bushing (like K&B does) does the bushing stay tight in the lower unit? I would think you have tried the bushings without using the epoxy to seal everything up i order to retain your lube? When you use it without the epoxy and just use a gravity-drip oiling system, how well does it hold up?

 

Thanks!

-Carl

Hi Carl, I trust you and your family are well. Herewith my answers to your questions.

  1. I simply use pliers to hold and tighten the new bushing into the lower. However, the secret is to screw the bush into the lower before any oil or grease is present on the components. I use a drop of CA-glue on the thread of the bronze bush and have found it to hold everything in place a lot better than any thread-lock product.
  2. I have found that a 'gravity only' oiler setup mostly does not provide sufficient oil delivery to the prop-shaft/bushing surface. In fact you will often find that the water pressure around the PSA will then actually push water back up past the prop-shaft. I know that sounds strange but it happens and water obviously has no lubrication properties in such an instance. And without the epoxy most of the oil in a 'gravity oiler' setup will end up leaking out of the lower before it gets to the prop-shaft. So the fact that your gravity oiler is empty after a heat, does not mean that the oil got down to the prop-shaft. For the record, the flex-shafts in my boats have actually also been in the lowers now for more than two seasons!!!

    That's it for now,
    Regards, 

 

Sounds good. We have pounds of old K&B 7.5cc prop stubs.  Two years is great! I worked for Hartzell Prop where we drew the 3d models of the turbo chargers for aircraft and to my surprise, the turbo chargers that turned 135,000 rpm were not spinning on ball bearings but were turning that high RPM in "bushings" and a pressurized oil system similar to what you are doing.  Cool stuff for sure.

-Carl


Edited by Carl Van Houten, 09 August 2017 - 10:19 AM.


#7 CapeTown73

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Posted 09 August 2017 - 04:08 AM

Wennie

 

That's a great solution to the K&B bushing problems.  I once had a K&B lower fitted with a ball bearing that used a similar shaft oiler.  I ran pipe pressure into the lower through a  nipple with a small diameter hole.  Your pressurized setup is a better idea.

 

Lohring Miller

PS I'm looking forward to meeting you this summer.

Hi Lohring, Thanks for your comments, and the same sentiments here about us meeting you towards the end of next month. Just over 6 weeks left before we depart on our long-awaited trip to the North-West USA !!! Becoming more excited every day ! I include two photos of my new 7.5 OB tunnels for our new racing season. I think you will recognize the second hull but I'll tell you more about it when we meet.

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#8 LohringMiller

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Posted 09 August 2017 - 06:09 AM

It looks like a Leecraft cowl, but not Leecraft sponsons.

 

Lohring Miller



#9 CapeTown73

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Posted 09 August 2017 - 06:27 AM

You are 100% correct !  Will bring more photos of the boat with me and then share the whole background with you next month.



#10 Carl Van Houten

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Posted 03 October 2017 - 05:59 PM

Winnie,

 

How is your prop shaft and lower unit modification working out?

 

-Carl



#11 Jim Allen

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Posted 09 October 2017 - 12:38 PM

I replaced the two standard 1/4" ID lead teflon bushings with a one piece, precision machined (.2470" ID), B-10 Bearium Metal bushing. The bushing is pressed into the original threaded piece & it has thrust bearing surfaces on the front & back. I rebuilt two of these housings but have never had to use the second unit. I added a teflon thrust washer to the drive dog end & replaced the standard drive dog with a set screwed, pressed on, stainless steel drive dog. I use a grease gun with #2 Lubriplate Special Marine Grease & a grease fitting to lubricate the enitre hard brass stuffing tube & the propeller shaft's bushing.

 

Jim Allen

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Edited by Jim Allen, 09 October 2017 - 12:43 PM.


#12 Carl Van Houten

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Posted 11 October 2017 - 12:58 PM

I replaced the two standard 1/4" ID lead teflon bushings with a one piece, precision machined (.2470" ID), B-10 Bearium Metal bushing. The bushing is pressed into the original threaded piece & it has thrust bearing surfaces on the front & back. I rebuilt two of these housings but have never had to use the second unit. I added a teflon thrust washer to the drive dog end & replaced the standard drive dog with a set screwed, pressed on, stainless steel drive dog. I use a grease gun with #2 Lubriplate Special Marine Grease & a grease fitting to lubricate the enitre hard brass stuffing tube & the propeller shaft's bushing.

 

Jim Allen

That has to be better than the stock prop shaft asembly.



#13 Rudy Formanek

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Posted 13 October 2017 - 10:38 AM

Jim, is Bearium B10 used as a conrod bushing material also??



#14 Jim Allen

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Posted 13 October 2017 - 06:56 PM

Jim, is Bearium B10 used as a conrod bushing material also??

How are you doing Rudy? B-8 or B-10 Bearium metal cannot be used as a bearing material for a connecting rod bushing. Compare the data spec sheets for #544 Phosphorus bronze & B-8 or B-10 Bearium Metal; the reason will become obvious.

 

Jim Allen



#15 Rudy Formanek

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Posted 15 October 2017 - 11:24 AM

Was wondering why you had this relatively hard to get bearing material around. (From the Aerospace side of the businesss??)




Edited by Rudy Formanek, 15 October 2017 - 11:27 AM.