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.
The next simple modification is to reduce the pressure on the exhaust push rod by changing or shortening the spring. However, take note that there needs to be enough tension to push the contact breaker back for the points. I used a spring from a ballpoint pen, which had less tension and was shorter than the original.
Another pen spring was employed to replace the standard exhaust spring as this is quite stiff as standard. This is very fiddly but has made a marked improvement.
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 https://www.stirlingkit.com if you have the spare funds.
Before video: https://youtu.be/KtDYahggu_Q?si=tbZpxYnN4B0Bj1Sw
After video: https://youtu.be/eEHHU7Nc8Q8?si=KuOnLeMHygpOHjSc
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Country: GB
