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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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#59 ·
How NOT to rebuild your Onan engine

Next we pull the pan. Notice he did install the rods correctly? They are offset towards the outside of the block. This is to provide extra clearance from the bottom of the opposite cylinder. One rod cap removed, now what? STOP!
 

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#60 ·
How NOT to rebuild your Onan engine

Before removing the piston and rod put rod bolt boots on the studs to protect the crank journal from dings and scratches. No rod boots handy? Rubber vacuum or fuel line will work fine. Most all bolts were not very tight on this engine. Oil pump, flywheel, pan, oil filter adapter, head etc. except the rod bolts! Over tightened! 14 foot pounds or 168 inch pounds on the rod bolts. Lube, torque, loosen and re-torque. Don't forget the loctite!

In most cases there will be carbon built up at the top of the cylinders. Carefully remove it before trying to push the rod and piston out of the bore.
 

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#61 ·
How NOT to rebuild your Onan engine

Both rods out. Remove rear bearing plate bolts. Carefully tap the crank with a plastic mallet or block of wood etc. to break the bearing plate loose. Do NOT try to pry it off with a screwdriver or knife etc. Tap the crank from the opposite end until it comes free. Push the crank back in and hold it there while pulling the bearing plate off.
 

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#62 ·
How NOT to rebuild your Onan engine

Now for some problems. The crank had ZERO end play. Specs call for .006-.012 end play. Shims were installed on both sides of the crank thrust washers at both ends. Shims are only installed under the rear main bearing thrust washer. Washers are installed with the notched area facing the crank as pictured. This was engine was not going to last very long the way it was put together. Bad shop teacher! Bad Bad shop teacher.....

Next up. The correct way to assemble your Onan. :)

Stay tuned!
 

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#64 ·
What do you use to carefully remove carbon deposits on top of piston and inside head? I've already done mine but wonder what you use.

Mine has 750 hours on 1990 vintage motor. Never had any valve adj or vavle service done. What would you recomend particularly relating to loose vavle seat issues. I'm not aware that I have valve issues but would like to prevent this from happening.

Only work I've done is replace intake and exhaust manifold gasket that were causing lean surging on intake side. Compression test. Also r&r plugs/oil/filters; dis-assembling carb/cleaning; re-assembling cleaning fins and repainting engine shrouds exhaust baffling to restore it somewhat to its fomer glory.

Sorry to jump in but you know a lot on onans and appreciate your opinion. thanks
 
#65 ·
Gasket remover and a plastic spatula work really good for removing carbon from the pistons without scratching them up. Spray on, wait 10 minutes......scrape....repeat several times until clean. For those of you wondering why not just hit the piston with a wire wheel on a drill? Because that will leave lots of scratches for future carbon buildup to get a grip on. Not good!

The loose seat issue is typically caused by over heating due to crud blocking the cooling fins and from being shut down before the engine has cooled off a bit. A couple minutes at idle after you're done working it is all that's needed.

At 750 hours it's due for a valve adjustment.

The gasket remover works on the heads also but will take a lot more time and effort to clean that way. I use a cheapo portable sand blaster outside with regular table salt for media. Table salt? Are you kidding! Nope! Cleans the carbon off really good and then wash with soap and hot water. Glass bead works too but will leave micro pits in the head that will grab and hold onto the carbon. Table salt won't do that. The salt blasting is also the way I clean the block after it's been stripped down. Table salt will dissolve when washed off but it's important to wash it off really good and get the block painted ASAP.

Have a dishwasher in the kitchen?...Wife/Girlfriend not around?....Easy on the soap and put it in for a nice cleaning:) Then head for the oven that is set to 250 and dry it out in there for an hour. Don't do any of this unless the block has been degreased first and has no gas or solvent residue. Stink up the kitchen and you will never hear the end of it!

If it's going to be awhile before you assemble then hit the internal areas with some WD40 to prevent any corrosion from starting.

DO NOT SAND OR GLASS BEAD THE BLOCK! One little grain of sand is all it will take to mess up your fresh rebuild. There is no way to be certain every last grain has been cleaned out!

