Flat head Onans and mod heads

I have been away fellows…

Working in far north mines and travelling to places the sun don’t shine.
But I am back up here at home base and even though all my summer projects went to hell I still managed to tinker and make some interesting things for you to look at.

First off I screwed up here on what I am about to show you.
it was never meant to produce this stratospheric compression level.
We have managed to achieve 150 PSI compression on an LK205 using the BGE head modified to fit.

This was more than expected and I am still working out why…
I clearly measured the wrong heads ( I have a CCKB head mixed up here I think ).

Very sloppy work on my part, but this is probably as hot as you might ever be able to push a K Onan for a wood gas conversion.
So if yyou are mad enough to try you can fit the BGE or BG43 tractor heads to a LK of CCK



This is one of those times when an excess of nitrogen is your friend.
In this case it would cool the tip of that plug a touch more


Wow, that’s awesome!! Do you know what that would mean for compression ratios? And you could possibly pull some more power from it by polishing the combustion areas, as long as you’re only using charcoal gas, because liquid fuels will make that pointless real fast :stuck_out_tongue:


I just noticed this so sorry for being late in reply.
just so busy…

To answer a couple of questions.
The BGE head was modified real slack and lazy
The stock contractor head on the K engines are 5:1 the high compression raises this to 6:1.

The B43 - BGE head has a factory compression ration of 6.2:1

So just a stock head can make a big difference. I was trying and tinkering with the B43 head to see if I could make a quench area and boost it a little further for natural gas. I compared it to a couple of heads I have hear without doing any proper documentation or testing. What I can tell you is its probably better to not modify the head and use a stock on on a LK or CCK and be prepared to modify tin work for proper cooling.

I just posted this information as a proof of concept and to show it could be done, I am not sure if its worth doing though. The head is in a box, the engine is back together in stock form and oiled up for storage ( maybe it gets a new piston and rings, maybe valves and guides to )

Phase 3:
I have collected a few more BGE engines.
This is an interesting unloved geneset because the electronics in them make them less desirable to RV folks ( BTW guys the BGE -B43 heads will bolt on the BF series and you can avoid all those old electronic issues and make a good high compression easy retrofit engine with a heavy duty exciter cranked generator among other advantages ).

Where am I going?
Well the unloved BGE has a few tricks up its sleeve. Its rated below what it can really put out. 5kw from a 4 kw is not a problem at all on gasoline. And unlike other earlier engines its full pressure lubed and has a filter system as built.

One of these BGE I have is going to be taped to see if it can run a small turbo and I am going to go back to the idea of using it to blow operate a charcoal gasifier, to a gasometer and improvise some sort demand regulator system to feed it gas.

I think this might be the set I use, but I am waiting on a nice lady in China to sell me some boards to bring the gen end back to life.

Yup that was as hard to get out of the trunk as it was to slide in there…
As far as polishing and porting or any hot rod tricks. This is a 50 cubic in flat head turning 1800 rpm. There is not much you can do to it other than compression increase and even then a thinner head gasket and more careful milling are the words of the day not what I did on that LK


Some things about the LK CCK MCCK ( last one is a marine engine water cooled )
I know guys that tinker and were able to pull 3000 watts out of the 205LK ( rated at 2000 watts or 2500 in some cases ) There are several version of the CCK rated at 3000 4000 and 5000 watts and these are all the same unit just different combinations of factory heads. Then there is a the MCCK rated at 6000 watts in boat generators…

There is enough displacement there to make for a lot of reserve power.
These engines sets are probably some of the best candidates for wood conversion, and being flat heads they are pretty easy to service.

SO speaking strictly about the CCK if you swap out the heads on that unit from a set on a lawn tractor you get the higher compression you want and you might get 3500 watts from a set drawing as gasifier without any hotrod stuff.

And I will leave it at than.

Have a great days fellows, be safe out there with this this new covid wave thats on us and I will try and reply to any questions faster in the future

Here’s a CCK 4000 watt with all the tins off. You can see the heads and reverse flow cooling system. All of the OTC engines are built this way so you can see why I think they are pretty nice to work on.


What do you think the best way to make this into electric start?

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Hello Bruce, on the older Gudbrod tractor there was a built-in starter and dynamo both in one, connected to a V-belt, … maybe it also has something similar in you with a garden tractor

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Excellent suggestion Tone.
Here, USA and Canada it would be the Hitachi 12VDC four pole starter-generator units.
Used a lot on golf carts.

