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.

2017-02-15 – Setting up

Today I started working myself up to making some valves. I ground a fresh tool with a radius for the turning the head - stem radius, and refined my 47½° tool for turning BA threads. I tried turning a few oddments of stainless, and the turning tool seems to be working well. (3 hours)

2017-02-18 – More prevarication

Experimenting with making the valve stem threads, I found roughing with the 7BA die, and finishing with a chaser produced good results. The tapped nuts do not need anything like full thread depth on the stems.

I made up a copper lap for the valve stems as they need to be a good fit in the valve guides, and it will be difficult to turn them to size, or even parallel, with sufficient accuracy.

I turned an experimental valve stem 0.0005″ over size and started lapping. I found 0.0985″ too big, but 0.0984″ just right. In making a trial valve out of this piece, I turned the head a bit under size, so compensated with a slightly shallow seating chamfer. Trying it in place showed that it was a little short over all, but the threaded length was quite a bit too long. I found adjusting the tappet clearance not too difficult with the right spanners. (4¾ hours)