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Onan 7.5JB siezed. Advice. Is it worth it?


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  #1  
Old 01-18-2016, 03:51:02 PM
Tschapman Tschapman is offline
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Default Onan 7.5JB siezed. Advice. Is it worth it?

I have a Onan 7.5 JB FL30644D. Serial number C900303211.
It was given to me free. I put a socket on the crank and it wouldn't turn. Then pulled off the case and found the flex tube on the exhaust was broken. That's where the water entered. Pulled the plugs and found water on the Pistons. Sucked it off with the shop vac and filled it with PB Blaster. Here's where I am. Now is the time for the experts. Is this worth it? Suggestions please?????
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  #2  
Old 01-18-2016, 04:48:06 PM
YellowLister YellowLister is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

Drain the oil and tell us what it looks like.. many have been saved.. any idea how long it has been sitting?

Prob best to pull the head off if its been over a few months..

Its worth a few hundred in parts.. carb can easily bring 60 bucks or over 100 if its tri fuel..

Made march of 1990.. what the inside of the control box look like? Any rodent nests?
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Old 01-18-2016, 04:56:41 PM
len k len k is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

Pull the head, gotta see how rustly cylinders are, that'll tell the story. My other ideas here don't give you as good view.Even rustly can be saved or rebored.

I suppose a borescope down the plug hole might be good enough to see things, if you have one, I don't.

I haven't tried it on a JB , but on other things I wire up a 1/4 inch dia 12V dash bulb and sneak it down a hole to light the inside of things so I can see what's going on. Done it on engines, walls, steel sign poles, heating oil tank with 55watt halogen headlight..... Might not see much down plug hole.

Last edited by len k; 01-18-2016 at 05:08:09 PM.
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Old 01-18-2016, 05:31:10 PM
YellowLister YellowLister is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

another thing to try if you don't want to pull the head.. poor a good amount of atf in the cylinders.. leave the spark plugs out and light a stick on fire and try to set the atf on fire.. will heat up the cylinder walls and expand them, may be able to break the rust lose if its not bad..

never tried it with the head on.. but it could work..

I guess the first thing to do is see if you can see a lot of rust down the plug hole..
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Old 01-18-2016, 06:48:03 PM
sdowney717 sdowney717 is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

ATF mixed with acetone is one of the best rust busters.
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Old 01-18-2016, 06:49:05 PM
len k len k is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

I think first step is inspection. Think you'ld want to see how rusty the walls are first before you try forcing anything. Helps you decide what to do next.

IF heavily rusted and pitted likely need to remove crankshaft, soak for couple weeks , heat, and pound pistons down/out. Then rebore and oversize pistons. That's what I did with an old car.
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Old 01-18-2016, 08:13:13 PM
Ray Lynch Ray Lynch is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

Pull the head and check condition of cylinders and pistons. This will also let you inspect the oil line located in the valve cover as well as the valves themselves.
Everybody has their favorites for rust busting. I had a stuck jb piston and PB wasn't that effective for mine. ATF and AeroKroil soaking with occasional persuasion from a wood block and 3lb sledge worked for me. Sure you will find other effective methods here on the site.
Good luck.
Ray
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Old 01-18-2016, 09:45:32 PM
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

Here's a way to easily exert a tremendous amount of evenly distributed pressure on the piston:
With the head off (or on) fill the cylinder as completely as possible with grease. Drill a 14mm bolt and tap it for a Zerk fitting. Screw it into the 'plug hole (use a sparkplug gasket) Tighten. Use a grease gun to force the piston down.
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Old 01-18-2016, 10:57:41 PM
len k len k is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

If other piston is badly stuck , you might break the other piston, connecting rod, ect.

Pull head first and see how bad rust is first.

Last edited by len k; 01-18-2016 at 11:22:44 PM.
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Old 01-18-2016, 11:47:22 PM
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Jim McIntyre Jim McIntyre is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

^^ Right - pull the head and have a look-see. JB sets are really nice, but not rare, have expensive parts, and can bought all day long for $500 or so.

So, unless you just want to do this as entertainment (and I'm NOT knocking that aspect) it's doubtful it'll make economic sense...

And if you do part it out, I might be interested in the head...

Oh, and welcome to the Stak! - In case it wasn't obvious, you've hit the mother lode of old Onan info here.
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Old 01-24-2016, 04:41:55 PM
Tschapman Tschapman is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

Hey guys I'm back. Five kids, sports, old farmhouse and 2500 head of cattle on feed. I have limited time. So here goes the status on the JB. The crank case oil is perfect. Very clear and no water. The control boxes are immaculate. Looks like no one has ever been in there. I sucked out the PB Blaster and filled it to the top with acetone and ATF, with the head still on. Think I'll let it set awhile and soak with daily gurgling of the fluid with air. I know that the unit was functional in 2010. I'm hoping to get it without taking too much apart. What do you all think? Other than I'm crazy!😃
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Old 01-24-2016, 05:06:29 PM
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

I would give you about a 25% chance without pulling the head. It really isn't that hard to get off. Your chances of getting it running increase quite a bit by taking the head off. Honestly it couldn't take more than an hour. You can do a fair bit of cleanup in the cylinders from the top , and hand hone the cylinder walls with a piece of fine sand paper. The easy way is the hard way , take it a part a bit and it becomes easy . Cheers Dan
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Old 01-24-2016, 06:29:49 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

A couple of ideas. Note both Pistons go up and down I unison. One is on the compression stroke while the other is on the exhaust stroke. I would strongly recommend that you remove the single cylinder head which covers both pistons. That way, once you free up the pistons do not initially move them over the rusted cylinder walls but rotate the Pistons in the opposite direction to hopefully expose the unrusted portions of the cylinder walls in both cylinders at the same time.

