Stirling Engine Bengs Kit "Laura"

This page last updated: 14 December 2016.

Introduction

Disclaimer: I executed this project in order to make use of equipment that is new to me, build up my workshop and re-learn machining.  The practices I adopted aren't perfect, may not even be good, they just worked for me. Now that's said...

Having bought an Emco Unimat SL lathe a few years ago, I'd been looking for a starter project for it.  At lunch John had already suggested making a Stirling Engine and, while bored waiting for someone in China to fix the [non source-code part of the] driver for the QHY guide camera used in my astrophotography project, I found the perfect thing: a gamma configuration Stirling Engine kit made by Patrick Bengs in Germany, called Laura, in which no milling is required but some machining is required.  In retrospect I could have picked an easier starter project: Stirling Engines require exacting tolerances (0.02 mm on the working piston), no friction and utterly perpendicular joints.  Construction took 4 months of "evenings when I found the time".

Note: the flywheel in this project doesn't fit in the Emco Unimat SL lathe as an 82 mm turning diameter is required and the SL only has 70 mm; the MK3 has 92 mm so would be OK I think. I was originally going to get a friend to turn this part for me but instead decided to purchase a Warco Super Mini lathe as it has a maximum turning diameter of 180 mm (100 mm over the cross slide) in the smallest possible package and will be useful for future projects.

Here are all the parts, as they arrived:



The first thing worth mentioning is that the kit requires some very specific drill sizes, in 0.1 mm steps.  Since I also lacked the necessary metric reamers and metric taps/dies, I ended up spending around three times the price of the kit on tools (also bought from Bengs for convenience).  Then I went way over the top and bought a cheap-yet-huge band saw capable of cutting metal (definitely not required for this project), so that I can get a nice clean cut. Expensive but my workshop is now well kitted-out.  A complete list of the tools I actually used can be found at the bottom of this page.

Making The Parts

• for a nice easy start, make the brass parts that require only drilling/tapping and deburring: 1 to 6, 16 and 19,
• set the speed of the lathe for turning steel and make parts: 7, 10, 11, 13 and 21,
• make the intricate brass parts that do not require turning: 12, 18 and 14,
• set the speed of the lathe for brass turning and make the simpler brass parts: 17 and 20,
• make the crank arms: 8 and 9,
• cut the glass tube (don't leave this to the end in case you mess both up and need to order more): 28,
• make the remaining parts that are cut from the 22 mm diameter brass rod: 23 and 26,
• make the remaining parts that are cut from the 30 mm diameter brass rod that form part of the engine: 24, 27 and 25,
• make the working piston: 15,
• make the displacement bolt: 22,
• make the columns: 30,
• finish the flywheel: 29,
• and finally, make the parts that form the burner (cut from the 30 mm diameter brass rod): 31 and 32.
  

