ETW's Wyvern Gas Engine

[Jo]

...
Just at the moment I think my next model is going to be all fabricated!

  And Fabrication has its challenges. You are doing well Rod 

Nick: Yes there are good sets of castings, many ok and sadly some really awful ones. Looking at Rod's problem I would have said that was a drawing fault that should have been corrected years ago.

Jo

[tangler]

Well, I guess it's these little challenges that make it interesting 

So, on to the next one - the cylinder head.  There's a chucking piece/ sprue on this so I thought the first job was to turn a flat on the base .





I then bedded the flat in the 4 jaw and adjusted until the chucking piece was running as true as possible before turning it cylindrical.



I chose to hold this in a collet so that I could move it about and replace as necessary.  The head needs a recess for the flange of the cylinder to fit into which makes the head/cylinder joint gas tight



But the cylinder head also needs to be in good contact with the cylinder jacket so that there is a good thermal transfer to keep the head as cool as possible.  So quite a bit of shaving and trying until a smear of blue showed that everything was snug.



A taper was then turned to make the combustion space.  You can see that there is some gas porosity in the casting but I don't think this will be a problem



The design shows an inserted valve seat/guide for the inlet valve which has a flange seat on the cylinder head.  The exhaust valve is shown as seating  directly in the cylinder head - fine for a cast iron head but not, I think acceptable in (porous) cast ally.  One of the seats need to be removable to get the valves in.  There isn't much of a flange seat cast into the head on the exhaust side so I need to think about a design.  I'll root around the other Wyvern builds on the net for inspiration - michaelr managed it.

[tangler]

Hello again, I'm afraid I've been a bit distracted by Murray mania and the good weather here in the UK - I need to see a bit of sunshine occasionaly lest my transformation to Gollum becomes complete.  Anyway, a bit more progress on the cylinder head.  The inlet and exhaust holes for the valve housings need  to be bored to intersect the compression space and then the inlet and exhaust ports are drilled to intersect the valve housing holes.  The chucking piece on the cylinder head is true with the cylinder axis so this can be transferred to a collet in the dividing head mounted on the mill for drilling and boring.  Here is my quick and dirty method for centering the dividing head.  The two bars are 1/2" precision ground MS milled to exactly half depth and held in collets - ER25 in the DH and Myford patent in the mill.  It's good enough where extreme accuracy isn't required.  Direct indexing was used to rotate the cylinder head in 90 degree steps for each face to be dealt with.



With the centre found and using the cylinder face as a datum the various holes can be faced, drilled, counterbored and bored as appropriate





The depth of the intersecting holes needs careful monitoring



The inlet and outlet ports were drilled but the valve housings were bored to give a good fit with the inserts



The seat end of the valve housing isn't bored through so as to provide to provide a gas tight seal



Next, the valve housings...

[tangler]

This is the casting as provided for the inlet valve housing



registration of the 2 halves isn't great but I can get the part out of it



Oh, and in amongst the bits I found the casting for the marine type bearing for the big end bearing.  Doh!



The casting was mounted as true as possible in the 4 jaw and then turned up so that I could then mount it in collets so that the job can be turned end for end. 



The housing was drilled and reamed 1/8" for the valve spindle and then counter-bored from both ends using a 5/16" slot drill.



I made up a filing jig from 3/32" steel to shape the flange




As mentioned, the valve housing needs to seal in the bottom of its pocket so I used feeler gauges to determine how much need to be removed so that the flange would also seal.



I've left a gap of 1 thou.



ETW's design  tapers the external housing for aesthetic effect but I'll leave that until I'm sure I don't want to return it to the collet - I still need to turn the valve seat but I'll do that when I turn the valves

Again, as mentioned, the exhaust valve in the design seats directly in a cast iron head.  For this ally head I need to make an exhaust bronze housing similar to the inlet one, constrained by a) there isn't room on the head casting for a similar flange and b) the biggest piece of bronze I've got is 3/4" diameter.  I decided on a square flange.





I drilled the mounting holes for the flange using the milling spindle and the HDA





There's just room to turn a 1-size-smaller 8Ba nut



I'm thinking I might use a 6BA thread on the stud in the head casting, reduced to 8BA through the flange mount.  I'm also thinking that I'd like to smear something suitable around the housings to ensure a gas tight fit on final assembly.  Any thoughts, bearing in mind this is going to get hot?

[tangler]

Next up, the valves.  I'll make these by using a length of 1/8" precision ground stainless steel brazed to a stainless disc for the head.  The first problem was that a 3-4mm ER25 collet won't hold a 1/8" (3.175mm) length of rod tightly enough to screw 5BA with a die.  Not major, I used a 1/8" Myford collet instead but I'll need to investigate alternative ER collets in these smaller sizes.



