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Onan Pistons

Dan Wiese

New member
Hello,
Does anyone make, or have, aluminum pistons for Onan's? I need 2 that are 3" in dia. And I do have an old one that can be used as a pattern. I have located 3"+.010 for Briggs but have not got the other necessary measurements yet. Compression height, etc. to see if they will work.
Thanks,\.
Dan
 

turtmaster

Subscriber
Re: onan pistons

Can you please post the make and model of the generator or engine, that you're looking for the Pistons for? If you don't have any identifying numbers, can you please post some overall pictures of the complete unit? Because with some do some pictures we should be able to get a pretty close identification of your unit.
 

Dan Wiese

New member
Re: onan pistons

Hello. It is a model number W2S 550 and is badged as Fairbanks Morse. I hope this helps. You can see one of the pistons. The cylinders and valves are sitting upside down beside the wheel. The head and radiator are elsewhere in the building.
 

Attachments

AngrySailor

Subscriber
Re: onan pistons

I have on my list of things to do, make a pair of these pistons. Or attempt to... I have two of these W series generators with bad cylinders so was going to make oversized slugs. Probably 0.060-0.100 over depending where the bores clean up. I MIGHT have an extra std bore piston but I’d have to look and I’m away from my shop for a few weeks.
 

Zephyr7

Active member
Re: onan pistons

Angry, as our resident machinist, do you think someone with a CNC lathe would be able to run a batch of pistons with minimal effort? Just a thought...

At some point making new parts will be our only option to keep these older units running.

Bill
 

AngrySailor

Subscriber
Re: onan pistons

I’ve been thinking of how to go about making these... it’s really more of a milling project. While these aren’t exactly high performance pistons there are some interesting issues to consider. Pistons are not round, they’re oblong with a smaller diameter across the pin. This is due to the expansion of the extra material in the pin boss. In my case this will be rotary table work on the mill to relieve this area. Also rotary table mill work to hollow the underside. The only real lathe work is the general shape and ring lands... the pistons I have in mind to make probably won’t look exactly like the original due to not being a casting and having to come reasonably close on weight. They will probably have scalloped skirts for this reason. I think the long skirt and 1800 rpm will make any vertical imbalance a non issue (the crown being heavier than the original casting). I hope.

It would be interesting to see what a piston manufacturer would charge to make a small run. I’ve had custom BBC pistons made reasonably but that what these companies specialize in, then again they are equipped to make PISTONS which aren’t all that much different from each other when it comes down to it...
 

Dan Wiese

New member
Thanks for all of your replies. As I understand it, from searching the internet, aluminum internal combustion pistons are of a special alloy because the beer can/soda can aluminum expands too much when hot and seizes to the cast iron cylinders. That would be a disaster!
 

len k

Subscriber
Can you post a link to that techical info please?

In past I've only beliefly scanned engineering texts and "Material Selector" (3/4 inch thick yearly magazine with specs on EVERY metal made) when looking for info on other things.

But usually all metals of a similar type have very similar "coefficient of thermal expansion" ( be it aluminum, steel , brass, ect)
 

AngrySailor

Subscriber
Len, I will try to find the links to some material I was reading, most discuss the hypereutectic alloys used in modern cast pistons, and the 2618 or 4032 alloys used in forged modern pistons. There were also many SAE technical reports which will surely make your head hurt! Thermal expansions varied only slightly and the big factor seemed to be in design, where the material was placed in the piston. How the pin boss and any skirt bracing expand to deform the piston to fit the cylinder. Much of the differences between the alloys was their properties at high temperatures, in the range that a W series Onan would see if launched into the sun.

For this application I grabbed some 6061-T6 off the shelf, while it’s not a common piston alloy, I’m pretty sure it will be superior to the 1940’s cast that the original was made from...

These are low compression, low rpm, low HP engines and also water cooled. Given reasonable clearance I don’t see any issues. Also the piston crown where the ring lands are is undersized compared to the skirt, most heat will be localized here and will have room to expand.

The Onan spec for piston to wall clearance for this model is 0.003”-0.005”. Even with factory pistons I would be hesitant to go 0.003”. I’m thinking 0.006” should be safe. Would be interesting to run some number first though and even simply heat the piston and measure across several points to be sure it’s not expanding more than acceptable in some places:shrug:

I’ve never made an IC piston before so if anyone has suggestions fire away!

OP, I will look when I get home, I may have a factory std. bore piston for you.
 

Zephyr7

Active member
Angry, note that all of the “T” numbers on aluminum alloys are for different heat treatments. All of those are done at pretty low temperatures, but for very long (many hours) time periods. You can actually go between T0 (soft) and T6 (fairly hard) in a kitchen oven if you want to.

In a piston, the fact that these heat treatments don’t need very high temperatures may be a problem. You may find 6063 to work better, and it’s not that difficult to find.

The 202x alloys may be good too, but they are much different from the 606x alloys and are also less common and more expensive.

Aluminum is my favorite metal to work with. Very versatile, and no need to paint it :)

BTW, I like your idea for heating the piston and measuring the expansion to test you theory. That makes my inner engineer very happy! I recommend heating it and holding the elevated temperature for a long time, maybe an hour, to ensure it’s stable before measuring. You’ll want to measure it very quickly after taking it out of the oven (I’m assuming you use an oven here). If you do the test at several temperatures, that will let you determine if it’s linear with temperature or not which will let you make predictions for percent expansion at other temperatures that you do not, or can not, test.

Bill
 

AngrySailor

Subscriber
I don’t think it would be necessary to heat the piston much beyond 212* as the only areas expansion will be of concern would be the skirt areas which will run much cooler than the crown. I’m wondering about wear properties though, but again I think this is a very low stress application...
 

Zephyr7

Active member
I think most of the wear will be taken by the rings, and whatever you use as a pin/bearing for the tie rod. I don’t wear-type durability is much of an issue for the piston itself.

You’ll probably be fine with 6061-Y6, which is probably the most common of the easily machinable aluminum alloys. I was just pointing out that there are probably some slightly better options out there.

Bill
 

Dan Wiese

New member
I finally found oversize pistons with all other pertinent measurements at EGGE. They had them in stock and are in the mail. Thanks for all your help.:)
 

Zeromedic

Subscriber
Dan,

Did they have these specifically for your W2? or did they pick by bore, oversize, weight, pin to deck height?

Steve
 
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