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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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#30 ·
I'll back up just a bit and show you the actual differences in the con rod weights Old Vs. New. The examples shown are an early style 18-20HP rod with a hole cast in the I-Beam section and the new later style with solid I-Beam.

These two rods have a weight difference of 13 grams at the big end. Small end weight is not critical and typically is about the same.

13 grams does not sound like much but it is! A matching pair of old or new style rods will usually be within 2-3 grams or less of each other. That amount is OK, but 13 grams will make a very nice paint shaker out of your Onan.

It won't shake enough to walk away on it's own if bolted to your tractor but the life of the engine will be much shorter.

Last pic is an overview of my home brew rod balancing setup.

TJ
 

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#31 ·
Oil pressure bypass valve & spring.

The oil bypass valve and spring are what regulates the oil pressure. The bolt in the first pic is removed and the spring will typically fall out after you turn the block upside down. Sometimes the bypass valve will come out with it, sometimes not.

If not use a small magnetic pickup tool or spray some carb cleaner down the hole to breakup the oil that is holding it in place. If using compressed air to launch it out of the block and you don't put a rag over the opening to catch it and hours later it's still hiding someplace you will never find it.....Part number is 120-0398.

The valves hold up well but it's always a good idea to replace the spring, part number 120-0140 when rebuilding.

When putting it back in the "flat" end goes in first. The smaller rounded end on the top is what keeps the spring centered. Oil up the valve before installing it, drop it in the hole and then put the spring in. The copper washer can be reused if annealed. Heat it up cherry red and drop it into some water.

What? You lost the bolt too? It's a 3/8-24 X 7/8. The length is what sets the oil pressure. Torque it to 10 foot pounds. Don't over tighten or you risk cracking the block!
 

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#32 ·
Onan question... I bought a Gravely 30 years old and looks like it was never used... Parts of the engine is missing... The crank on the flywheel moves up and down like a bearing is missing... Should the seal be on the outside ? I'm guessing there shoud be a bearing behind the seal ?
12 hp single cly... I think someone from the factory forgot to put a bearing in this engine...

 
#34 ·
That should be an LK series engine. If the crank nose moves up/down etc. then you may be correct and have an example of how NOT to build/rebuild an Onan!

Pull the timing cover and if there is a large gap all the way around the crank main bearing area then the bearing could actually be missing. Here is a pic of what it should look like without the crank installed.

If the bearing is there then it's possible it was not "clocked" correctly when installed and the oil feed hole was blocked. This will cause a lot of clearance real fast!

There are a number of holes and or slots in replacement bearing's to allow for different applications. The most common is pictured. Oil feed comes in almost directly from the pump or the filter. One hole in the bearing has to be lined up with the feed hole and another hole in the bearing lined up with the feed hole to the cam bearing.

The groove in the main bearing is what allows the oil to get to the cam bearing feed hole.

TJ
 

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#36 ·
All the small gas air-cooled one and two cylinder models use the sleeve type main bearings. C series: CCK, CCKA, CCKB. B series: BG, BF, B43, B48 etc. N series: NH, NHA, NHB, NHC, NHD, NHE etc. P series: P216, P218, P220, P224 etc.

Extremely tough bearings that can last many thousands of hours.
 

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#37 ·
wow...awesome!

Ive got a P218G on my Deere that is sitting on about 1700 hours, and it's recently started using a good bit of oil and smoking, but oddly enough it never ever has any buildup on either plug, none, in fact Ive had the same plugs in it for 200+ hours.

It smokes on startup, and continues to have a haze as it runs, if I hit tall grass it smokes pretty good though.

I tried to tear it down once before but could never get the oil pan off it, I pried and hammered and prayed for a few hours, couldnt get it loose, and all the bolts were out.

How much does a set of P218G rings, pistons, rods, valves and valve guides run these days?

I dont have ANY rod noise at all, just the oil use, so Im sure my crank is great.


great post, lose these old giants.
 
#38 ·
I've been catching up on those late summer projects I need to finish before the weather starts to turn bad so more rebuild tips and tricks will be along soon...

The oil base: If you have a "tin" cover on the bottom then that has to be removed to expose the four long oil base mounting bolts. Once those big bolts are removed a careful whack with a plastic hammer or piece of wood and a regular hammer to the mounting feet and off it comes. Did I say "careful" whack? You don't want to bust a foot off!

