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Onan 7.5 jb high vs. low compression engine


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  #1  
Old 12-17-2014, 10:36:03 PM
Ray Lynch Ray Lynch is online now
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Default Onan 7.5 jb high vs. low compression engine

Model 7.5jb-3cr/1ad
s/n 1850778356
Thanks to everyone for past help with my questions while converting(slowly) from gasoline to lpg.
New question...
My jb is a gasoline only unit and therefore I think probably low compression engine.
According to the jb manual 967-0754 the valve seats and exhaust valves are Stellite. Can I burn lpg in a low compression engine without damaging it?
Thanks
Ray
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  #2  
Old 12-17-2014, 11:20:49 PM
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Jim McIntyre Jim McIntyre is offline
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Default Re: 7.5 jb high vs. low compression engine

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Originally Posted by ray lynch View Post
...Can I burn lpg in a low compression engine without damaging it?
Yep. Not a problem.
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Old 12-19-2014, 12:57:39 PM
Leon N. Leon N. is offline
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Default Re: 7.5 jb high vs. low compression engine

Good Morning. With all due respect, I beg to differ with Jim WRT converting a Onan J-Line engine designed for "gasoline-only" to LPG.

Yes, you can convert the carburetor and run it on LPG. However, unless Onan changed the design since the earlier version, for example those built to factory spec through S and even later, you do so at the risk of cutting the useful life of the engine by an undetermined amount.

The reason being, certain internal parts for gaseous fueled engines are different the identical part(s) for "gasoline-only" fueled engines. Just take a look at the PLs and it will be obvious. I believe the main reason is because, the gasoline-only parts take into consideration the fact that there is (or was) lead in the gasoline which help lubricate the parts and where as the LPG fuel does not have any additives and LPG burns hotter than the gasoline.

The parts in question include the cylinder head assembly, valves, valve guides, rocker cover, rocker cover oil line, piston and rod assemblies, to mention a few of the more obvious parts.

If I were the owner of the JB plant and are the least bit mechanically inclined, get a copy of the pertinent Onan parts list for your specific machine and check out what I am saying.

On the other hand, if you only expect light infrequent duty of your JB then what the hell, just connect it to LPG and be done with it.
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Old 12-19-2014, 08:16:18 PM
soundbos soundbos is offline
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Default Re: Onan 7.5 jb high vs. low compression engine

As someone that HAS converted many engines to LPG the ONLY thing that wears faster is the valve seats in older engines
designed for leaded gas since we don't have lead in our gas any more they wear anyway.
The rest of the engine may never wear out on LPG
We usually got about 60-80k miles until a valve job was needed
which should be a long time with a standby genset
With solid lifters I would check lash more often.
I ran a Ford 351 drag car on lpg for more than 1500 1/4 mi passes and 50,000 street miles with almost NO wear

Butch
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Old 12-19-2014, 08:37:50 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 jb high vs. low compression engine

Butch I will not argue with what you are saying. I see where you are coming from. However, here we are talking about a machine that runs at a constant RPM and in an application where reliability may be paramount. So you are apparently not aware that the old Onan power plant designs went to extra precautions to make sure the machine did not wear out prematurely.

One example that most folks are not aware of is the cooling oil supply to the valve chambers is greater for gaseous fueled engines than gasoline-only designs.

When I refurbished my JB, I found excessively worn valve guides and valves simply because the plant was run on propane and the oil line to the cylinder head was plugged and this engine was designed to run on propane.

Ray, you could be very correct wrt your later model J plant having stellite valves and valve seats because the leaded gasoline phase out started in 1970. It makes sense that Onan would employ the harder material knowing this. Therefore, I agree with the thought that you could use LPG. It would be interesting to see if Onan still listed a gasoline-only design.

Last edited by Leon N.; 12-19-2014 at 10:17:16 PM.
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Old 12-19-2014, 11:08:27 PM
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Default Re: 7.5 jb high vs. low compression engine

Quote:
Originally Posted by Leon N. View Post
...
The parts in question include the cylinder head assembly, valves, valve guides, rocker cover, rocker cover oil line, piston and rod assemblies, to mention a few of the more obvious parts...
You are correct - The Onan parts list does reveal some differences:

It doesn't show a gasoline-only piston or rod assembly. There are only two types of pistons shown. One is for natural gas only, and the other covers all other variants.

The gasoline-only sets are shown to have different exhaust valve guides.

The gasoline-only sets are shown to have different intake valve seats.(probably NOT stellite?)

The gasoline-only sets are shown to have different intake valves. (probably NOT stellite?)

The gasoline-only sets have a different rocker cover oil line.

Prior to Spec P, there were different cylinder head oil lines.

I don't see any other gasoline-only differences, and it's still my opinion that he'd do no harm operating his set on LP. And as a practical matter, what's he gonna do? Overhaul a perfectly good engine to preemptively replace a handful of super expensive Onan parts?
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Old 12-19-2014, 11:23:20 PM
nothingbutdarts nothingbutdarts is online now
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Default Re: Onan 7.5 jb high vs. low compression engine

When I get my 1982 vintage JB close to being in running condition, I want to find a trifuel carb. for it and run it on LP. The guy I bought it from said it had a gas carb on it but, he couldn't find the carb. and said it got broken anyway!
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Old 12-19-2014, 11:23:25 PM
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Default Re: Onan 7.5 jb high vs. low compression engine

Most of the LP/NG units I worked on. Didn't run enough to really matter. As a prime power unit it might. But I have never seen prime power units that much.
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Old 12-19-2014, 11:36:20 PM
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Default Re: Onan 7.5 jb high vs. low compression engine

Leon after looking over the JLine Brochure under gasoline it makes note the valves at least are stellite faced and the seat is solid stellite. One less thing to worry about at least the part about needing leaded gas. Not as easy to burn a valve either. See description item #17
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Old 12-20-2014, 10:52:06 AM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 jb high vs. low compression engine

I agree with what is being said here. It's just that I became aware of the subtle differences in part numbers when I refurbished my JB back in 1970 or so. At that time I could order the genuine Onan replacement parts as Onan was still in business.

But today, fellow Stak members who are in need of parts may not be aware of these seemingly small differences. That's what is good about this forum in that we can inform and discuss with our members these ideas and let them decide if a used part from another machine is OK for their use.

BTW, I wonder if anyone who has torn down a gasoline-only plant ever noticed if there were valve rotators installed under the valve spring retainers in the gasoline only plants? My notes from the J line service manager indicates that was the case at that time.

Jim and Mike are correct. See, for example, the JB PL, page 25 in 967-320.
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