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Old 08-02-2009, 04:29 AM   #1
onanparts.com
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Default How to rebuild your Onan engine

This thread will be a complete step by step on how to rebuild your Onan B & P series engine. B43, B48, P216, P218 & P220. Tips and tricks on tearing the engine down and putting it all back together along with everything in between.

I will include anything possible that will save you $$ along the way without taking shortcuts that will come back to haunt you later.

I'll start out by skipping past the removal of the Onan from your tractor as that is fairly straight forward along with unbolting all the easy parts and jump right to the one thing that will stop you in your tracks. The crankshaft timing gear.

Everything is stripped down on the Onan block but how do you get that dang crank timing gear off? No room behind it to give the gear puller arms a place to grip and until that gear is off the crank is not coming out!

Time for a "special tool". I made this adapter years ago and never got around to making it perfect.....if it works don't fix it! This was a chunk of steel I made to bolt on to the gear face via the two 10X32 threaded holes already tapped in the gear. Use allen style 10X32 as they typically are grade 8. Cheap or low grade screws will break! 20 years on these and they are in perfect shape.

Grind a small notch on the back side for clearance of the gear woodruff key. You can see where it sticks out slightly in the second pic. Put the flywheel bolt back in all the way for the puller bolt to bear against. The gear is a press fit and on VERY tight.
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Old 08-02-2009, 04:31 AM   #2
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Default Re: How to rebuild your Onan engine

Adapter bolted on to gear. Just tighten the bolts until they are snug. Puller setup and ready to go and then gear almost halfway off. I did this one by hand with a plastic mallet handle wedged into the puller arms to keep the block from trying to roll and get away! The tougher ones require Mr. Impact!
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Old 08-02-2009, 04:33 AM   #3
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Default Re: How to rebuild your Onan engine

OK, Mr. crank timing gear is now separated from the crank and we can now remove the crank from the block.
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Old 08-02-2009, 04:36 AM   #4
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Default Re: How to rebuild your Onan engine

Rear bearing plate bolts removed. Do NOT pry this plate off with a screwdriver! Doing so will in most cases damage the gasket sealing surface! Put that screwdriver down! NOW! OK, grab a plastic or rawhide mallet or a block of wood and "gently" tap on the flywheel end of the crank. Keyword is "gently". You just want to get the rear bearing plate to separate from the block not launch it across the garage floor!

Tap tap, ok it's free now but still with the block. Push the crank back in and hold it in while carefully pulling the bearing plate out and off.
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Old 08-02-2009, 04:39 AM   #5
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Default Re: How to rebuild your Onan engine

Line up the crank with the area pictured on the block that is relieved for just this purpose and very carefully and slowly remove the crank. Slow and easy so you don't ding or damage the journal surfaces.
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Old 08-02-2009, 05:13 AM   #6
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Default Re: How to rebuild your Onan engine

If the crank has no visible damage give it a good cleaning and then check the journals for wear and the rod journals in particular for any "egg" shape. Measure the rod journals in four places. If all locations are the same the journal is round and this is a good thing!

Main bearing journal specs are: 1.9992-2.0000 Con rod journal specs are: 1.6252-1.6260

If possible take the crank to a machine shop and have them polish it and double check your measurements. Shortcut: Do it yourself with long narrow strips of sandpaper. Start with some 400 grit and finish with 600. Keep moving around the journal and don't stop in any one place.

If one or several journals are damaged the crank will need to be reground undersize. Main bearings and rods are available down to .030" undersize.

Valves: The B series valves have a lot of meat on them and if the stems are in good shape the faces can be reground several times. The P series valves have less metal and typically can only be reground once or twice on the exhaust and 2-3 times on the intakes. Most automotive machine shops will reface all four valves for around $25.00 or less.

Stay tuned for more updates as they happen.
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Old 08-02-2009, 07:13 AM   #7
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Default Re: How to rebuild your Onan engine

This should be put in a how-to Onan file. Good work and thanks. A lot of us Onan guys appreciate your work on this subject.
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Old 08-02-2009, 07:24 AM   #8
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Default Re: How to rebuild your Onan engine

Thanks, nice post! great info!
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Old 08-02-2009, 07:57 AM   #9
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Default Re: How to rebuild your Onan engine

onanparts,

Thanks for the informative post. I could see this becoming a sticky.
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Old 08-02-2009, 08:34 AM   #10
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Default Re: How to rebuild your Onan engine

Excellent writeup. I want to personally thank you for this.
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Old 08-02-2009, 08:35 AM   #11
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Default Re: How to rebuild your Onan engine

Great post!! Keep it coming.
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Old 08-02-2009, 08:40 AM   #12
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Default Re: How to rebuild your Onan engine

This is an invaluable post for newbies with Onan problems. And you certainly got it right that those crank timing gears are TIGHT!
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Old 08-02-2009, 08:44 AM   #13
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Default Re: How to rebuild your Onan engine

Post like this is what keeps this forum going...
Good show....
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Old 08-02-2009, 09:56 AM   #14
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Default Re: How to rebuild your Onan engine

Great presentation! Love the clear pics and well phrased instructions.
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Old 08-02-2009, 10:32 PM   #15
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Default Re: How to rebuild your Onan engine

Thanks for the positive feedback! I know a few folks will get some use out of it.

Next up: Valve guide RnR.

Onan valve guides hold up really well but at some point they will need to be replaced. If your valves have a normal wear pattern on the faces, not offset or oblong and you don't get any smoke on start ups or when shutting down then they are probably OK. Tired stem seals will cause smoking too.

For those of you that are going to replace them here are the part numbers, specs and how to remove and replace.

B series: B43E, B43G, B43M, B48G, B48M etc.

Intake guide: Part # 110-3161. Takes intake seal 110-3604
Exhaust guide: Part # 110-1935 This # supersedes 110-1807

P series: P216G, P218G & P220G.

Intake guide: Part # 110-3526. Takes intake seal 509-0289
Exhaust guide: Part # 110-3527

B & P series Intake valve clearance (stem to guide) 0.0010-0.0025

B series exhaust valve clearance (stem to guide) 0.0025-0.0040

P series exhaust clearance (stem to guide) 0.0020-0.0035

First part is removing the old guides.

Tools required: Hammer and a stepped punch that will fit down the guide bore.
No stepped punch in the toolbox? No problem. Use a long bolt with a nut threaded at least an inch up with a small washer as a shoulder and drive the guides into the valve box. Lifters need to be removed before attempting this!
You should be able to see the guide around your punch/homemade driver. You don't want to scar up the guide bore here!

Put a small rag in the valve box to catch the guide and prevent damage to the lifter bore. A couple of good whacks to get the guide moving and then easy tap tap tap until it falls into the valve box.

Intake guides have the raised shoulder for the stem seal and the intake valve/guide is always at the rear end of the block. Exhaust valve/guide always at the flywheel end of block. Intakes valves are larger than the exhaust.

First pic is of my home made copy of the factory stepped seat guide tool with some 1/4" all thread and washers. Use a lock nut at one end with correct size washers built up for intake or exhaust seat for the do it yourself super cheap tool. Don't go oversize because you want the washer to rest on the seat shoulder not the block. Seats and guides are at an angle compared to the head gasket surface and you want the guide to be pulled in nice and straight.

Small washer with oversize nut and regular nut for the guide or valve box end. Second pic is of the all washer cheap tool. Washers work fine if you pay attention to the next part.

When you are turning the locknut/all thread go easy and keep it centered so you don't mess up the inside of the new guide with the all thread. You may or may not have to put a wrench on the nut inside the valve box.

Third pic ready to start pulling the guide in.
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