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How to rebuild your Onan engine

278K views 133 replies 44 participants last post by  N2 Tractors 
#1 ·
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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#2 ·
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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#4 ·
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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#6 ·
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. :)
 
#9 ·
onanparts,

Thanks for the informative post. I could see this becoming a sticky. :)
 
#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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#16 ·
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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#17 ·
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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#18 ·
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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#19 ·
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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#20 ·
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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#22 ·
Great! Now I'm stuck! :) Guess I better get back to sticking stuff in here....

Lets get to checking the cylinder wear and see if we need to visit the machine shop and spend $$ or stay home and just put in some fresh STD rings.

Even a cheap made in China bore gauge seen here will be more accurate than snap or T gauges. If all you have or can get are the snap T style gauge they will
at least give you a ballpark measurement but are a pain to use. :(

In the first pic we see the "bottom" unworn portion of the cylinder. This is where you take your first measurement so you can determine the amount of wear in the worn part of the cylinder. With the bore gauge zeroed in the bottom of the cylinder you then pull it up to about 1/2" or slightly less from the top of the block.

Rock the guage back and forth a little to find the center and see what reading you have. In this block just a hair under .002" wear on both cylinders. We lucked out here! No scoring of the bores and the wear is within specs of .005" maximum allowable taper from top to bottom. You want to take measurements at 3-4 different places in the cylinder to check for out of round that is a max of .003"

We were under both numbers on each cylinder so it's deglaze the bores and new STD rings. A "stone" type hone is the best way to deglaze and get the proper cross hatch finish for best ring seating and oil control.

I will admit having good luck with a "flex hone". You know, the bottle brush style. Not worth a *** if using chrome rings but onan rings are cast iron and will seat after using a flex hone. It may take awhile though.....:)

If you have access to an automotive machine shop, having them put a quick finish hone on the block would be ideal.
 

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#23 ·
If you lucked out and don't have to replace your pistons and just need new rings you have to be aware that Onan used two different styles of piston rings on at least 5-6 different piston designs on the B & P series engines!

You have to measure the thickness of the compression rings to be certain of the correct style to get.

The following applies to B & P series engines with 3 1/4" bore.

Part # 113-0314 is for the early style rings and the compression rings measure out at .078" or 5/64. In metric that's about 2.0 MM.

Part # 113-0310 is for the later style rings and the compression rings measure out at .058" or just under 1/16. That's about 1.5 MM.

Think I'm kidding about all those different pistons? Here is a pic of four of the 5-6 different ones out there. Notice the two at the bottom appear to be identical but they each use a different ring style.

The one at top left is the only type currently available from Onan, casting # 112-0277. All other designs are obsolete and no longer available new. If you only have one bad piston and it matches the 112-0277 cast piston, no problem. But if yours look like the others you will need to replace both pistons with new ones or begin a long search for a match to your original ones.

Mixing old and new styles will cause an out of balance engine due to weight difference resulting in a paint shaker that will shorten the life of your engine. You can use any two matching styles of pistons if you are hunting around for a deal on used ones. As long as the casting numbers match and the top ring groove clearance is within specs. If the seller can't or won't tell you what the groove clearance is keep looking or get them to promise a no hassle return/refund if they turn out to be junk.

112-0264 is the current part number for new replacement STD pistons. It supersedes the old number of 112-0186.

For any oversize just add a -01, -02 or -03. A +.010" piston would be a 112-0264-01 for example.
 

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#24 ·
Old and new style rods.

The new style rods with a casting number of 170-4444A, B, C etc for the 16HP engines & Casting numbers of 170-4447A, B, C etc. for the 18-20HP engines have different weights compared to the early style rods. If you only need to replace one rod and it has the same casting number as above then no problem.


But if you have the older style with different casting numbers you must replace them in pairs to avoid an out of balance engine. New style have a solid I-Beam and old style have a hole in the I-beam near the big end. Common casting numbers on old style rods that can NOT be mixed with new style are: 170-2429, 170-3439 etc. for the 16HP engines & 170-4105 A, B C etc.for the 18-20HP engines.

114-0394 is the current part number for new 16HP conrods. It superseds the old numbers of 114-0225 & 114-0300.

114-0397 is the current part number for new 18-20HP conrods. It supersedes the 114-0257.

Same rules apply when hunting for used rods as pistons. They have to match and even if the big ends look great the small end may be shot.

