Is it possible to use a dial indicator to check the valve to piston clearance on a Desmo head? This is wrt an old bevel but suspect the question is relevant to all Desmo valve trains.
lovely subject - in theory, yes it is but you would need to carefully study your camshaft timing/overlap and full lift specifications to get an idea. To confirm, you are concerned about TDC at end of exhaust stroke? Although laborious, we always resorted to a dummy build with plasticine or similar but I expect there is an equivalent (thick) plastigauge substance available now.
Hmmm... so with the inconsistency of bevel cams that would mean having an accurate measurement(s) of where the valve is in 'space' for particular degree rotations. And this would also require replicating the exact position of the measurement equipment after removing the closing shim ?? And then taking it apart after you let the valve drop into the bore... ahem... I'll be doing this check after a rebore & new (standard Omega 86.5) pistons so, yes, this is definitely the area that needs checking, say +/- 20 degrees around TDC. But checking the complete cycle would probably be a wise choice. I think this would be the easiest way to go. With quoted clearances of 2 & 2.5 mm it appears we are not looking for race engine tolerances.
I can't picture this without looking at assembled head, but you might be able to set up to follow valve movement without dismantling possibly?
Chris, from what I understand, and as per your picture, with 'normal' engines the valve spring is replaced with a much lighter spring. So at any point in the cycle the dial gauge can be zeroed to the top of the valve and then the valve can be manually pressed down until it touches the piston crown with the distance moved measured by the gauge. But with a desmo the valve is 'locked' in position by the closing shim/rocker/cam lobe that prevents the valve from being pushed down. So as I see it for various degrees of crank rotation you'd have to be able to measure and make a note of where the valve is in 'space' i.e. how far it had moved down, remove the closing shim & then, with the crank at the same degree rotation points & the valve now free to rest on the piston crown, obtain a measurement. But this would also require the gauge to either remain in or exactly replicate it's position - see photo of a bevel valve opening to see how tough this could be. Even with the engine out of the frame.
^ of course you are right, it's been such a long time and I am more car-orientated than bike, and particularly desmo - totally skipped my mind for a moment*/will think again, but sounds as though you have got it covered re:- correct procedure anyway. * that picture you included alone would have jogged my memory!
Thinking about it, if you do want a second confirmation besides a dummy build, then I probably would strip shims off the valve and allow to drop on TDC (as you originally outlined) as it still won't drop so far that you won't be able to retrieve.
Thanks Chris, but besides the practicalities of doing it myself it is still an interesting question I wonder how race teams etc deal with this when installing higher comp piston and/or new cams ? Probably, as you first mentioned, they would have accurate profile/lift data and some how do it via computer modelling or some such. I have just bought some play doh...
