Powerhead rebuild tips

[cvlngineer]

I am getting ready to tackle the chore of replacing the knuckle bearings in this 1975 Johnson 70 hp motor. I have never opened one of these up before, and I am wondering what pitfalls may await me. What all should I replace while I have it opened up, within reason? I don't want to do a half-&%$ job on it, but I don't want to pour any more money than absolutely necessary into it. <br /><br />Thanks,<br />cvlngineer

 

[Hooty]

Re: Powerhead rebuild tips

I checked your foto album out last night. That's some project. Good job.<br />I'm not sure which bearings you're referring to as "knuckle bearings".<br />First, I would recommend using the factory manual. Next, do a search for "rebuilding tips". It's there with lots of good juju.<br /><br />c/6<br /><br />Hooty

 

[cvlngineer]

Re: Powerhead rebuild tips

Thanks Hooty, I will do a search.<br /><br />I hope I can do as well working on this powerhead! The bearing that I am referring to is the one that connects the piston head to the rod. Wristpin bearing??<br /><br />cvlngineer

 

[G DANE]

Re: Powerhead rebuild tips

cvlngineer<br /><br />I´v read some of your posts. Its a very nice boat your got, I know its lots og work, always more than anyone expects. I read your posts about the motor too. Why do you want to change the bearings ? Inless some failure occurs, they usually easyli outlast two sets of pistons. Did something specially happen ? Bearing has three elements, cranck and rod journals and the bearing needle pins. Far the most wear is of coaece at the pins due to the little diameter, but alle three share the wear. The problem you had getting on plane usually is carbs, compression, timing or ignition.

 

[G DANE]

Re: Powerhead rebuild tips

Hi again<br /><br />I just read end of your last post - I see the point now - sorry. Good luck.

 

[Yepblaze]

Re: Powerhead rebuild tips

Without reading through them, I see the majority of your recent post dealt with, an accelleration issue, and powerpack testing. <br /><br />I wonder why you have decided to make the issue wristpin related?

 

[cvlngineer]

Re: Powerhead rebuild tips

Yepblaze,<br /><br />It is not related. I believe the acceleration problem is fixed. <br /><br />The guy I took it to pointed out a knock in the engine to me. He used his stethoscope to isolate it to the top end of the #1 cylinder.<br /><br />cvlngineer

 

[cvlngineer]

Re: Powerhead rebuild tips

I have the powerhead off, and all but one of the head bolts loose. One will not break free and I am afraid that If I put too much torque on it, that it will break off. I have wrapped on it and it is now soaking in WD-40. Are there any tricks to getting these things loose?<br /><br />Also, before I break this bolt off, do I need to remove the head to change out the wristpin, or can I just break apart the crankshaft case and do all of the work from that end?<br /><br />Thanks,<br />cvlngineer

 

[G DANE]

Re: Powerhead rebuild tips

Hi cvlngineer<br /><br />Unfortunately pistons has to get up and out. A lower rod bearing can be changed from buttom. I understand you want to change a wrist pin bearing, so you have to get the head off, you have to anyway as you loosened some of the bolts and have to renew gasket.

 

[cvlngineer]

Re: Powerhead rebuild tips

Thanks G Dane,<br /><br />I finally got it off. I inspected the cylinders and they look good. The compression was good to start with. There is carbon on top of the pistons, should I clean it off, and with what?<br /><br />cvlngineer

 

[G DANE]

Re: Powerhead rebuild tips

If its only a thin layer of carbon and only on top of pistons, I usually leave it. Next time you decarbon, it will disappear, and a thin layer is only natural from combustion. If there is much carbon at ring area it should be cleaned, or motor decarbed when you get it running again. Decarbing it the best way to remove it.

 

[cvlngineer]

Re: Powerhead rebuild tips

Thanks G Dane!<br /><br />I have a couple more questions. The compression in the motor was right at 120 psi in all cylinders. I can see a couple of tiny marks in the cylinder, but I can not feel them with my fingernail. I shouldn't have to rehone the cylinders should I? Should I go ahead and replace the rings? Also, do I have to pull the flywheel to do the job, or can I leave it on and just open the crankshaft cover.<br /><br />Thanks,<br />cvlngineer