Improvements to the Retrol-HM01
The little Chinese Retrol HM-01 from Stirlingkit runs quite well out of the box, but a few simple modifications can slow it down to a more restrained pace.
The first and simplest modification is to use the extra length of spring provided to create a custom spring. Made longer so the spring is at rest when the governor latch is disengaged this makes a big difference to how the engine runs. I’ve made a custom spring out of 0.3mm piano wire, which works but the engine needs a few other modifications, and it is a bit less reliable with it fitted.
This concept was proposed by British inventor Herbert Akroyd Stuart in the late 19th century. The first prototypes were built in 1886, and by 1891, they were licensed for production by Richard Hornsby & Sons of Grantham, England, under the name Hornsby-Akroyd Patent Oil Engine. Later, German immigrants Mietz and Weiss developed it in the United States, combining it with Joseph Day’s two-stroke engine.
To reduce interaction between the cam and follower, a 1mm bit of brass was cut, shaped and super glued to the governor latch. This was done so the latch holds out the exhaust push rod further and the cam hits the follower less, reducing friction.
After a bit of time compression built up quite considerably, which would cause the engine to stall out. I made a head gasket out of some thick Klinger gasket I had spare. This required adjustment to the rocker and valve clearance but successfully reduced the compression for slow running.
In all, these little engines are quite customisable; out of the box they run quite fast for us in the preservation hobby, I can assume this is for easy starting and less trouble as they start to get temperamental the slower they get; you do need to be prepared for this if you decide to carry out these mods. They are a nice little toy and worth investigating if you have the spare funds.
