IN BALANCE: The seed of doubt

Normally us off-roaders are pretty unflappable… at least until piston circlips are mentioned…

There I was, happily enjoying a small libation after a superb Christmas dinner and watching the Strictly Christmas Special with Dr Who and Maigret to come, when I had the bright idea of running through the pics of assembling our Project IT engine. Don’t ask me why, I just did and it seemed like a good idea at the time. Normally, for shoots in the workshop my other half is drafted in to wield the camera but, for the engine assembly, I did it myself. It takes a little longer but it was cold outside anyway, assembling the engine and doing the pics…

Now, no matter how many pics are taken for a feature there are only so many that can go in the mag and the choosing process is simply a matter of scrolling through the pics and copying the ones that tell the tale well enough. The rest of the pics serve a useful purpose as reference to how things were or how they went back together. Sort of a modern version of a sketch, it also helps when trying to solve an issue such as the location of swinging arm pivot bearings… see the IT feature later on in this issue.


So, there I was flicking through the pics and congratulating myself on a job well done when it appeared… a side on shot of the gudgeon pin – or wrist pin if you’re reading this in the USA – it is an interesting name ‘gudgeon’ and has its origins in middle English and French with references to a pin and socket for a hinge or a fish depending on the context. Actually, given some of the attention this engine has had in the past, I wouldn’t have been all that surprised to find a fish inside it!

Okay, we all know or should know that the gudgeon pin joins a piston to a con-rod by sliding through the little end eye and being located in bores in the piston. It should be obvious if the pin moves that great damage will occur to the barrel… I’ve seen score marks in barrels where this has happened and in a worst-case scenario it can destroy an engine. There are a couple of methods used to prevent a gudgeon pin from moving, one involves PTFE buttons that slide on the barrel bore but the other, more common method, is to use a circlip of some type at either end of the pin.

The circlips locate in machined grooves in the gudgeon pin bore in the piston and prevent its lateral movement.


They are fiddly little things to fit and the nightmare scenario is dropping one in the crankcase mouth of an assembled engine, but a clean cloth covering said hole works wonders for peace of mind. Some of these circlips have ears on them, so circlip pliers can be used, while others are just spring wire and need to be pressed into place with some form of progger. It is a wise engine builder who takes a few moments to check all is well at this department, as the possible damage caused if the circlip isn’t seated properly doesn’t bear thinking about, but makes grown men shudder. And there it was, a pic of the circlip… not properly seated… I blew the pic up to check… no, it wasn’t in, I moved on to the next pic, which showed the barrel in place… then back to the previous pic and the circlip still wasn’t seated properly. Surely I must have seated it correctly… surely I wouldn’t have assembled the rest of the engine without checking the circlip was in place…? Or would I?

That was enough, the seed of doubt was planted, all thoughts of a relaxing Christmas night went out of the window and out to the workshop I went. This isn’t as simple a task as could be imagined, as despite popular opinion, an editor’s workshop is likely to be as cold as anyone else’s and gone are the days when I did actually have a heated workshop. Also in order to do anything in my workspace almost everything in it has to come out, so it can be a demanding task.

Luckily, the engine was on the bench by the door so only my TY250 and my 650 Triumph had to be pulled out to allow me to stand at the bench.


Even more luckily, the engine in question is a two-stroke single, so whipping the head and barrel off isn’t a huge task and simply involved a few nuts for the head and four base nuts for the barrel. I could also lift it high enough to put my piston supports under the skirt and then lift the barrel further up to reveal the gudgeon pin and circlips, but not expose the rings.

With a powerful lamp aimed at the piston and my freshly cleaned glasses on I could closely inspect the circlips… and collapse against the bench while breathing a sigh of relief, I had ensured the circlips were properly seated before fitting the barrel… but you know what the seed of doubt is like – once planted it flowers into something huge and it is best to check.


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