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4 stroking a Merco 61...

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Martin Field13/08/2019 20:07:47
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Hi,

I have a couple of Merco 61s and would like to try 4 stroking one of them for use in a model aircraft.

I was thinking of going the usual pushrod and rocker method, but then started thinking about doing a belt driven OHC version, possibly even a twin cam.

Can anyone direct me to a thread of any similarity or articles in ME over the years? I have a small lathe, but no milling machine so part would have to be fairly small.

TIA,

Martin

JasonB13/08/2019 20:16:53
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Nemett Lynx would be a good one to take details from though for a conversion you would probably want to put the belt at the back of the engine.

Twin cam 4-valve something like a HEMO would look good but I've not seen drawings for that.

Edited By JasonB on 13/08/2019 20:34:04

Martin Field13/08/2019 22:19:09
59 forum posts

Ah, I think I have some details of that somewhere.

My problem is I can't do major components on my lathe and I don't have a mill, so it really needs to be lesser bits as part of a conversion.

Thanks for your response.

Martin

Barrie Lever13/08/2019 23:23:55
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Martin

Given the restriction of having no true milling facilities, how about taking a look at the layout of the Webra 4T 40, I think that might be a bit more lathe friendly.

Drive the rotary valve from something that goes into replace the old backplate.

I think there is quite a lot of work converting the Merco's and it will always have the exhaust stack in place.

How about doing a Vega, they ran sweetly as I remember.

Regards

Barrie

Martin Field14/08/2019 00:33:07
59 forum posts

I'm assuming, Barrie, that I would need to make a new liner and piston with no 2 stroke holes in them. That's just a start. I'll check out the Webra and Vega.

Thanks,

Martin

JasonB14/08/2019 07:01:34
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I don't see why the small amount of milling could not be done on a lathe with the use of a vertical slide.

Barrie's suggestion of a rotary valve would save on making the cams but you are still going to meed to make a new back plate and head in either case. Just the cams that you would save on.

Barrie Lever14/08/2019 08:02:44
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Martin

You will certainly need to make a new liner, you might be able to reuse the piston after removing the piston crown baffle.

For this project to work you will need some fairly creative ideas. You will certainly need to do some cross slide milling.

Jason

The rotary valve layout might save on making a crankshaft as well, I think it might be possible to pick up drive for the belt from the crankpin by getting a hole into the pin and pressing a drive peg into the hole.

Regards

Barrie

JasonB14/08/2019 08:14:47
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Posted by Barrie Lever on 14/08/2019 08:02:44:

The rotary valve layout might save on making a crankshaft as well, I think it might be possible to pick up drive for the belt from the crankpin by getting a hole into the pin and pressing a drive peg into the hole.

Yes, that is what I was thinking to drive it off the crank pin, if there was any spare length on the pin then a socket to fit over it would save having to drill the pin. I'm assuming the pin was part of the crank rather than a separate pin inserted into the web which would have made it easy to just fit a longer pin.

Looking on the web it seems that there was a commercial conversion made by a company called Stebro but that used the traditional push rod arrangement. Would be worth searching to see how that drove the gears and cam.

John Pace14/08/2019 09:10:24
156 forum posts
156 photos

Some useful and interesting threads in the RCME model flying forum

Engine projects
or I THINK it's all there ? By jeff2wings
and

4 stroking a Merco...
By Foxfan

If you are going to superimpose an existing valve timing
layout from another engine it is as well to remember the
Merco engines have an offset alignment from the crankshaft
to cylinder bore (Desax) so it may be wise to work this
out before cutting metal

John

Niels Abildgaard14/08/2019 09:26:18
253 forum posts
79 photos

Why not try a worlds first:saml.jpg

Martin Field14/08/2019 09:38:45
59 forum posts

Thanks, gents. I do like the neat approach of the Webra rotary valve, but I know nothing about them and would feel a bit lost.

The Stebro has all its valve gear at the back so must have taken the drive off the pin somehow or was perhaps a new crankshaft.

Niels, the Vega was a side valve, like your drawing, but where on your drawing is the drive to a twice as big gear to get 1/2 speed rotation for the cams?