16HP rods are slightly longer than the 18 or 20HP rods. 16HP cranks have a stroke of 2.625 and the 18-20HP cranks are 2.875. Start mixing these up and your piston will either come up short, 16HP crank with 18-20HP rods and have VERY low compression or stick out the top of the block and hit the head, 18-20HP crank with 16HP rods. :(

Note: The B43G is rated at 18HP but it has the 16HP crank and rods. It has a raised compression ratio to get the 18HP rating.
 

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#25 ·
Since we are on the subject of conrods might as well jump ahead a bit and show the correct installed position of the rods. In the pic you can see the rod bolts are "offset" slightly to provide clearance at the base of the cylinders. Rule of thumb for early style rods with an oil hole drilled in them is that hole always faces "up" towards the cam.

Newer rods do not have this hole for reference so always make sure the offset is towards the "outside" of the block.
 

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#26 ·
Next up: The oil pump. The heart of any infernal combustion engine. If your Onan has the oil filter option odds are very good that your oil pump is in good shape and will not have to be replaced. No oil filter? Inside of that pump may not look so good.

Two bolts hold the cover on. Remove those and carefully take the pump cover off. Slowly...you want the gears to stay put, OK one gear is retained in place with a very small C clip. You want to carefully wipe the oil off them so you can use a permanent marker on both gears so they can go right back exactly where you found them. This goes for any parts that mate or move together that you will not replace. Always mark, bag and tag parts so they get put back in the same place.
 

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#27 ·
If the gear faces look nice without any ugly wear pattern then in most cases you can put it back together replacing the only part that typically shows any real wear and that's the woodruff key. Before removing the small oil pump gear that is retained by the C clip hold the big drive gear and then see how much "play" or slop the small gear has when you try to turn it back and forth.

Very little play the key is ok. Lots of play it's tired and you will see that when you remove the gear. With the gear off.....don't lose that little C clip...you can check the drive gear shaft for play in the housing. A little is OK but a lot with obvious wear on the shaft means it's toast. If all is good time to re-assemble it. A new woodruff key if needed and gasket kit # 120-0161 that will come with three different thickness gaskets. .005" .007" & .009".

Don't use a metal scraper or screwdriver to clean the gasket surface! I use really fine scotch brite and some solvent on both the cover and body. Put any nasty scratches here and you may as well give it up and get a new pump. Some of that spray gasket remover stuff comes in handy here if the gasket won't let go without a fight.

Oil up that big shim washer that goes back on the drive gear shaft first, then put it back in the housing. New key, it may not want to go back in the shaft slot. NO metal tools here! A small wood dowel to get the key seated works best. Hit the dowel with a small hammer but not the key! Install small pump gear and make sure the C clip has the "rough" edge side facing away from the gear.

Be certain the C clip is actually fully seated on the shaft. It will click on and appear to be all the way there but most likely it will need a second little push.
Triple check the C clip!

Set the other gear in place with the marks you made lined up. Now comes the fun part. You want to use the thinnest gasket possible that allows the pump to turn freely. Start with the thickest gasket, typically the white one and put the cover on and just finger tighten the bolts. You will need to move the cover around a bit while turning the drive gear to find a position that has no resistance or binding.

Play with it until you have the pump turning smoothly with one finger turning it. Now try the next size thinner gasket, the yellowish one. same deal. Still moving freely? ok go for the Blue one and try for some smooth action.

Remember, finger tight only on the cover bolts when doing this. You have to be able to move the cover but have it stay put at the same time. When you feel confident the pump is turning smooth and not binding at all carefully tighten the cover bolts one side then the other, repeat. Back and forth nice and easy and keep checking the pump for smooth turning. Tighten some more, still smooth? Snug them up some more, still smooth? About 10 foot pounds is good for a final torque but good and tight works too. Just don't put your weight into it!

If it bound up as you were tightening the cover you probably need to go to the next thicker gasket. When all is good put some oil down the pickup/inlet and see how it feels. Still smooth? A little resistance is ok as it is now squishing the oil you put in there. Put some more oil in and keep turning the gear counter-clockwise as you are facing it. The oil should start to come out the feed hole.

Final test is screw on the pickup tube and drop the pickup in some fresh oil and turn that drive gear until oil is coming out the feed hole really nice. Cover the feed hole with your finger and see if you get some pressure.
 

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