Seagull Engine Construction Diary

Making a 10cc twin 4-stroke to the E T Westbury design


With the intended change from 8-BA to 7-BA threaded valve stems, the 116″ thick valve-spring locknut would contain only a little over 3 threads. Fortunately, the slightly larger hole in the tappet means there is comfortably room for a shoulder to extend inside it. With an overall thickness increased to 332″ it allows just under 5 threads, a lot better. I used 4-BA hexagon stock for the nuts, nominally 0.002″ less than the originally specified ¼″ A/F.

2015-05-23 – Trying one

I made a trial locknut. I found I would need some means of ensuring true running for the second operation. I must have done other things too, since I have logged for today: (2 hours)

2015-05-26 – Making a start

In a brief session today I first-operation turned two nuts. They are faced, chamfered, drilled, tapped and parted-off. (½ hours)

2015-05-27 – Starting the rest

Firstly, I first-op turned the two remaining thick nuts and four locknuts. For the tapping operation I felt the mandrel handle would be too insensitive for this job so instead I used a carrier to turn the tap, with its shank loosely held in the tailstock chuck.

Next, I made a 7-BA stub mandrel for the second operation. For some reason I forget, the thread was poor so I decided to try screwcutting one, and started by grinding a 47½° tool. (3¼ hours)

2015-05-28 – Making a mandrel

I screw cut a new mandrel, finishing the thread with a 7-BA die. The result was not bad this time. I used the ML7 gear program to work out a gear train for 0.48 mm pitch. It offered a 0.4799943 mm pitch using these wheels:

7 BA Cluster 1st stud 2nd stud leadscrew
Driver 35 38 20
Driven 45 46 85

2015-05-30 – Finishing the nuts

Simply mounting the nuts on the threaded mandrel against a face would probably not give the concentricity I wanted so turned a shallow recess on the end of a stub of bar to a diameter to just accept the nut corners. I drilled and tapped the stub to accept the 7-BA stud.

I faced the parted side of each nut to length and turned the shoulders, then cut the 30° chamfers on the hexagons as a third operation, and finally put a 0.004″ chamfer on the edges of all the shoulders. Changing the nuts for these jobs seemed quicker than changing tools. (3¾ hours)



A length of stainless steel is provided in the kit for the valves. I have increased the stem diameter a few thou from 332″ to 2.5mm. This allows the threaded end to be made 7-BA rather than 8-BA, giving a 70% increase in core area of the thread. I have also made them a little longer than original design to allow for the longer thread length in the locknuts and for a male centre at the end of the stem.