Item	Description	Supplied Material	To Do	Result
1	Base plate: 95.5 mm x 42 mm pre-milled brass plate.	(both sides shown)	Deburr, drill 2.2 mm holes, drill & tap M2 holes (1.6 mm drill).
2 (2 off)	Junction plates: 14 mm x 7 mm pre-milled brass plate.		Deburr.
3	Cylinder holder, front: 73 mm x 41 mm pre-milled brass plate.	(both sides shown)	Deburr.
4	Cylinder holder, back: 73 mm x 42 mm pre-milled brass plate.	(both sides shown)	Deburr, drill & tap M2 hole (1.6 mm drill).
5 & 6	Main bearing support & cover: 24 mm x 26 mm pre-milled brass plate.		Deburr, drill 2.2 mm holes, drill & tap M2 holes (1.6 mm drill). Note: drill 2.2 mm holes then clamp top and bottom halves and drill 1.6 mm holes through 2.2 mm holes to ensure alignment.
7	Crank shaft: 6 mm diameter, 38 mm long, silver steel rod.	6 mm diameter, 40 mm long, silver steel rod.	Turn down to required length.
8	Crank arm, work: 19 mm diameter, 4 mm thick, brass rod.	22 mm diameter, 46 mm long, brass rod (also used to make parts 23 and 26).	Turn down original material to 19 mm diameter then cut disk free. In the lathe, drill (5.7 mm drill) & ream 6 mm hole. Very carefully drill & tap M3 hole (2.5 mm drill), using a center drill to start it off so that the drill doesn't drift.  Drill & tap M2 hole (1.6 mm drill), cut and file the piece to shape.
9	Crank arm, displacement: 25 mm diameter, 4 mm thick, brass rod.	30 mm diameter, 101 mm long, brass rod (also used to make parts 24, 25, 27, 31 and 32).	Turn down original material to 25 mm diameter then cut disk free.  In the lathe, drill (5.7 mm drill) & ream 6 mm hole. Very carefully drill & tap M3 hole (2.5 mm drill), using a centre drill to start it off so that the drill doesn't drift. Drill & tap M2 hole (1.6 mm drill), cut and file the piece to shape.
10	Crank bolt, work: 3 mm diameter, 15 mm long, silver steel rod.	3 mm diameter, 200 mm long, silver steel rod (also used to make parts 11, 13 and 21).	Turn down to 2 mm diameter on one end to cut M2 thread, cut to length required, cut M3 thread on the other end.
11	Crank bolt, displacement: 3 mm diameter, 20 mm long, silver steel rod.	3 mm diameter, 200 mm long, silver steel rod (also used to make parts 10, 13 and 21).	Turn down to 2 mm diameter on one end to cut M2 thread, cut to length required, cut M3 thread on the other end.
12	Connecting rod joint: 6 mm square section, 12 mm long, brass rod.	6 mm square section, 42 mm long, brass rod (also used to make part 18).	Drill (2.8 mm drill) & ream 3 mm hole (part 10 should turn smoothly inside it), cut to size, drill & tap M3 hole (2.5 mm drill), file to shape.  Note: use a centre drill to start each hole so that the drill bit doesn't drift.
13	Connecting rod:  3 mm diameter, 58 mm long, silver steel rod.	3 mm diameter, 200 mm long, silver steel rod (also used to make parts 10, 11 and 21).	Grind end flat and VERY CAREFULLY drill 2 mm hole as centrally as you can, using a centre drill to start the hole.  If it goes wrong you've got enough material to cut off your mistake and try again three times. Cut to length required and cut M3 thread on the other end.
14	Working piston joint: 5 mm square section, 8 mm long, brass rod.	5 mm square section, 22 mm long brass rod.	Drill & tap M2 hole (1.6 mm drill), drill (1.8 mm drill) & ream 2 mm hole, cut from original material, cut either side of slot with hacksaw then file down the remainder with a 1 mm needle file.  Don't try drilling it out, such tiny drills wander too much.
15	Working piston: 10 mm diameter, 10 mm long, gun metal rod.	13 mm diameter, 42 mm long, gun metal rod.	Turn 10 mm outer diameter, taking great care to ensure a perfect fit in part 26 (get it to within 0.5 mm, then take off the remainder with abrasive paper, test fitting into part 26 frequently, then later put metal polish on the piston and run it through to achieve a final finish) turn 8 mm inner diameter, 8 mm deep, drill 2.2 mm hole, cut from original material.  You have enough material for three goes if the fit turns out wrong; see running.
16 (2 off)	Driving rods:  6 mm x 35 mm pre-milled, brass plate.		Deburr.
17	Driving rod bush: 6 mm diameter, 11 mm long, brass rod.	6 mm diameter, 27 mm long brass rod.	In the lathe drill a 2.8 mm hole (using a centre drill to start the hole off) then ream out to 3 mm, very carefully turn down one end, cut from original material, very carefully turn down the other end. The ends should fit tightly inside the larger holes in parts 16.  Part 11 should turn smoothly inside this part.
18	Cross piece: 6 mm square section, 6 mm long, brass rod.	6 mm square section, 42 mm long, brass rod (also used to make part 12).	Drill (2.8 mm drill) & ream 3 mm hole (part 21 should fit inside it), drill & tap M2 hole (1.6 mm drill), cut from original material.  Note: be generous with the length along the axis of the M2 hole as a bolt will need to bite on that thread without stripping it; it can be filed down later. I used a 4 jaw chuck and did all this in the lathe. If you use a drill press, use a centre drill to start each hole so that the drill bit doesn't drift. If it goes wrong you have enough material for a few attempts
19	Piston rod guidance: 21 mm x 5 mm, pre-milled, brass plate.		Deburr. Consider drilling the centre hole out to 3.3 mm (see assembly).
20 (2 off)	Guidance bolts: 5 mm diameter, 37 mm long, brass rod.	5 mm diameter, 100 mm long brass rod.	Cut two 40 mm lengths of rod, turn down to 37 mm long, turn down ends to 2 mm diameter and then cut M2 threads.  Make sure that the shorter threaded ends are quite square, introduce no distortion while cutting the thread.
