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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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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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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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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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onanparts,

Thanks for the informative post. I could see this becoming a sticky. :)
 

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Excellent writeup. I want to personally thank you for this.:thThumbsU :thThumbsU
 

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Discussion Starter #15
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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First pic of guide halfway in and second pic it's all the way in and done! You will feel when the guide bottoms out, it's not a lug nut so stop turning the wrench at this point. Don't forget to put some clean engine oil on the outside of the guides before installing.

Checking the pistons and rods up next.
 

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Checking out the pistons.

Your pistons might appear to be in really good condition. Until you check the top ring groove clearance. On Onan's that have been well taken care of and ones with the oil filter option the pistons and rods may be ok and can be reused if within specs.

The top ring groove clearance is 0.003-0.008 anything beyond that and you get blow by issues and a very good chance of the ring eventually breaking and putting a nasty groove in the cylinder. The quick check is if you can "see" the ring move up and down in the groove using your finger. A very small amount of movement is OK but if it's obvious, the piston is toast. A feeler gauge is the correct way to determine exactly what the clearance is.

First pic is a typical piston that looks OK. But in the second pic we see it has 0.014" clearance. :( Scrap at that point unless you know somebody that can re-groove the piston to the next size up ring. Possible but not cost effective.
 

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Checking out the rods part 1.

OK, so the big end looks good with no scoring or visible damage. Not so fast there speedy! With piston still on the rod clamp the rod in a vise with a rag wrapped around it and get it snug but not too tight. This is where you check the "small" end or wrist pin hole. Grasp the piston firmly and try to rock it back and forth the direction it should not travel. In line with the pin. Movement should be barely perceptible if at all.

Pin clearance in con rod is 0.0002-0.0007 so if you can feel and see it move the small end is worn. $$ saver: if you can only "feel" it move slightly but can't "see" it move then there is still some decent amount of life left in the rod and reusing it is possible without any problems in the near future. It won't be perfect but it will provide hundreds of hours more of service.
 

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Checking out the rods part 2.

So if your crank checked out OK it's time to check the con rod big end clearance. Assuming your rod big ends look good like the one pictured, clean and smooth, grab the crank and mount it in a vise clamping it at the throw area as shown and a rag between the jaws. Clamp it so when the rod is installed it will rest against the vise side where you can hold it from moving while torquing the rod bolts down for the plastigauge check.

You want the plastigauge positioned on the journal so the rod cap is centered over it. The journal needs to be "clean" with nothing on it. A very small dab of clean oil in the are where the plastigauge will be is OK to hold it in place.

Carefully install the rod from underneath without scratching the journal with the rod bolts. Hold it up against the journal firmly while also holding it against the vise base/side to keep it from moving. Any rod movement from this point on will cause an incorrect measurement. Install rod cap and carefully torque it down to 14 foot pounds or 168 inch pounds alternating from one nut to the other.
 

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Keep holding the rod firmly while removing the rod cap nuts and rod cap. Remove rod and take a look at the cap or the rod journal. One or the other will have the plasti gauge stuck to it.

Clearance is 0.0020-0.0033

This rod checks out right at the minimum spec of 0.0020 going by the plastigauge scale. Plastigauge comes in about a 1 foot length and the scale is built into the paper wrapper it comes in. Available at any decent auto parts store. For Onan engines you want the "green" plastigauge good for .001-.003

Next up will be how to check the cylinder bores for wear.

TJ
 

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