Scott vacuum engine (flame gulper)

[Jasonb]

I'd start with at least honing the cylinder before offering up the piston, a turned finish is not going to be smooth enough as your starting point for "fitting"

[Sanjay F]

I was going to ask that, so I have one of the 3 stone honing gizmos, so a quick blast with that 1st then make the piston, followed by lapping the cylinder with the piston and grit.

The build I’m following suggested honing after lapping, which seemed counterintuitive, but guess that’s to get a surface to hold oil?

[Charles Lamont]

I think the parts need to be lapped separately. A hone will not necessarily correct parallelism errors in the bore. If you try to lap the two parts together, as you would for, say, a valve seat, you will get far more than your desired 0.0005" clearance.

[Jasonb]

In the past I have used a brake hone to take a slight taper out of a bore by concentrating the stones at the thinner end but a lap will generally be the better tool to take out any taper and also refine the finish at the same time.

The piston can be brought to a good finish so that it just enters the bore either with an external hone or lap or you can use fine emery backed up by a flat piece of MDF or flat metal bar. Again this can be used to correct any slight taper in the piston.

Once they start to enter then use the finest lapping powder and oil too do the final fitting. This is how many a small IC model aero engine is done and the way I also tend to do my Flame lickers and hot air engines all where you have no piston rings so the fit is your seal and also where minimum friction is required.

This thread has quite a lot of useful information, though you don't have to make the fancy laps as even a piece of hardwood carefully turned to size and charged with oil/power will do.

https://www.modelenginemaker.com/index.php/topic,1908.0.html

[Sanjay F]

You have described exactly what happened the stones took the taper out but did nothing for the surface finish. I remembered the thread you sent from a previous engine I built and made some hardwood laps with 400 grit on them.

Its really hard to photograph the result but lets just say I'm pleased with the finish, its reflecting light and feels smooth. I think its good enough for now until I have a piston and am ready to lap. I have plenty more hardwood t make 600 and 1000 grit batons if need be!

1st picture is the after and 2nd picture is before

[Sanjay F]

I started on the piston and am confused by how this mandrel works. I've made it and as you can see the spigot is sticking out and I've drilled out as much as I can before the boring exercise.

The build states:

"I need to get the heaviest of the machining done first, as the screw will be gradually shortened to a couple of threads by the time the centre is bored out"

How do I get the boring bar to the bottom with the spigot sticking out and the 'wall' of the drilled bottom? The bottom of the cylinder is only 0.094 thick and there's 0.075 betwee the end of the tool tip and boring bar (see photo)

Finally I've messed up and cut it too short, does the metal need to be cast iron or can I use the steel in the photo?

[Jasonb]

You just turn away the screw was you use th eboring bar to flatten the bottom of the hole in the piston. Needs a bar less than half the diameter of the hole so you can face the bottom as well as increase the diameter of the hole.

Does not really matter if the piston is a bit short, so long as you keep all dimensions to the top of the piston correct it will be OK

[PaulR]

What's that swan doing in there?! 

[Sanjay F]

You just turn away the screw was you use th eboring bar to flatten the bottom of the hole in the piston. Needs a bar less than half the diameter of the hole so you can face the bottom as well as increase the diameter of the hole.

Does not really matter if the piston is a bit short, so long as you keep all dimensions to the top of the piston correct it will be OK

Ah ha .........got it thanks; I think my brain was thinking about the big boring bar I used for the cylinder and got myself in a pickle! I cut the piston 1.125 instead of 1.250 in length  - I spent too long in the workshop today and lost focus 


Paul  - I think if it is a swan it's a Picasso swan

[Sanjay F]

This is where I've got to  - I can gently push the cylinder about 1/2" one end and 1/4" the other  - should I call it a day and use some fine emery or take more off?

Obviously don't want to over do it!

[Sanjay F]

Actually - I think I'm there, there's a tight spot towards the flange but the piston is just sliding in nicely about 3/4 of the way down!

[Jasonb]

If it is tightening at one end then adjust the cylinder.

[Sanjay F]

I don't want to mess this up at this stage, so just checking the sequence:

- I think I'll use the stick I made with the emery to free the tight end of the cylinder
- Should I have a piston that travels up and down the cylinder before the lapping begins?
- then lapping where I'm using the finest grade powder and working the cylinder up and down the piston mounted in the chuck rotating as slow as I can go

I've just read the lapping article again and it suggests 'thin oil' - no sure what is thin is and what the consitency the paste should be when mixed? I presume I keep cleaning it off and redoing it, but when do I stop?

[Jasonb]

That would be about it, once the piston is a firm push all through the cylinder you can start lapping teh two together.

1000g if you have it. I use the 32 weight oil that goes in the headstock or something a bit thinner like 3in1 would do. A lot more oil than powder maybe 20 drops of oil to a similar 1 drop size of powder . I usually apply several drops of oil then pick up a little powder with the end of a 3mm flat screwdriver and add that to the oil. Too much powder and it will just jam the piston in the cylinder.

[Sanjay F]

Thank you - let's give a try and see what happens!