21	Piston rod: 3 mm diameter, 70 mm long, silver steel rod.	3 mm diameter, 200 mm long, silver steel rod (also used to make parts 10, 11 and 13).	Cut to length and deburr.
22	Displacement bolt: 12.5 mm diameter, 45 mm long, aluminium rod.	14 mm diameter, 52 mm long aluminium rod.	Turn down to intended size, drill (2.8 mm drill) & ream 3 mm hole, cut from original material, shape the end with an EMCO Unimat SL1000 radius turning attachment, drill (2.5 mm drill) & tap M3 hole.	nbsp;
23	Cylinder cover, work: 22 mm diameter, 4 mm thick, brass rod.	22 mm diameter, 46 mm long, brass rod (also used to make parts 8 and 26).	Turn ends to size (I used a parting tool to cut the inner edge), cut from original material, drill 2.2 mm holes.
24	Cylinder cover, displacement: 30 mm diameter, 10 mm long, brass rod.	30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 25, 27, 31 and 32).	Cut from original material, turn the end with the larger diameter sticky-out bit and then, without unclamping, drill (2.8 mm drill) & ream 3 mm hole so as to ensure the hole is exactly perpendicular to this side, then turn the other end to size and make sure that the main disk is no more than 2 mm thick, mark hole positions using parts 3 and 4 as a template, drill 2.2 mm holes, drill & tap M2 holes (1.6 mm drill).  It's really critical to get the M2 holes in exactly the correct position and the central hole exactly perpendicular if all is to be aligned and the displacement piston held centrally when assembled (see running).
25	Displacement cylinder: 30 mm diameter, 27 mm long, brass rod.	30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 24, 27, 31 and 32).	Turn outer pattern, separate from original material, drill inner with a 6.5 mm drill and then bore out to correct diameter, drill & tap M2 holes (1.6 mm drill), using part 4 as a template for the thicker end and part 27 as a template for the other end. Use a bottom-tap and be careful not to strip the threads when the tap hits the rest of the material.
26	Working cylinder: 22 mm diameter, 27 mm long, brass rod.	22 mm diameter, 46 mm long, brass rod (also used to make parts 23 and 26).	Turn outer pattern (don't go deeper than specified as you will foul the M2 holes), turn down for the front (this should fit into the smaller of the two holes in the cylinder holder plates), drill (a 6.5 mm drill and then a 9.5 mm drill) & very carefully ream (by hand with the reamer fixed in the lathe) 10 mm central hole (ensuring the smoothest possible surface), separate from original material, drill 3 mm hole, make 2 mm bite out of back using grinder and then a round needle file, drill & tap M2 holes (1.6 mm drill) using a bottom-tap and being careful not to strip the thread in the shallow holes (they need to be 3 mm deep on the back and 6.5 mm deep on the front).  The holes in the front can be marked by pushing the part into the cylinder front plate, aligning the 3 mm hole and scribing through the other holes.
27	Glass tube muff: 30 mm diameter, 3 mm long, brass rod.	30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 24, 25, 31 and 32).	Turn inner (the hole sized to be 0.2 mm wider in diameter than the measured glass tube and the diameter of the step so that one of the thick seals provided just fits inside it), separate from original material, drill 2.2 mm holes. I used the left-over piece of glass tube to check the size of the hole.
28	Glass tube: 15.5 mm diameter, 41 mm long, glass tube.	(2 off supplied)	Cut to length.  I marked the desired length, leaving ~3 mm extra to allow for finishing, with an indelible pen and then cut the glass with a tiny tile cutting diamond disk intended for use in a Dremmel, held in the lathe grinder but in an extension piece so that I could accommodate the required length of glass tube.  Once cut, finish the end of the part on the same Dremmel diamond disk.	nbsp;
29	Flywheel: cast/milled steel.		Clamp into lathe at the axle, turn the outer edge and the visible rim/axle to nice shiny steel, drill & ream 6 mm hole (5.8 mm drill), turn the piece around and turn the other rim/axle to nice shiny steel, in a drill press start the angled M3 hole with a chunky centre drill then drill & M3 tap it (2.5 mm drill).
30 (6 off)	Columns: 8 mm diameter, 14 mm long, aluminum rod.	8 mm diameter, 151 mm long aluminium rod.	I worked in batches of two: cut a 30 mm length, turn outer of two parts, cut into two, drill (1.6 mm drill) centre hole, trim ends to length, M2 tap the centre hole half-way from one end and then half-way from the other end.
31	Burner bottom: 30 mm diameter, 20 mm long, brass rod.	30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 24, 25, 27 and 32).	Turn outer, use the parting tool you used to cut the slots in part 25, but mounted at a 45 degree angle, to cut the slot for the rubber seal, separate from original material, drill 6.5 mm hole to 18 mm depth (being careful not to breach the base by accident) and then use a boring bar to complete turning of inner.
32	Burner cover: 30 mm diameter, 16 mm long, brass rod.	30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 24, 25, 27 and 31).	Turn outer (the outer portion is at an angle of 13.25 degrees from the horizontal and the inner portion at an angle of 16 degrees from the vertical), drill centre hole, separate from original material, turn inner, drill 1.5 mm hole.

