Camshaft Timing

[Andy 105 1 Cars]

A general query that I am sure someone will answer. The query centres on cam timing on the 16v engine with hydraulic lifters. Posted elsewhere is an excellent description of timing both cams using lift at tdc. I understand the process, even though I have always preferred to time at full lift, using the same 'before' and 'after' full lift process as one does to find true tdc on the crank, via the flywheel.

What I am struggling with is the notion of measuring cam lift at tdc as this measurement must depend on the cam to follower clearance. With solid lifters, no problem. Set the valve clearance, then time the cam for a specific lift at tdc and job done. However, with hydraulic lifters, especially on a newly built engine when the lifters will not have taken up the cam to follower clearance, measuring the required lift at tdc could make the timing very late , as the cam may well be quite a way up the ramp before the valve begins to move, because of the afore- mentioned clearance.

So, what am I missing ? Any advice gratefully received.

[petert 586]

You measure the top of the lifter not the valve. Consider solid lifters first. If the lift @ TDC for a given lobe centre line was 0.100" and the valve clearance was 0.010", then the lift @ TDC measurement should be 0.090" to set up the cam correctly.

 

With a hydraulic lifter, the preload of the lifter spring will ensure that the bucket remains in contact with the cam lobe at all times, even in a freshly built engine. Thus you can ignore any valve lash etc. and set the given TDC figure.

 

Any cam grinder worth their salt can tell you what lift @ TDC value corresponds to an LCL, allowing you to install a cam using either method.

[SweetBadger 94]

With hydraulic lifters you should have zero clearance between the cam lobe and the lifter face, so you measure how far the top of the lifter moves.

 

Even if the lifters haven't filled with oil, the spring in the lifter should keep it in contact with the cam lobe at all times.

 

Edit - beaten to it by petert!

Edited November 15, 2016 by SweetBadger

[Andy 105 1 Cars]

Thank you both. The bit I had not appreciated is that the lifter would be in contact with the cam lobe even when not oil filled. I had assumed that there would be significant clearance.

Cheers

[Andy 105 1 Cars]

One further thought which I now realise prompted my question in the first place. Both Peter and yourself assume that the dial gauge is reading from the lifter, in which case your joint explanations are fine. I was working from the valve , hence the query, which still applies . However, as I am timing on full lift, valve to follower clearance doesn't matter .