OS FS120 or Saito copy. Bar stock glow engine.

[pat_pending]

Hi HMEM,

I wanted to run something by all you knowledgeable sorts out there. I have just completed a Webster engine and toying with the idea of having a go at designing an amateur workshop-friendly copy of the popular OS FS120 4-stroke glow engine (un-pumped).

https://www.osengines.com/engines-airplane/osmg0935/index.html

I got my hands on one (actually the Chinese copy Super Custom SC) from fleabay and it runs goes like crazy. Starts firs flick and turns a large propeller with enough power to pull over a workmate (don't ask me how I know).

Inside:

• Ringed piston so no taper lapping etc required (I used cast iron piston rings on my Webster with great success)
• Steel liner
• 1-piece crankshaft (that could be made in 2 to save on swarf)
• Pushrod OHV design (cant see how the rockers get enough lubrication but its a design that seems to have been around for years so obviously works OK)
• Lubrication from 2-stroke oil blowby in the crank case

The only complicated bit I see is the cam gears which are a very neat helical gear that allows the camshaft to run perpendicular to the crankshaft making this really compact up front. I think even with some amateur CNC this will be impossible to do.

My thought is to instead use an inline cam gear/camshaft like the Saitos

https://alshobbies.co.uk/saito-engines

which could be done simply-ish with an involute gear cutter.

I have seen some similar amateur designs but have not been happy with using a timing belt to drive the camshaft as it would limit the practical application of the engine beyond stationary engine.

The purpose of this project for me is both honing the machining skills but also (FINALLY) take the plunge and learn a CAD package or die trying. I've not really spent enough time with Autocad Fusion360 to get past the tearing hair out phase.

Sooo.. the question to this forum is whether there are any other such copies already out there (I couldnt find any) and, in your opinion, how feasible this would be/where the main challenges would be (gears, piston rings I've already just about figured out)

 

[pat_pending]

 

[Cogsy]

I can't look it up right now but George Britnell has/had a video on YouTube showing the making of helical gears manually (no CNC) using a design that I think was from Chuck Fellows. It's all a bit hazy in my head but seemed very manageable so you may want to have a look into that before you right off the helical gear idea.

 

[pat_pending]

I can't look it up right now but George Britnell has/had a video on YouTube showing the making of helical gears manually (no CNC) using a design that I think was from Chuck Fellows. It's all a bit hazy in my head but seemed very manageable so you may want to have a look into that before you right off the helical gear idea.

Thanks Cogsy, I'll have a look. Cant even get my head around how they work let alone making them at the moment!

 

[pat_pending]

OK so its one of these I'd need to make (spiral/perpendicular helical gears) to copy the OS design. Ill see if anyone out there has already tried. I found George Britnell's video which will be very helpful in figuring this out although I think that's just a straight Helical gear.

 

[mayfield]

I'm not familiar with internal combustion engines so apologies if this is a stupid comment, but could you achieve the same using simple bevel gears?

 

[pat_pending]

I'm not familiar with internal combustion engines so apologies if this is a stupid comment, but could you achieve the same using simple bevel gears?

Hi Mayfield, no such thing as a stupid comment! A bevel gear would transfer rotation to the vertical. I’m trying to get the camshaft rotating how OS did it perpendicular to the crankshaft. I’m rapidly thinking this is out of the realms of amateur manufacturers.

 

[Cogsy]

I've never seen a gear-train like that, looks very cool though. If you can figure out how to get it done it would be a great achievement.

 

[pat_pending]

I've never seen a gear-train like that, looks very cool though. If you can figure out how to get it done it would be a great achievement.

Yeah that's what I thought. I can't even get my head around how the gears mesh let alone how they happily spin along at 14,000 RPM with a 2:1 reduction. Ill keep doing homework and update this thread on what I find. I think cracking the technique of making them could open the doors to some neat amateur engine designs. Having said that, using simple spur gears and angling the rockers like the Saito design seems a lot easier to achieve.

 

[ixb1]

If I should choose , it would be a design similar to the Webra T4 engines.Its simple with timing belt ,no cams ,pushrods , rockers, springs etc.

 

[Aerostar55]

 

[Aerostar55]

You might want to look at Ron's Modelengine News website. Ron Chernich has passed away but his web site is very informative. Here is a link to his Feeney Four Stroke engine build http://modelenginenews.org/cardfile/feeney.html The Feeney was a spark ignition engine sold complete in the 1930's and '40s Art DeKalb of Precision

 

[Aerostar55]

http://precisionserv.com/model-aero/ This is a link to Art DeKalb's site with casting kits

 

[WOB]

If push comes to shove, pairs of small helical gears with a 45 deg. helix angle and 2:1 ratio are made by http://sdp-si.com/D805/D805_PDFS/sections/gears.pdf ( page 136) hardened steel , no idea of cost


WOB

 

[kadora]

Buy gear train including camshaft separately as spare part.

 

[pat_pending]

Buy gear train including camshaft separately as spare part.

hi kadora, good plan but feels like not manufacturing the crankshaft and camshaft myself feels like cheating + they would add £65 to my build.

 

[pat_pending]

If push comes to shove, pairs of small helical gears with a 45 deg. helix angle and 2:1 ratio are made by http://sdp-si.com/D805/D805_PDFS/sections/gears.pdf ( page 136) hardened steel , no idea of cost


WOB

thanks for this. Ill take a look

 

[TonyM]

Saito gears use a special helix angle. Standard 90 degree cross helix are usually 45/45 degrees and therefore the ratio of the diameters is the same as the tooth ratio. Not sure exactly what angles Saito use but 30/60 angle would give 2:1 gear ratio for 1:1 diameter ratio.

 

[pat_pending]

You might want to look at Ron's Modelengine News website. Ron Chernich has passed away but his web site is very informative. Here is a link to his Feeney Four Stroke engine build http://modelenginenews.org/cardfile/feeney.html The Feeney was a spark ignition engine sold complete in the 1930's and '40s Art DeKalb of Precision

I love that site. Some great resources there. Feeney was on the shortlist.

 

[pat_pending]

Saito gears use a special helix angle. Standard 90 degree cross helix are usually 45/45 degrees and therefore the ratio of the diameters is the same as the tooth ratio. Not sure exactly what angles Saito use but 30/60 angle would give 2:1 gear ratio for 1:1 diameter ratio.

Thanks Tony. I think these are above my machining skill level.