• first I filed down the surfaces using a diamond needle file where necessary and gave them an initial coarse abrading,
• then I went down to a finer grade of abrasive paper and abraded with wire wool to leave only tiny scratches,
• then I polished the surfaces using a leiderfeile (a bit of wood with some leather stuck on it; watchmakers user them) with some buffing compound.

The steps are shown below. You could, of course, insert more steps of finer abrasive paper before the last step to get a true mirror finish, if you have the patience.  Round parts can be finished, carefully, in the lathe.

Original cut sides

After diamond needle filing and initial abrasive

After finer abrasive paper and wire wool

After polishing with a leiderfeile and compound

Assembly

The Bengs instructions are relatively light on how to go about this, it should largely be obvious.  For the soldering of the base I used a paste that contains both solder and flux, then simply applied a blow torch; definitely the best way to go about it.  Make sure all the parts fit correctly, apply the paste inside the joints, put the parts in position and then heat with the blow torch. I didn't clamp the pieces, relying on the paste to hold things together.  I suggest keeping a couple of sticks of wood around to gently apply pressure if anything comes adrift while soldering.  In any case, the base itself warped in the heat and so straightening was necessary anyway (gently, using a vice to hold the assembly).  The joints are pretty good and it certainly isn't going to come apart [edit: famous last words, see below].