Reason I suggest this is so that you will not put undue stress and maybe scoring on the piston rings and the piston ring lands. That way, and if you are lucky, you can maybe remove the rust with some very fine emery paper or the like.

If you do this please request a copy of the JB repair manual, 967-500 which lists the cylinder head bolt torque requirements as you install a new head gasket. Note, if water got inside, you may also find the valve stems have rusted so once again a little TLC here to minimize damage to the valve guides.
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Old 01-24-2016, 06:49:50 PM
Ray Lynch Ray Lynch is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

Quote:
Originally Posted by BigBlockChev View Post
I would give you about a 25% chance without pulling the head. It really isn't that hard to get off. Your chances of getting it running increase quite a bit by taking the head off. Honestly it couldn't take more than an hour. You can do a fair bit of cleanup in the cylinders from the top , and hand hone the cylinder walls with a piece of fine sand paper. The easy way is the hard way , take it a part a bit and it becomes easy . Cheers Dan
Good advice
Ray

Quote:
Originally Posted by Leon N. View Post
A couple of ideas. Note both Pistons go up and down I unison. One is on the compression stroke while the other is on the exhaust stroke. I would strongly recommend that you remove the single cylinder head which covers both pistons. That way, once you free up the pistons do not initially move them over the rusted cylinder walls but rotate the Pistons in the opposite direction to hopefully expose the unrusted portions of the cylinder walls in both cylinders at the same time.

Reason I suggest this is so that you will not put undue stress and maybe scoring on the piston rings and the piston ring lands. That way, and if you are lucky, you can maybe remove the rust with some very fine emery paper or the like.


If you do this please request a copy of the JB repair manual, 967-500 which lists the cylinder head bolt torque requirements as you install a new head gasket. Note, if water got inside, you may also find the valve stems have rusted so once again a little TLC here to minimize damage to the valve guides.
Tschapman
More good advice.
A little extra time spent now will pay off later. The JB is a very good generator to have.
Ray
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Old 01-24-2016, 07:05:17 PM
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

May be able to get it broke free that way and run however if the rings are stuck/rusted that will destroy the bore or rings in short order. I agree, take the head off and carefully inspect what you have. I would venture to say its not too bad. I have NEVER been able to un-stick an engine and have it run without severe blow-by or excessive oil consumption. If you decide to give up on the project, I would be interested in buying it from you.
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Old 01-24-2016, 09:34:36 PM
len k len k is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

It's well worth the little bit of extra time to pull the head and check for rust. While you may be able to get it running with out doing so it may cost you a quick death if walls are rusty,,, rings will quickly wear and cylinder wall likely will get scoured up.

PULL THE HEAD........ do it right and clean up any rust first, then the gen may well outlive you.
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Old 01-24-2016, 09:56:33 PM
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

A lot of the time when you pull the head you can reuse the head gasket and if really carefull the valve cover gasket as well.
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Old 01-24-2016, 10:46:19 PM
YellowLister YellowLister is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

I had a 3 phase 277 480 v jb that was locked up when I got it.. from sitting in the desert with the exhaust open.. I pulled the head and found alot of rust and some pitting in the bore.. got it unstuck pretty easy and cleaned up the bore.. has a few tiny pits in it right below tdc.. put it back together and it did not use any oil or smoke.. I got lucky.. but I find that doing things the right way, yet time consuming saves you money and time waiting around..

Being you have so much to do on the farm I can see that waiting might not be an issue.. but like said without removing the head and cleaning it up can cause a failure when your lights are out.. and make all your work a waist of time..
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Old 01-25-2016, 11:12:34 PM
David C David C is offline
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Default Re: Onan 7.5JB siezed. Advice. Is it worth it?

I am in the same camp as a couple of the others posters. Pull the heads to see what you have. All of the water damaged Onans that I have picked up over the years all had piston rings that were stuck in the ring grooves. If you end up with little pitting of the walls, it still should be a reliable genset. I have a 2.5LK that should have been scrapped or bored due to the water damage it had, but for an experiment I cleaned it up to see how it would do. It saw 24/7 running for about 12 days after hurricane Ike. It used about a quart of oil a day and it never fouled a plug. It keep several neighbors from losing food in their refrigerators and freezers.

David C.
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Old 02-15-2016, 12:59:58 PM
Tschapman Tschapman is offline
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Smile Re: Onan 7.5JB siezed. Advice. Is it worth it?

Hello all. Im back with a progress report.
Im excited to let you all know that it broke free on Jan. 26. I took the starter of and pushed on the fly wheel a little and it moved. I then moved it a little each day for a couple of weeks to soak all of the walls. It rotates freely and i do not feel anything when i turn it. Also when looking into the cylinders i cant see much of anything. Finally, against all of your advice; i put a battery on it to see what would happen and.......................suspense................ it turns over nicely but i have no spark. I may not be doing things correctly but im enjoying the learning experience and will be anxiously waiting my next lengthy posts of valued advice!
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