Anybody please feel free to jump in with questions or answers. That's what will make this thread more worthwhile...:)
 
#66 ·
Thanks o great ONAN oracle!

I used gasket remover on piston top and head, forgot what tool it was but something fairly soft. Glad to hear about what causes the seat issue. Would have done the valve adj last winter when doing intake gaskets but was not sure gaskets were definately the surge cause and wanted to limit my time/$ investment if tractor was a waste of time.

Thanks again
 
#67 ·
I was digging around my stash of literature and found some goodies. Even if you are not rebuilding your Onan engine these brochures have some interesting info. The performance graphs with power and torque curves are a nice touch. Keep in mind 20 years ago when these were published Onan was not in a HP war with anybody. The specs shown here are real, not made up to impress or fool the public.

P216G up first:
 

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#72 ·
I'm digging deeper for more technical B series info but for now I can say the B43E & B43M models are identical or very close in power and torque outputs to the P216.

B43G & B48M the same or close to a P218.

B48G same or close to the P220


B43E...16HP @ 3600 RPM
B43M...16HP @ 3600 RPM
B43G...18HP @ 3600 RPM
B48M...18HP @ 3600 RPM
B48G...20HP @ 3600 RPM
 
#78 ·
This is an excellent thread, and I am using it as a reference as I rebuild a P220.

One question about camshaft end-play...

I noticed on my block that there is evidence of some wear on the surface that mates with the thrust washer behind the camshaft gear. When I check the end play on the installed cam, I get .051". I see the spec for end-play is .011 - .048", but the JD/Onan manual (in the downloads section here) gives a maximum wear limit of .065"

I don't see a selection of thrust-washer thickness's offered, I would like a thicker one to reduce this end-play. I am tempted to make one from Phosphor bronze, but I'm a little worried that this might "eat" its way into the block itself as there is no provision to stop this thrust washer spinning. I have wondered about pinning it to the block (similar to how the crankshaft thrusts are retained).

I realize that I am within the limits given in the manual, but it seems like a lot of play to me. And I wonder if all this play is enough to upset the operation of the governor, as the whole camshaft moves backward and forward...

Any thoughts on this?
 
#79 ·
Helical cut gears keep the cam from moving out while turning. Only one cam thrust washer size is offered and a new one will add a few thou to the clearance. The plastic/nylon cam thrust washers have never been a problem on the Onan engines. It's not an area to be concerned about.

Onan part number for the cam thrust washer is a 105-0541.
 
#82 ·
A basic and maybe dumb question........
This pertains to a P218.
After re-assembling the heads with new head gaskets, how long should the engine be run before re-torqueing the head bolts? I have run the tractor a little, but not put it under much of a load yet.
 
#83 ·
That is a great post. Wish I'd have seen this before I paid someone to do it. I am trying to figure out how to set the timing on my B43, the manual says 21 deg. bftdc. Can you tell me how to identify the #1 cylinder? Any help will be greatly appreciated!!!

The grass is growin, gonna hafto get me a goat.
 
#84 ·
Remove the tube going from the blower housing to the air cleaner. Shine your light down the hole and you will see timing marks on the timing gear cover. They should have numbers on every fifth mark. The largest mark in the center is TDC. It should be labeled BTDC and ATDC on either side of that mark. Rotate the flywheel until you see a line scribed in it. That is the timing mark. Align it with the 21 BTDC mark. I am not sure about the B43 but the manual for my BG says to rotate the engine another 1/4 turn to ensure the points are fully open before setting the gap. Hope this helps, Joe.
 
#85 ·
Onan Parts

My Sideline Onan parts Business is growing; I deal in New (Old Stock) and Used.

Just received three brand new short blocks.

NHC 1 7/16ths Crank (Application: Skid Steer Engine)
T-260 - 7Kw Generator Taper Crank
b-43G 1 1/8th Crank

I also have new short block T-260 13 Spline Crank


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Will Ship any and all parts
Call for more information: 651-437-2826

Thank you, Boomer
 
#86 ·
Can't wait to see the assembly section!! Got two i need to refresh right now!!
 
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