• mark the positions of the holes in the wooden base, using part 1 as a template, before you start assembling the machine (taking into account the length of part 25 when positioning it so that the burner can sit sensibly underneath the glass tube); bits will get in your way if you leave this to last,
  
• parts 16 and 17 didn't hold together tightly for me so I, very carefully, soft soldered them,
• the silver steel rods of parts 10, 11 and 21 need to be very smooth to slide/rotate without snagging, so abrade and then wire-wool them to a smooth finish and use a very fine machine oil (e.g. sewing machine oil) to prevent corrosion,
  
• parts 19, 20 and 24 need to be extremely square if part 21 is to slide smoothly through them; it is worth test-assembling just parts 19, 20, 24 and 21/22 to make sure that everything runs smoothly.  You may need to carefully file the hole in part 19 slightly larger with a round needle file to avoid snagging,
• if you apply pressure to parts 20 when screwed into part 24 to line everything up, be very careful as the tiny threaded ends easily break off,
• getting the holes in parts 24, 3, 4 and 25 to line up is non trivial; you may need to hold the parts together and then run a 2.2 mm drill through the holes to make sure all is lined-up and to give you a fair amount of play,
  
• apply a high temperature sealant (e.g. Loctite 574) when assembling the following:
• parts 3 and 4 (to seal the air channel),
• parts 3 and 26 (ditto),
• parts 26 and 23 (being careful to avoid blocking the air channel),
• parts 3 and 24 (ditto),
• parts 4 and 25,
• my (English) version of the Bengs instructions talk of soft soldering part 13 into part 12; I think this must be a hang-over from an earlier version of the design where the two parts weren't threaded, just ignore it, they simply screw together,
  
• you will need to make a fair number of adjustments to the end of part 13 in order to get it to fit properly into part 14 and move smoothly with the rivet holding the two together while still leaving room for the end of the bolt holding part 14 to part 15; here is part 13 in final form and then fitted into position with the other parts:

• assembling the crank arms, the axle and then getting everything to move smoothly will take a lot of fiddling, adjusting etc.: I found I had to shorten part 13 by several millimetres, open up the smaller holes in parts 16, file thinner the ends of parts 16 (the ends with the smaller hole), very slightly turn down/smooth part 11 and reduce the width of part 18 somewhat; you may find that different adjustments are necessary,
• don't bolt parts 5 and parts 6 together too tightly, the ball race will not run smoothly if you do,
• when bolting parts 30 through the wooden base, I added some washers on the underside to make sure the heads didn't pull through,
• I painted the wooden base with three coats of teak-stained varnish, rubbing down between each coat to achieve a smooth finish.
  

Here are a few more views of the machine coming together:

	nbsp;

This is what the whole thing looks like (refresh this page if no YouTube video appears below, sometimes they don't appear on first loading):

As you can see, there are plenty of improvements that could be made to the finish but this is my first attempt to machine something since I left school.  This is how freely it runs (the Bengs instructions suggest it should run for five revolutions when "activated"):

Running (1)

In my first attempt to run the engine I used lamp oil, since that is what I had to hand.  It was rather smoky.

The engine did not run so I rubbed off the soot and tried once more but this time using methylated spirit, just in case the flame temperature has an influence; no soot but still no running.

Next thing to try was making another piston as my first attempt was loose, as you might be able to see below.  Bengs says the clearance needs to be 0.02 mm.

That helped but it didn't help enough and, on further investigation, I found that the aluminium displacement bolt was rubbing against the edge of the glass tube.  I polished the surface of the bolt and filed down the edge of the glass tube to ensure it didn't catch but still no dice. I started to wonder if the central hole in the working cylinder was not as properly finished as it should be, since the motion of the piston was not smooth, and then, in attempting to remove part 23 (the brass cover of the working cylinder), I managed to break the solder joint that is holding the base together; Loctite 574 is strong stuff.  Since I needed to take the whole machine apart to re-solder, I took a closer look at all the potentially binding parts:

Working cylinder

Working piston

Displacement cylinder cover bore hole

Displacement bolt

Glass tube

The working cylinder and piston should be as smooth as the proverbial, as should the hole in the displacement cylinder cover and, of course, the hole in the displacement cylinder cover should be absolutely perpendicular to the face shown if all the parts are to line up.  So I had another go at making the displacement cylinder cover (part 24, buying some more brass rod to do so), revisited the bore in the working cylinder (part 26) and made another working piston (part 15, using up my last chance with the gun metal) to fit the revisited bore.  The aluminium displacement bolt can rub against the edge of the glass tube so I put some further effort into polishing it, and the displacement rod, to reduce friction.

