Custom Piston source for old Onan JB Generator (also a PipeMAX software question)

[KT_NorCal]

Does anyone happen to have any experience rebuilding old industrial internal combustion engines where you had to source non OEM or non standard pistons? Looking for some recommendations for suppliers. Most custom piston manufacturers are oriented towards high performance or racing which this is not definitely not.

I've recently picked up a great old Onan JB Generator. Onan used to offer an option for higher compression in order to not have to derate the engine when used on natural gas. The kit was basically just a pair of domed pistons and I would like to find some pistons that could be modified or just get some custom made as long as it doesn't break the bank, hence the question.

 

[thunderalley3]

I am guessing that it was a kit the way that you described it so more than likely a limited run and not many sitting around now. I am sure most would order when needed and did not stock them which makes it harder to find in later years.

With that said I do a lot of vintage motorcycle hot rodding and I have researched and found pistons that have worked by searching by bore size not engine and then digging into stroke to determine rod length to get the correct pin placement in the piston.

If all you are looking to do is boost compression the first thing I would look at would be to rework the cylinder head and gasket to obtain the compression needed. Most heads like that on the generator have plenty of meat to machine, obviously there was a larger than needed clearance if domed pistons could be added so think about reducing the chamber size of the head to obtain the same compression. Those are likely cast heads which are easily machined. I have welded up aluminum heads before and cut them to get the chamber volume I wanted.

 

[KT_NorCal]

I am guessing that it was a kit the way that you described it so more than likely a limited run and not many sitting around now. I am sure most would order when needed and did not stock them which makes it harder to find in later years.

With that said I do a lot of vintage motorcycle hot rodding and I have researched and found pistons that have worked by searching by bore size not engine and then digging into stroke to determine rod length to get the correct pin placement in the piston.

If all you are looking to do is boost compression the first thing I would look at would be to rework the cylinder head and gasket to obtain the compression needed. Most heads like that on the generator have plenty of meat to machine, obviously there was a larger than needed clearance if domed pistons could be added so think about reducing the chamber size of the head to obtain the same compression. Those are likely cast heads which are easily machined. I have welded up aluminum heads before and cut them to get the chamber volume I wanted.

Thanks for the feedback. It was an original Onan option, kind of like you would pick an option on a car when you are buying it, but it was easily retrofitted as it was literally just a different set of pistons. I have a picture somewhere and will post it when I find it again. They are basically unobtanium now-a-days as spares have long since run dry. The JB gen set I have is from 1967 and the type stopped production in the early nineties (I think).

The standard pistons are already essentially at zero deck height and I'm pretty sure there is not enough meat in the cylinder head (which is cast iron) to fully make up the difference in compression. There is a lot to make up in this case as the "standard" engine compression was about 6.5:1 and the higher compression pistons raise it to about 9.2:1. I'm going to disassemble the engine and while it is apart I'll take some detailed readings of the chamber size and see what I can get out of milling it down, but I suspect it won't be a ton.

I'll take the piston measurements and start a search for matching pistons from other engines. I hadn't considered how rod length could impact things though, so I'll need to do some more research on that bit. Also, a complication is that I do believe the pistons are directional, which means the piston pin could be offset by a currently unknown amount.

 

[Provincial]

Back in the mid-1990's I rebuilt a Lincoln welder that used a CCKA engine. Even then, parts were hard to find.

I rebuilt a P218 a few years ago. It took over a year to get the right pistons, and they were regular ones. I had to buy undersize rod that were honed to standard, and had to settle for +.010 pistons because +.005 ones were no longer available.

Is your engine similar to the P218?

 

[KT_NorCal]

Back in the mid-1990's I rebuilt a Lincoln welder that used a CCKA engine. Even then, parts were hard to find.

I rebuilt a P218 a few years ago. It took over a year to get the right pistons, and they were regular ones. I had to buy undersize rod that were honed to standard, and had to settle for +.010 pistons because +.005 ones were no longer available.

Is your engine similar to the P218?

