As those who have been unlucky enough to meet me in the flesh will testify, I’m slightly long in the tooth, but will readily admit, the world of the desmo is a fairly new one to me.
It’s not that I’m just a stingy old git, as more that I dont buy new parts if I can a machine up an alternative from bits in the scrap-box. So I need to lean on the experience of members who have ‘been there, done that, and know what works’ when it comes to getting the best from my air cooled Desmo lump……
I’m slowly but surely getting a pair of heads ready to allow the engine to make best use of a pair of FCRs and some HC pistons…… ports are done (well, they definitely flow more now!), suitable seals are waiting, and now there’s just the little matter of shimming the valves.
I’ve scoured the interweb thingy, watched the CA vids (very handy), and then set about the tasks…… and am now mildly confused, and so need some advice (even if it’s just someone to say “Yep, mine were that far out as well” )
Tappet/Shim clearances on old 4 stroke engines are usually, in my experience, around the 4 to 12 thou range dependant on engine type, valve layout, materials, cam drive, etc. I wasn’t totally surprised to see that Desmo’s should have tighter settings of 0.003” openers and 0.001” closers ……… until the heads I was checking gave me gaps of between 0.007” and 0.009” on the openers. I then used the ‘press down on closer and remeasure opener, then deduct orig. opener gap’ method, and got a further 0.010” to 0.023” gaps on the closers…… way too big, even if they were on an old Ford X-flow.
Now these heads came off a running engine…… so, either the 0.002” tolerances most advice gives before needing to change shims is way off, or these heads must’ve been very noisy, and the engine was probably lucky to even start - unfortunately I never got to hear it running, let alone check it for power.
Firstly, are these gaps common, or have I completely screwed up the measurements (I never discount human error) . And secondly, has anyone tried to bridge these kinds of gaps with what would effectively be 8mm diameter inserts and 15mm washers for the opener/closer shims repectively. If the gaps were a couple of thou out then separate shims would be too thin to last any length of time, but on gaps of 20 thou, that’s tempting to make up some oversized (to ensure enough strength) inserts, and then polish the original shims down to required size. If you can see where my brain is leading here; if a supplier where to make up a batch of case hardened washers of 0.010”/ 0.25mm or 0.020” / 0.5mm, then it would just be a matter of picking the most suitable one and polishing down the original closer shims to suit…… think of all the old discarded thin shims that could be reused – there, biking can be environmentally friendly.
So, has anyone else tried it? What grade material did you use? And was the end result a cheap fix or a shortcut to an expensive engine failure…… ?!?