Belt driving/driven with ball bearings so no engagement-overrunning clutch needed. Wire in a charge regulator-relay to be able to charge up a separate, independent starting battery.
~$50. USD used take-offs.
Steve Unruh


About to be some electric start conversions going on at my house! Why did I never think to use one of those? IV seen and played with them, just never thought to take it off the gold cart for something else

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You have a contractor set there, a stand alone portable.
The CCK-B used on tractors would have this flywheel.

And a conventional starter.

The golf kart starter generator idea might work.
Kohler and Wisconsin used that system and it worked on engines as large as 35 cubic inches.
The CCK is 50 ci and that might be a problem.

The upside is just about every application from John deere tractors to clark forklifts used a CCK at one point so finding a dead engine with the right flywheel starter tins and parts would not be that difficult. ( also a tractor would have the CCK-B high compression heads )

What is the name plate on that set?
I will tell you more about what you have if I can get that info


I like the way the guy on Project Farm uses a drill motor to start small engines. Just an idea. Start watching at minute 5:00.


I do it the project farm way as well, call it the shoulder saver method :grin:

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Just make sure you use a drill and not an electric impact gun. Can shear the flywheel key.

Don’t ask me how I know :zipper_mouth_face:


YOu can make or buy an starter that runs on 12 volt like this.

Just watch your wrist if they kick back you can end up going to the hospital.
Charles is a big boy, makes that starter look small.


Can do more than that, can snap the threaded end clean off a crank.


Thanks Tone, Steve, Wallace, Marcus, and everyone else who responded.
I think the starter generator idea would work if I added decompressors to the heads.
The best course is obviously finding the correct ring gear and starter, which I never even suspected was available, so thanks Wallace for that!
For my own satisfaction, I am going to put a Chevy small block flex plate against the pull starter gear and see if a Chevy starter would fit in that area somewhere.
Before my arm chair thrusters kick in here, it’s probably better to just sell the cck and get something with electric start. Although the damn thing is a wonderful generator able to power the MIG welder with no problem.

I have a different Onan generator with Exciter start and it seems like the brushes are dirty or not making contact. It was starting now it isn’t oh, and I have to pick up the generator and bring the brushes to eye level because I cannot bend over right now to work on it.
Wallace, if you could speak to Exciter start and it’s foiables I would appreciate it.


Well its pretty simple:
The exciter is simple a DC generator built on top of the Ac windings in the armature. Its purpose is to supply a lot of DC to the field coils, and by a lot I mean full saturation. This type of generator does not need a regulator because the exciter makes so much flux its assumed it has enough to provide decent regulation ( about 10% maybe 15% full load to no load not bad by standards of the day )

So if you have a look at your brushes you will see there are two sets. The slip ring set draw the AC power the DC set are for obviously 12 volt DC ( more actually about 18 to 20 before the regulator for he charging system)

When you apply DC supplied from the battery to the exciter it will motorize and spin the whole generator. This is exciter cranking…

Onan wiring is a little quirky in that you tend to ground things to turn them on and off.
A photo of your controller inside will help and I can identify the parts…

There will be a starter solenoid, a couple of relays for control and a couple of relays for the battery charger. There will be a few resistors for controlling charging current.

No cranking or slow cranking has one of two causes if the controls are working one is the brushes are worn, sticking or broken and the commutator is in poor condition, worn dirty ect.
If you can get some photos of that I can probably make some better guesses.

The most common problem is a weak battery. These things really draw a lot of amps to crank and they are on par with a lot of cars with how much they demand of a battery

So next from you I need the model and spec number so I can find some prints. and photos of the condition of the wiring in the control box and brushes and slip rings.
We will work at this…
It has a the makings of a useful thread, thank you for asking these questions.

Armature in real good shape ( this one is three phase could 4 slip rings 3 line 1 common Add info it could be a re connectable type that is 120 - 240 or 120 only I can’t tell from the photo)

Here’s some field coils without the pole shoes or frame

Here’s a manual I just found, we need more details now on the model and spec to find the correct one.


Thank you Wallace. On my next lap past both generators, I will take photos.
I really like these guys. They come cheap, and are very well built.


This is me trying to start the 4.0 cck

I forgot to take pictures of the 5.0 cck.


AH HA … Brushes are lose!!!

Those things you are tickling should press down and clip in. There is a rolled up spring that applies pressure to force the brush down and make good contact…

If push them down and feel it click into place you know you are OK.
Without the correct amount of pressure you will burn up your commutator.
Yours does not look bad, a quick clean with a commutator stone will fix that right up.

How much brush you have left might also be an issue they could be too short and spring and clip can no longer hold them in place.

I will try and find some photo/video to help.

Maybe my LK can stand in and for the video ( need to dig it out )

Looking online today I think this is the actual LK CCK bush clip.
You can kind of see how it needs to be pushed in and then hooked into the brush holder