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The Much Maligned And Misunderstood Onan


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#1 mtoney OFFLINE  

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Posted January 17, 2013 - 12:24 AM

Get around any WH or JD guys and the talk will eventualy come to the Onan B or P series power plants that come in the popular 316/318, 420 JDs and several WH models among others such as Case and Sears.  Many tractor guys will shy away from the Onans due to what I feel are several misconceptions about them.  Where as the RV folks highly desire the Onan gen-sets, many of which use the B and P series engines or the older CCK cast iron flat twins.  I have personaly seen RV gen-sets with upwards of 4000+ hours without any thrown rods or popped valves.  Onans were originaly designed for gen-set operation in tight quarters like RV's, boats ect.  So they move boat loads of air with thier flywheel fan set up.  The are a high performance engine as this is what is demanded of a generator engine.  High performance comes at a price, this is higher maintance costs or demands.  Regular valve adjustments, decarboning heads if run on gasoline and proper oil changes.  Other brands of flat twins have unadjustable valve settings once assembled, valves must be ground or replaced to change the valve stem to tappet clearance.  Not so on Onans, they are adjustable and as such will get out of adjustment.  Valves out of adjustment can cause hard starting, cause hot spots in the block around the hardened seats and even contribute to an out of balance situation that in the exteam will lead to broke rods.  When an RV gen-set goes "off line/off load" it will run for a pre determinted length of time to cool down the block and valves a bit before shutting down.  RV's dont normaly live in dusty enviroments with all kinds of grass and chaff being inhaled by the cooling fan.  So on a GT they lead a hard life.  Most guys shut them down way to soon, lack of proper maintance and just overall abuse leads them to break rods, pop valve seats ect.  But properly maintained, an Onan will out live and out torque a similar Kohler or Briggs.  In the last year I worked for a dealership in the repair shop, Kohler parts for older KT and Mag series were almost or equil in cost to OEM Onan parts. But you had relief in the form of Stens aftermarket parts.  Until recently, this wasnt the case for Onan.  But recent word on the street is Cummins/Onan has release parts rights to the aftermarket.  So the expense of overhauling this engines should come down a bit.  If the Onan B and P series were such a bad design, they wouldnt be highly desired for the RV industry, where generator cost is a small drop in the bucket in the whole RV picture.  There is an excellent write up over on the myfavoritetractor forum on proper tear down, measurement and rebuilding of Onans.  I highly recommend someone read this before attempting to overhaul thier Onan.  Its better to pay more for a proper overhaul, then bodger it up and pay more later when it doesnt last or blows up completely.  Cheers  Mike


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#2 tractorgarden ONLINE  

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Posted January 17, 2013 - 12:43 AM

I myself ,Love Onans. The thing that gets most people in trouble is that they buy a used

piece of equipment that  is Onan powered which was NOT maintained properly.

When they are correct,they are the sweetest out there. But the majority of ads I see

for Onan power GT's say just needs a tune up, which in turn could be a loose valve seat

or worse.       


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#3 John@Reliable OFFLINE  

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Posted January 17, 2013 - 01:27 AM

 A little TLC goes a long way, they are great motors. The one on my Case 446 runs very well. Glad to hear that part prices may come down, some parts took a big jump in the last two years, but still worth it.


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#4 KennyP ONLINE  

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Posted January 17, 2013 - 02:20 AM

I am now the proud owner of 2 Onans. The first, I have had since the early 90's. Though not run a lot, it has yet to be rebuilt. I am very happy with the engine.



#5 JDBrian OFFLINE  

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Posted January 17, 2013 - 06:39 AM

Thanks for this info. I'm just making a deal now on a Onan RV genset with a P218 on it. It's got some sort of problem where it runs for a few minutes and shuts down. The owner has put on a new carb and coil so far and just wants to shift it. I'm looking forward to getting it. It has low hours and the price is also low.



#6 Alc ONLINE  

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Posted January 17, 2013 - 06:51 AM

I had the Onan main bearing on the flywheel side go in my 318 ( 1983) with under 1000 hrs , not sure why it went with so little hours , my luck I guess lol , anyway replaced with another one but they uped the hp to 20 on the replacement now . Hope this one last longer , Al

#7 jms180 ONLINE  

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Posted January 17, 2013 - 07:05 AM

Mike thanks for the information. I like Onan engines because it is a well built engine. I worked at the Onan engine plant in Huntsville Alabama for 23 years until it closed in 1997. I am a proud owner of 2- P220 and always on look for more. The B-NH and P from 16 Hp to 24 Hp were built in Huntsville from 1974 to 1997.


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#8 mtoney OFFLINE  

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Posted January 17, 2013 - 09:22 AM

I want to find a small Onan get-set from an RV to set up for backup power for a few things here at the house.  Just dont have the $$ for one unless its super cheap.  Most any of the more common Onan blamed problems can be found in other brands, even Vtwins.  I have seen valve seats out of Vanguards from running with misadjusted valves for to long.  Most all of the newer V twins and even a few singles with OHV's have adjustable valves and must be checked from time to time.  We had hard starting Kohler OHV singles right out of the crate that we had to readjust the valves.  Briggs singles with OHV are some of the worst offenders, we actualy made it part of the normal yearly service on those, regardless of hours on the engine.  Its a shame that Onan didnt survive long into the Vtwin era.  I did get to see a late Onan/Cummins Vtwin on a Millermatic welder/generator that was supposed to be one of the last Onans off the line.  Cummins wanted it for the generator reputation, not the air cooled engines.  Thats why Linamar produced them till the EPA shut down importing the flat twins due to emissions. I really would like to swap my WH for one of the Sears SS16s with the BF/BG series Onans.  Mike


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#9 olcowhand ONLINE  

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Posted January 17, 2013 - 09:37 AM

Other than diesel, Onan is my #1 choice of engine!   I have 3 GT's currently sporting Onan engine, one 18hp, and two 20hp.  Love em!!!



#10 mtoney OFFLINE  

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Posted January 17, 2013 - 12:15 PM

Agreed, gotta love a diesel, but up here I have never seen a steal on a diesel.  Thats about the only way I could afford one right now.  Charging the battery right now, my bypass line had a chaffed area that was leaking, so I had to swap it out with another line off the lift control valve thats sitting in the garage.  I will have to get that line replaced before I can install the hyd lift on the 518.   Battery got low enough that she wouldnt start.  Should have my new VR first of next week.   Mike






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