The disc for the head was drilled 3.3mm to give a bit of clearance for the braze to penetrate and then parted off.





The last time I did this, on slightly smaller valves, I used brass wire but this time I couldn't seem to get it hot enough for the brass to flow.  I gave up and started again with new bits and some old C4 silver solder which has a higher melting range than some of the other compositions.  Here they are cooling down



The heads were then turned to shape, including the 45 degree seats.  While the topslide was at this angle I also turned the corresponding seats in the valve inserts



Valve springs were next.  As is common with ETW, there is no mention of the springs at all in the write up or on the drawings.  As most small i.c. designs seem to use 22SWG, this is what I used.  The valve stems are 1/8" diameter so I used a 5/32" mandrel to wind the springs.  12 tpi seemed to look be about right.  The winding bar is just a length of mild steel with a notch filed in it and there is a hole in the mandrel to tether the wire.



The gearbox on the lathe was set to 12 tpi and once the winding was started, the leadscrew was disengaged to give a couple of close turns



Then the leadscrew was engaged to wind the spiral (which I did by hand using the mandrel handle)



and then the half nuts disengaged again to close the spring





A touch on the grind stone finished it off



These are the components for the valve (inlet) assembly.  The spring cap was a simple turning job with a 5BA thread



This is the assembled exhaust valve.  The valve spring strength seems OK, I can't imagine it's at all critical on this type of engine.



I've still to cut the passages for the inlet from the carb and the outlet to the exhaust.  I'll do that when they are fastened in place but first I think it's about time I made some proper studs.

[Roger B]

Good and interesting work.  This is an engine I had often though about trying to build.

Have members found a lot of problems with casting sets?

[NickG]

Nice work Rod, looks very good. I've never heard of brass valve seats before but they've turned out well. Are you planning on any kind of leak testing device or just wait until final assembly and see what the compression is like. I have never done an ic before but I can
Imagine it helping pin point faults later.

[tangler]

The valve seats are bronze, not brass - quite common on small i.c. engines.  I'll see what the compression is like after assembly, one advantage of being able to remove the whole lot if it needs attention.
cheers,
Rod

[steamer]

Nicely done Rod!

Dave

[NickG]

Oh didn't realise that, good to know no exotic steels are necessary!

[vcutajar]

Attentively following your progress Rod and learning new things.  Thanks.

Vince

[Don1966]

Ron, I have been following you quietly and learning. Great work on your engine and I like your use of the HDA since I just completed mine. BTW great photos.

Don

[ths]

I've also found minor problems with holding certain imperial sizes in metric ER collets. I've decided to overcome that by getting some imperial collets, as they will generally require minimal tightening of the closing nut. At 3.175 mm, a 1/8" rod is asking for nearly maximum tightening of a 4 mm collet.

Nice work on the valve unit!

Hugh.

[tangler]

There's a little bit more to do on the head before I know how long the studs should be.  The combustion space is not cylindrical but is elongated 5/16" high (as bored) but 9/16" wide.  ETW suggests opening this out with a rotary file.  I don't know how he intended to do that since the end at the spark plug is semi-circular in plan view, 9/16" radius.  Anyway, I decided to open it out with a 5/16" ball ended mill.  The largest cutter I can hold in my milling spindle is 1/4" OD shank so the plan was to hold the cylinder head in the collet on the dividing head and the cutter in the lathe spindle.



The dividing head has 2 seats at right angles - in this configuration it sits at exactly centre height (it should do since it was bored between centres on this lathe) and the key fits snugly in the cross slide tee slots to keep it parallel to the lathe axis.  I was prompted by a comment by Jason on another forum to look out my electronic centering guide which was broken.  A couple of tweaks to the wires and a new spring to contact the battery and it worked again.  That's a 1/2" PGMS bar in the dividing head.



Centred in X,Y and Z I could now mount the head in the collet in the dividing head and the ball end mill in a collet in the spindle.  I know the inlet and exhaust face is at 90 degrees (6 holes on the indexer) so I levelled this and then rotated through 90 and widened the passage by milling 1/8" either side of the centre.





  Result?  Misery 



It looks like I've miscounted the number of holes on the indexer and milled at the wrong angle. 

It gets worse 



I've broken through the valve insert seats, which are required to make the valve assembly gas tight.  They must have been paper thin anyway - a miscalculation somewhere.  I suppose I could live with the slanty combustion space and I could bore out the valve pockets and press in a plug before re-cutting the pockets (properly this time) but in reality it's a scrapper.   I could fabricate a new head from solid and bolt on the rocker arms but I think a call to those nice people at Hemingway is in order.

Ho hum.

 

[Don1966]

Bummer Rod, and it looked like every thing was going so well. Its still a very interesting project and you have done well with it. Just regroup and attack it again we all get those moments.

Don