Martin

JasonB14/08/2019 09:38:49
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Posted by Niels Abildgaard on 14/08/2019 09:26:18:

Why not try a worlds first:

How do you get a 4 cycle from that with the cam being on the crankshaft?

Stripped down Stebro here which seems to show a smaller pin in the crankpin but the gear block is the wrong way round to see if there is a mating disc on the pinion.

Edited By JasonB on 14/08/2019 09:42:02

JasonB14/08/2019 09:53:04
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This Jan Ridders engine gives an idea of how the rotary valve works, ideally the valve would be placed in the head more like this. Probably offset the cam to one side of the head and have the inlet and exhaust on opposite sides then you would have room for the spark plug. Or keeping to the twin cam theme you could have twin rotary valves which may make sealing easier

Edited By JasonB on 14/08/2019 09:54:50

Martin Field14/08/2019 10:16:32
59 forum posts

Thanks, Jason. Interesting pictures. Pretty much as expected, except for the drive to the smaller gear. I can see a smaller pin pressed into the crankpin, but where does it engage the smaller cam gear? It is, after all, at full stroke, so to go then to a twice size gear with the cams on would suggest a much bigger box at the back. Pity we can't see the cams either.

But all in all I think this is the way to go for simplicity.

Interesting that the Stebro was slightly more powerful than the OS too.

Martin

Martin Field14/08/2019 10:26:44
59 forum posts

I imagine leakage is a problem with rotary valves. But thanks for the reference to Jan Ridders' apparently simple idea.

I was thinking like you that a drive disc with a smaller pinion behind it would have to be the answer on the Stebro. I can't see any dint where the smaller gear (pinion) takes the drive off the extra pin. Also, the backplate of the Merco is quite deep into the engine, whereas the Stebro gear casing is ON the back rather than IN the back so how does that little extra pin even reach the smaller gear?

Strange.

Martin

JasonB14/08/2019 10:44:44
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I would imagine that the pinion is mounted onto a shaft with integral disc and some form of bearing is trapped between gear and the back of the disc. To reach far enough inside the hole in the disc could be extended out from the surface of the disc., may want to add a small counter balance to the disc to compensate for the extension.Sketch shows two discs so the extended hole can be seen and how they may go together. Think I would be tempted to but a bearing on both sides of the pinion either ball or bronze.

stebro drive.jpg

Martin Field14/08/2019 10:51:32
59 forum posts

That arrangement wouldn't work , Jason as there is nowhere to house the ball race in the Stebro gear casing. I think there's a bronze bearing behind the gear. It shows on the assembled engine outside. I was wondering where an inner bearing could go...unless I'm looking at the gear casing effectively inside out (which I may well be, come to think of it!)

Martin

JasonB14/08/2019 11:20:34
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The bearing I show would fit in the backplate not the crankcase.

As you have a deep recess in the back of the crankcase the bearing(s) can sit in a spigot turned onto the front face of the backplate, this would then also move the disc further in towards the crank pin so you can then locate the drive easily.

stebro backplate.jpg

So the backplate would look something like this on the side that fixes to the crankcase with spigot to fit the case and a recess for a bearing and through hole.

 

Edited By JasonB on 14/08/2019 11:21:21

Martin Field14/08/2019 11:59:03
59 forum posts

Jason,

I'm very impressed by your rapid ability to draw on the old pootah!

I can see well what you're suggesting, but wonder what goes on on the gear side of the plate you show nearest to us...or, is that just a generic "face" to actually be part of the gear casing? In which case I understand. Then there would be the double sized gear with the cams on above it?

I have just received an astonishing offer from Jon who makes Laser engines to help by supplying a gear set and cams from one of their "deceased" engines. I'm hoping they will fit the Merco crankcase.

Martin

JasonB14/08/2019 12:34:16
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Yes it was just a generic plate that would be more of a block with hollowed out areas for the gears.

Have you gone off the belt idea as the shaft that the larger gear runs on would easily take a toothed belt.

The gear scan be placed back far enough so that they don't have to fit the Merco crankcase but ideally you would not want the enclosure to be much larger than the width across the case.

If you are going for gears, cam and pushrods then have a look at the head for the Nemett "Bobcat" which is quite simple to make

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