No, completely different. Vertical inline, air cooled two cylinder. All cast iron, ohv, and about 1 liter capacity. They were somewhat modular in that the JC engine was just a pair of JB cylinder "units" on a common crank and with a different block. They were rated for "prime" power not just stand-by so if you take care of them they will essentially last forever. The generator end of them was also very good. They also made diesel versions that are essentially the same.
That said, I'm sure the design philosophy wasn't hugely different. If you jumbled all the parts in a box you would probably confuse a lot of them.

I'm actually also trying to find a good P series engine to fabri-cobble a home made pressure washer set up out of. They are a great engine.

 

[Mr onetwo]

Go talk to the guys on Smokstak

Onan Generators

Restoring, operating, and maintaining vintage Onan generators. When asking questions about your generator, give your model number AND serial number.

www.smokstak.com

 

[KT_NorCal]

Go talk to the guys on Smokstak

Onan Generators

Restoring, operating, and maintaining vintage Onan generators. When asking questions about your generator, give your model number AND serial number.

www.smokstak.com

Already am. They are a huge help. Just figured I would post here about the pistons as the number of "car guys" wandering around here is significant...

 

[rsanter]

How about TIG weld a dome onto the top of the piston

 

[Firebrick43]

Making Pistons for Experimental and Restoration Engines: Steve Chastain, Stephen D. Chastain: 9780970220349: Amazon.com: Books

Making Pistons for Experimental and Restoration Engines [Steve Chastain, Stephen D. Chastain] on Amazon.com. *FREE* shipping on qualifying offers. Making Pistons for Experimental and Restoration Engines

www.amazon.com

https://ia803103.us.archive.org/9/items/ceb.ac.in-od-knowledge-center-ebooks/E-BOOKS/Making%20Pistons%20For%20Experimental%20And%20Restoration%20Engines%20-%20S.Chastain_text.pdf

Good little book on making your own

 

[OccupantRJ]

No, completely different. Vertical inline, air cooled two cylinder. All cast iron, ohv, and about 1 liter capacity. They were somewhat modular in that the JC engine was just a pair of JB cylinder "units" on a common crank and with a different block. They were rated for "prime" power not just stand-by so if you take care of them they will essentially last forever. The generator end of them was also very good. They also made diesel versions that are essentially the same.
That said, I'm sure the design philosophy wasn't hugely different. If you jumbled all the parts in a box you would probably confuse a lot of them.

I'm actually also trying to find a good P series engine to fabri-cobble a home made pressure washer set up out of. They are a great engine.

I am rebuilding a P218 now. I suggest staying away from lil’ Red Barn valve guides, as the four I received could easily be slipped into the engine block with a finger tip. Luckily I bought them third party through Amazon, so returning was free in this case and refund was not an issue.

 

[KT_NorCal]

How about TIG weld a dome onto the top of the piston

That is potentially interesting... It is a cast piston though and my experience with cast aluminum is that it can move around on you which might not be a good idea with a piston. I'll have to talk to my welding instructor about it though...

 

[KT_NorCal]

Making Pistons for Experimental and Restoration Engines: Steve Chastain, Stephen D. Chastain: 9780970220349: Amazon.com: Books

Making Pistons for Experimental and Restoration Engines [Steve Chastain, Stephen D. Chastain] on Amazon.com. *FREE* shipping on qualifying offers. Making Pistons for Experimental and Restoration Engines

www.amazon.com

Good little book on making your own

Cool! I'll take a look. I definitely won't be casting my own piston but machining them from alloy stock is definitely something I could do.

 

[KT_NorCal]

I am rebuilding a P218 now. I suggest staying away from lil’ Red Barn valve guides, as the four I received could easily be slipped into the engine block with a finger tip. Luckily I bought them third party through Amazon, so returning was free in this case and refund was not an issue.

Good to know. That definitely doesn't sound ideal. ha I think I could make my own phosphor bronze or silicon bronze valve guides. I would just have to take them to someone to properly hone to final valve fit. I looked into a valve guide hone a few months ago and was a bit shocked at the price, so that part of it doesn't make much sense for me to try to do myself.

