Winter build. Atom Minor MK III

[Basil]

I've got quite a few long term builds on the go at the moment and I was looking for a nice little engine I could finish up over the winter. The casting kit and plans are from Hemmingway. I have previously built the Westbury Seal Major 30cc from their castings. Something about just following through instructions and plans and making parts to the best of your ability attracts me to these kits.
After deciding on the first part to make (Ignition bracket) It looked like a no brainer! I spent a few hours working through the best way to go about the machining. Per plans the bracket has a 29/64th hole through it which was going to be very tight with the raw casting material available.
To cut a long story short a couple of hours in the worst happened. A hairline crack between the large hole and the front of the casting. I had gone to great lengths to center everything within the available casting. I could have ordered another casting, this one did have an undercut that made it very thin in one area. Kirk is very good about this kind of thing and I am sure he would have swapped it out.
Because this bracket is designed to be adjustable and be pinch clamped to the front housing I thought it sure would be better to make it out of some good old 6061. I have lots from my machining days in the states. Luckily this component is fully dimensioned on the plans. Sweet!
A day later and it is complete. No need to worry about this one cracking one day from a little over torqueing.
On to the next part......

 

[ShopShoe]

Good Save

I'm following this build.

--ShopShoe

 

[Basil]

I spent today machining the block for the Atom. I had a choice, either chuck the casting in a 4 Jaw and machine on the lathe or rig something up on the mill. I felt I had more control and room for adjustment as I was working up to the final crank hole diameter on the mill. Not an easy item to hold on to, I did not want to put pressure on the casting length, deciding to hold under the mounting lugs instead.
The wood might look sketchy but with a bit of grinding it fitted very well and was solid. Once I was happy with the positioning I firmed everything up with super glue. The casting does not leave much room for error and I was happy when this first cut was over. Now I could mount on a mandrel and cut the flange surfaces.
The only material I could find of a close size for the Mandrel was a piece of nice EN24. I decided not to be wasteful by cutting it, hence the steady rest. Back over to the mill to work on the cylinder diameter, deck height and mounting holes. The cylinder bore is 1/16th offset to the crank centerline which could definitely catch you out.
Turned it over and dialed in an accurate 180degrees and worked on the mounting lugs. Then onto the 10 degree exhaust port. Went for a 7/16 thread in this. I have converted all the other threads to metric.
A full day with some nice resting time. I find taking a good break every hour or two helps keep parts out of the junk bin.
A good day and happy with the result.

 

[Basil]

On to the front and rear housings. Front housing just consists of doing some preliminary machining and making a very slightly tapered spigot to assist with the rest of the machining processes.
With not much to hang onto with the rear housing I got the first part of the machining gingerly out of the way in the 4 jaw. Matched the holes up with the main case in the mill then made a jig for the rest of the machining. The making of the jig was well worth while allowing for easy setup in the mill and 4 Jaw in the lathe. Chose the bore the tapered intake tracts on the lathe instead of making D bits. Then onto some casting clean up. Cheers

 

[Basil]

Moving on with the Westbury Atom minor. The front housing bushings were the next in line. Very straight forward job. Then I decided to tackle the cylinder head. A casting had been included with the kit, at least I took it that's what it was for. The head machined from this had lots of porosity which might not be an issue but seeing as I had everything at hand I decided to make another from bar.. I had made a mandrel to hold onto the head and help with setup, this made machining the second one go much quicker.
Getting the spark plug as shown in the plans was looking difficult and its a whopping big thing anyway so I decide to go with 1/4-32 ME8 I had from another project. This also is of course a regular glow plug size so I can go that route if desired.
Decided to make some brass pieces for the carb also at this time. Phone was dead so did not get many pictures of this but it is pretty straight forward anyway just needed to be a bit extra careful due to the size and tolerances.
I rigged up my Proxxon with the attachment I made a while ago for grinding the end of the fuel needle. Completed the needle adjustment knob and sweated the 2 together.

 

[minh-thanh]

Interesting project

 

[Basil]

Moving onto the cylinder. I always enjoy the crisp machining of cast iron but boy it sure is mucky! Everything went very straight forward. Whenever I am machining something on the larger side that will require deep cuts like this I like to stiffen the cutting assembly up. I remove the compound slide and fit an adapter I made up that takes my big Harrison lathe tooling.
I got the sequence a little backwards and should have drilled the fixing holes first before machining the fins. Anyway I made an aluminium holder to get around this small issue. As per the instructions the part of the liner where the exhaust port area is is machined for a wringing fit to facilitate a good port seal. I have ordered the material to make a lap for the final finishing.
While waiting I decided to get to work on the crank. The main journal is only 1/4" and I did not like the idea of finishing this area first and then making cuts on the larger head of the crank. I decided to cut away most of the big end material on the mill also doing the scallops at the same time. Then spun between centers to complete the big end and undercut. Lastly machining the 1/4" main shaft. This is where you find out how out of line your tail stock is, I kept a close eye on measurements and was able to adjust it in as I went. It was only a few thou out but this was giving me a taper early on. To finish I polished the main shaft and will polish the big end to 2500 grit.