Working cylinder, improved bore finish

Working piston, final attempt

Displacement cylinder cover MKII, bore hole

Displacement bolt, more polished

Glass tube, polished edge

When reassembling the machine I had immense trouble getting the solder joint of the base to hold.  I ended up switching to another brand of solder paste, leaded and hence with a lower working temperature (180 C as opposed to 650 C), delivered through a syringe, which worked really well: clean the joint, flux inside the joint, assemble the parts, apply solder paste along the outside of the joint, heat the surrounding metal and then, when the flux is sizzling, heat the solder paste and it will be sucked into the joint.

Running (2)

During reassembly I made the effort once again to line up, parts 19, 20, 21 and 22 inside parts 25 and 27, i.e. all the displacement cylinder bits, having taken the time to re-make part 24 with a perpendicular hole.  I looked at parts 21 and 22 inside part 25 and rotated parts 21 and 22 until they were central and so not in contact with the glass tube while running.  The video on the left below shows how freely it runs when activated, which I think is slightly better than before, and the video on the right shows what happened when I tried to run it with a flame; definitely better but still no banana.

Tools Used

• Emco Unimat SL lathe with:
• two 3-jaw chucks (the 9.5 mm drill and 10 mm reamer used to bore the hole in part 26 wouldn't fit into my drill chuck, hence I used a second 3-jaw chuck for that case),
• drill chuck,
• live centre,
• steady rest for turning the 30 mm brass rod or, alternatively, cut the rod into smaller sections before turning,
• either (a) an inner turning tool that can cut a 14 mm bore or (b) a boring bar that can do the same job (these for the displacement cylinder (part 25)),
• ~ 7 cm long collet-holding rod with collet that takes the mandrel of the Dremmel tile cutting disk,
  
• a narrow (e.g. 1.5 mm) parting-off tool for cutting the 2 mm wide ridges in the cylinders,
  
• optionally, for turning the hemisphere on the end of the displacement bolt (part 22), a Unimat SL1000 radius turning attachment; otherwise just do your best,
• optionally, a 4 jaw chuck for making part 18,
• phone (to phone a friend who had a lathe with large enough throw to turn part 29), or optionally a Warco Super Mini lathe,
  
• stand drill with vice,
• optionally, a band saw (useful for cutting the thick brass bar neatly),
  
• drills: 1.5 mm, 1.6 mm, 1.8 mm, 2 mm, 2.2 mm, 2.5 mm, 2.8 mm, 3.3 mm, 5.7 mm, 6.5 mm and 9.5 mm,
• reamers: 2 mm, 3 mm, 6 mm and 10 mm (all H7 tolerance),
• taps: M3, M2 standard and M2 bottom tap for threading the short blind holes in the working cylinder (part 26),
  
• dies: M3 and M2,
• Dremmel tile cutting diamond disk and mandrel,
• hacksaw,
• bench vice,
• set of diamond needle files,
• set of centre drills,
• centre pop,
• steel rule,
• calipers (inner, outer and sliding),
• scriber,
• abrasive paper, fine and medium grade,
• wire wool,
• leiderfeile and buffing compound,
• metal polish (e.g. Autosol) and cloth,
  
• solder/flux paste,
• butane torch,
• small hammer plus piece of rounded-end bar (for shaping the end of the rivet that holds part 14 to part 15),
  
• fine machine oil (e.g. sewing machine oil),
• Loctite 574 high temperature sealant,
• small tin of teak-stained varnish,
• small paint brush,
  
• 1.5 mm allen key (fits all of the small bolts supplied with the kit),
• 4 mm (or, just, 8BA) spanner (fits all of the small nuts supplied in the kit),
  
• indelible pen.