 

[KT_NorCal]

About PipeMax...
I have to build an enclosure for this thing so I can leave it in-place outdoors. Given that, there really isn't any reason for me not to try and optimize the intake and exhaust manifolds so they are not constricted like they are. I will have to route them both outside the enclosure either way.
To that end... Does anyone here use PipeMax and if so would you be open to plugging in some numbers to see what the optimal intake and exhaust set-ups are for this Onan JB? Since it runs at a constant 1800 RPM it is kind of ideal for a single intake runner length and I've heard PIpeMax is really quite accurate when it comes to that sort of thing... Anyway, any help would be hugely appreciated!

 

[OccupantRJ]

Good to know. That definitely doesn't sound ideal. ha I think I could make my own phosphor bronze or silicon bronze valve guides. I would just have to take them to someone to properly hone to final valve fit. I looked into a valve guide hone a few months ago and was a bit shocked at the price, so that part of it doesn't make much sense for me to try to do myself.

I have a machining and mechanical repair background and see no normal need to hone a valve guide on that engine. Reamers are available in at least .001 increments over or undersize. I like spiral flute reamers. Various size reamers relative to a nominal size are used for slip fit or press fit on dowel pins in stamping dies.

 

[KT_NorCal]

I have a machining and mechanical repair background and see no normal need to hone a valve guide on that engine. Reamers are available in at least .001 increments over or undersize. I like spiral flute reamers. Various size reamers relative to a nominal size are used for slip fit or press fit on dowel pins in stamping dies.

I actually have the ability to re-size reamers so that would be great if it works just as well. I had understood that honing was ideal for the same reasons that honing the cylinders was... It helped with oil film retention. Also gets the bore very round and very straight if it is a quality hone.

 

[Beerhippie]

Burt Munro would just find a bunch of aluminum beer cans and sand cast it.

 

[Packard V8]

On flathead engines, surfacing the head removes an equivalent volume of transfer area, thus limiting the breathing. That's why Onan used domed pistons to reduce the volume of the combustion chamber and thus increasing the compression ratio for natural gas.

What is your finished bore diameter, compression height and piston pin diameter? With those key dimension, you might get some help finding pistons which might substitute. FWIW, we do this all the time on obsolete engines.

jack vines

 

[KT_NorCal]

On flathead engines, surfacing the head removes an equivalent volume of transfer area, thus limiting the breathing. That's why Onan used domed pistons to reduce the volume of the combustion chamber and thus increasing the compression ratio for natural gas.

What is your finished bore diameter, compression height and piston pin diameter? With those key dimension, you might get some help finding pistons which might substitute. FWIW, we do this all the time on obsolete engines.

jack vines

These engines are actually OHV, which was pretty advanced for an industrial engine designed in the late 50s and early 60s. Onan used a Continental flathead engine on the water cooled version of this generator.

I don't actually have the exact specs yet. I have to measure it all once I get the head off. I didn't want to do that quite yet as I wanted to test run it first and see how it behaved. I have the standard specs, but I want to confirm that there isn't anything that will require oversized or under sized anything.

I'll try to get that done this coming week then get the measurements posted. Thank you for the help!

 

[KT_NorCal]

More general info.

 

[Steve_P]

I worked at a very high volume OPE shop in the '80s- we had five full-time mechanics in the summer and three people working the front of the store. Even then, Onan was uncommon for us to see, and we had very limited parts access.

If you want to save it, then you can have custom pistons made and pay $$$ each. Sure, there is probably a set sitting somewhere on a shelf, but the reality is that most NOS stuff went on ebay in the first 10 years that it existed. There may be a set on there tomorrow, or never but something will have to be made or modified. Typical small engine pistons were in the ~7:1 range, so even if you find a Briggs, Clinton, Tecumseh... piston that's close, it 99% won't have the high CR you want.

I'll guess that you're not going to get more than 0.5 points of CR by milling the head.