Cost? That depends on where you get the parts and what actually needs to be replaced. The low end is fresh rings and a gasket kit, under $200 combined with a valve job and you are back in business.

High end? $600-$800 depending on what all is going to need to be replaced. Until it's torn down and inspected you really won't know. Rebuilding an Onan is a long term investment. They are an industrial engine designed to be rebuilt many times over and over again.

With only 1700 hours I'm betting you have stuck oil control rings. Rods are most likely in good shape. If the pistons are not worn at the top groove then a $200 overhaul might be all you need. Guides should be ok too and the valves can be refaced in most cases at least once.

Stay tuned! More to come....
 
#45 ·
TJ, the new rings for my B48M Onan came in yesterday. Today I installed the rings and then the pistons and rods. With the timing marks on the crank and cam gears lined up the pistons are not at the top of the bore. They are about a quarter inch from actual top dead center. Can that be right? Thanks. Mike
 
#46 ·
Cannot remove camshaft.

So I'm following along with you on my p218g. I have everything out but the valve guides and camshaft. The cam seems to be hanging up on the rear journal, as it tries to come through the front bearing. The back journal SEEMS larger that the front. Any ideas what I am doing wrong?
 
#48 ·
You need to have the last cam lobe lined up with the notched area of the cam bearing and then the cam will slide right out.
If you do have it lined up and it still won't come out then you may have nicked or gouged the bearing with some of the first three lobes and that's causing it to bind up.

When removing the cam it's important to take it really slow and easy even if you plan to replace the bearings.

TJ
 
#49 ·
Well I got it out and indeed the front bearing was nicked. I was very careful but still managed to damage it. If the manual had mentioned that I would have saved it. Anywho looks like you already shipped the new bearing and valve guides so I should be running in no time. Thanks guys.
 
#51 ·
How NOT to rebuild your Onan engine!

During a recent teardown of a B43M Onan I found many mistakes that were made by the previous owner when he overhauled it. For the record he did this many years ago in High School auto shop class. Can't say much for the teacher!

By showing the mistakes he made this will be a good way to help you avoid doing the same when rebuilding your Onan. I took many pics from start to finish for the teardown process and will point out the errors as we go along.

This B43M appears to have been started but only run a few minutes after the overhaul. Any longer and it would have been real ugly very fast.
 

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#56 ·
How NOT to rebuild your Onan engine

Timing cover removed and two problems discovered. First the alignment pin in the timing cover was not in the governor cup with the little plastic bushing. You can install the cover and tighten it down and not realize it missed the hole. OK, almost. The cover should fit flush to the block with gasket in place and no gaps prior to tightening the mounting bolts.

As shown in the first pic the cup needs to have the hole with the bushing lined up at 3 o'clock. Install cover with gasket and while holding it against the block check for free movement of the governor arm/shaft. Put a couple bolts in but do not tighten them. Tilt the block forward so the timing cover is at a 45 degree angle and move the arm again.

There should be a slight bit of resistance from the flyballs pushing the cup out when you move the arm. Very slight! As in barely but you can tell. If not free the governor will not work properly.

Second issue. See where he went BANG BANG BANG on the crank gear? This is not how to put it back on!

Heat the gear with a small torch until your spit, yes I said spit, balls off the gear. Too hot and you kill the hardness and have a soft gear that will wear quickly. Once heated it will slip right on the crank. Be fast and have it lined up with the key.

Next part was covered at the beginning of this thread so I'll skip on past removing the gear.
 

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#57 ·
How NOT to rebuild your Onan engine

Optical illusions included at no extra cost. :)

You can remove the cam before or after the crank gear. I usually do it before so I can stuff a rag in between the gears to help keep the crank in one place along with a hammer wedged as shown when pulling the gear. Gear removed, along with oil filter adapter and valves from one side.
 

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#58 ·
How NOT to rebuild your Onan engine

Remove lifters from both sides and ready to pull the cam out. Shown in a previous post but here it is again. The last cam lobe has to be lined up with the notched area of the cam bearing to remove the cam. Go slow and easy when pulling the cam out or you will nick/bang up the bearings making it a pain to remove. The bearings may or may not need to be replaced but ding them up and you could have two problems!
 

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