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Measuring clearance with variable timing?

Engine Building Fundamentals

Discussion and questions related to the course Engine Building Fundamentals


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When doing piston to valve clearance checks with clay, how would you deal with an engine with hydraulically actuated timing swing?

What you need to do is check the clearance with the intake cam in the most advanced position. If the engine has variable exhaust cam timing then you need to check this in the most retarded location. This is a bit fiddly as you need to physically keep the intake cam advanced as you turn the engine over.

Ok so on a K20 you can whip the VTC wheel cover off and wedge the paddles inside

I use a ring spanner or crescent on the cam itself to advance the cam as I'm rotating the crankshaft through a full engine cycle. You could choose to phycically lock the cam wheel in position but this is a little more time consuming.

On VTC I usually just jump the timig belt or chain by the number of theet which correspond best to the maximum advance respective retard.

What can be difficult to find is if there is enought valve to valve clearance during overlap. Has anyone a good sugestion how this can be tested?

Besides, on mechanical adjustable cams, I find it much too time consuming to check with clay and remove the head several times, just to find the max allowed adjustment range. It's important to check before cam timing changes are made on the dyno.

I was thinking about to write a Matlab script or a excel sheet which calculates intereference betweem piston and valve.

Input data whould be stroke,bore, valve recess pockets diameter and deep, valve diameter and valve angel and most important the valve lift curve.

Has anybody done something?

Jumping teeth would be effective to a point provided this matches the actual swing of the cam control movement. This may or may not be the case. Of course if you know the cam timing is close and you've got ample valve to piston clearance then there's little to worry about.

A technique I've used to confirm valve to valve clearance is to dummy assemble the engine with just one piston and rod. You can then use a bore scope to view the valve to valve clearance during the overlap period from the underside of the block (Spin the block upside down on your engine stand). This will let you know if you're getting marginal and further investigation is needed or if you have ample clearance.