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[boatmadman]

I have machined an oscillator twin (its a stuart), when I run it, its leaks from the mating faces on each cylinder.

I lapped in the 2 parts with fine paste- improved but not tight by any means.

What should I use to get these things to seal?

Ian

[tobyker]

In my limited experience of these dear little things, not only have the faces to be absolutely flat, but the pin and hole in the standard must be at a right angle to the faces to hold them flat to each other, and there is often a small countersink where the pin is screwed into the cylinder, so they don't rock about the centre. Some nice gooey oil will always help, too - possibly delivered by a displacement lubricator. Also check that the big end is not pulling the cylinder face away from the standard either by twisting as it rotates, or by being pushed back by the crank disc. You could wipe it all clean and rotate the engine by hand against a strong light , looking at the interface for any signs of displacement. You've probably done all this!

[Bunkerbarge]

tobyker has come up with just about all the things I would have suggested as I suspect that it may have something to do with alaignment.  If possible try to eliminate things by steaming the engine with the piston not connected to the con rod and try to eliminate that etc..etc..

It is also very important that you are lapping the faces on a perfectly flat surface and as a final process very gently and lightly lap the faces together and see how they are touching.  Use some engineers blue if necessary but if you don't have any of that then you could try inks, dyes or graphite grease.  I lap mine on a piece of glass with a small square of 2000 grit wet and dry glued to it.

[bogstandard]

Hi Ian,
The usual cause of this problem is the hole thru the big end is not square with the piston rod. Take off the cylinder and put a piece of rod the same size as the crankpin thru the big end and measure by comparison with the port face using a good square. It only needs to be a minute bit out to cause the problem you are having.
If it is out, you can try to eliminate the out of squareness by slightly recutting the hole with a drill or reamer by hand until it is totally square. Then retry it and if it cures the problem you can run the engine like this as long as you haven't had to remove too much material, but it would be better to make new big ends.
If they are perfectly square when you check them, and done as the others have suggested, you just might be over pressurising the engine and blowing the cylinders off the ports. Try it on something like 10 psi and gradually increase pressure until it starts to blow out of the ports, this will show the max pressure you should be using with the engine. 25 to 30 psi is usually the max range these little engines should be efficiently run at. In fact I would try this before anything else.

John

[walrus]

Lap them in as Bunkerbarge suggests but work in a figure of 8 - not back and forth.
I am not familiar with Stuart engines but if you have retaining springs on the cylinders make sure they are clear of the standard as they oscillate. I assume that you have recessed the area around the pivot.

[boatmadman]

Thanks guys,

lapping in - done figure of 8 style

pivot recess - not needed - stuart standards have port faces that stand proud of the pivot entry through the standard.

I found with careful measurement that I had somehow drilled the crank hole out of place - doh! So, bushed it, silver soldered and re drilled.

did a test run, still a bit of leakage but no where near as bad as before, most coming from the control valve.

Engine runs smoother, and down to 10psi before it stops, previously was 20 psi. water consumption greatly reduced from yesterday - so must be doing something right!

Next problem is the burner, it runs ok for a while then draws the flame under the ceramic instead of on top of it. Tried as many positions of the nozzle tip as I can - sliding in and out, next thought is, is it an air flow problem, as it happens when pressure is up.

I have an air gap of about 8 mm around the bottom of the boiler, plus an ignition hole of about 15mm on one side. Think I will increase the air gap next see what happens.

Ian

[HS93 (RIP)]

You should not need a gap or hole Cheddar and M H B boilers do not have any just light down the chimney.

Peter

[boatmadman]

Hmm - maybe it's too much air flow causing the problem?

Ian

[HS93 (RIP)]

I can only find a picture of my mini vap engine boiler / burner but it may give you an idea  , I can take some of a bigger boiler if you want. Peter

[Bunkerbarge]

Ian a couple of things to play with here.  If it is only happening when the boiler is up to pressure then do you have some sort of burner control such as an electronic gas valve or an attenuator type of valve?  Do you have anything that turns down the flame when the boiler is up to pressure?

If not then it looks like the hot boiler is affecting the flame and this is most likely because the hotter air going into the burner does not have enough oxygen in it.  Play around with the mixture by either fitting smaller jets to the burner or supplying more air to the burner by either a fan or just blowing compressed air gently onto it from a compressor.

If the flame is going round the ceramic it sounds like the flame is weak so poor combustion as a result of mixture sounds like a possibility.

[HS93 (RIP)]

question , does this happen when it is on the bench or only when in the boat with the supperstructure on if so it needs more air holes a burner needs a massive amount of air to keep burning, I have had to have all the doors and portholes and bents and grills etc open to allow as much air in as possible.

Peter

[boatmadman]

Bunker - this is only happening when the boiler gets to about 20psi, there is no gas modulation fitted, this happens on a constant gas flow rate. The bolier safety is set to 50psi. I have had it up there a number of times, but this problem manifested itself after fixing the boiler to a brass plate supported by angle brackets giving about 8mm gap between the brass plate and boiler bottom. I need this gap to move the ceramic far enough away from the bottom boiler tubes so the flame doesnt impinge on the tubes.

I wondered if the heat in the boiler was drawing up air at a velocity high enough to disrupt the flame.

At the moment I only have a no. 8 burner tip, but I think i need to experiment with others.

Peter - the boiler and engine is on the bench, no boat for it yet.

Ian

[boatmadman]

Next installment!!

Today, I took the boiler off the mounting and tried the burner on its own as per the suppliers instructions - no problems at all.

Then remounted the boiler, wrapped a length of lagging around the air gap below the boiler and tried again - burner problem re occurred.

I then blanked the ignition port as well, a bit better - got to 40 psi - safety valve territory - but burner still misbehaved occasionally.

Next I tried lifting the boiler in steps to increase the air gap under it, again got to full pressure, but occasionally the burner coughed and misfired again.

This is a real head scratcher!  :-\

Ian

[EXBoat]

Re Burner, Fitting a small steam blower into the boiler chimney would help to stabilise the flame if you can arrange a take off point on your boiler through a small wheel valve, turn it on when you have a little pressure in the boiler, it would use very little steam as only a very small bore pipe would be needed.
If you have the exhaust from the engine going into the chimney that will help the flame, and then you can turn the blower off once the engine is running.

[boatmadman]

Would the gas butane/propane mix be a factor here? If so, what mix should I be using?

Ian

[Bunkerbarge]

I think most of these burners are designed to use th 70-30 mix found in most camping shops.  I would expect the burner performance to be consistent unless you are using a relatively small gas tank and you are sufferring from the effects of the tank cooling down.

This can happen if the cooling effect of the gas evaporating off actually cools the remaining liquid and the evaporation rate slows down accordingly.  What type of gas canister or cylinder are you using?

[boatmadman]

Its a disposable camping gas canister - cant remember size or mix - its at home, I am at work!

Ian

[Bartapuss]

Oscillators will always have some leakage and as one person has already stated you'll get to a point where the pressure will start to lift cylinder off the valve plate, not being a very efficient design they're only meant to work at low pressures.  Move onto the next stage and get a piston valve engine.

[Bunkerbarge]

Its a disposable camping gas canister - cant remember size or mix - its at home, I am at work!

Ian

That should be big enough to not have any effect unless after prolonged heavy use so I think you are OK there.

[bogstandard]

Your not getting neat liquid gas thru by any chance, is the cylinder at an angle. This would give you the symptoms you are getting.

[boatmadman]

The can gets cold, but not frosty.

The cylnder isnt at an angle, so i dont think neat liquid is getting through.

Ian