In Balance

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

Classic Dirtbike - - Contents -

The seed of doubt, once planted, grows in to the ab­so­lute cer­tainty of an er­ror and the only re­course is to check.

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.

…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… Tim Brit­ton

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