raylish said:
I assume then that as the valve seat recession reduces the gap, you simply reduce the thickness of the shims to reopen the gap?
Got it in one.
Adjustment by shims makes adjusting clearances a very difficult task from a maintenance perspective, but for an OHC engine where the cam is acting directly onto the valve (the Rover setup), it's probably the most effective way of doing things.
In a SOHC engine with a hemi-head (necessitating inclined valves), rocker gear is needed which could have the screw up/down adjusters you get on push-rod engines incorporated into them, thus making adjustment far easier.
The only similar solution for a direct acting cam is the off-the-wall Alfa Romeo approach: instead of having one big fat cam pushing on the whole top surface of the bucket, they had two very thin cams pushing on the extreme left and extreme right of the top surface of the bucket. In the gap in between, you slotted in an Allen key and screwed an adjuster that was contained within the bucket. A clever way of doing things, but it's much harder to get the buckets to rotate consistently, and if the cams wear at different rates you're buggered!
I suppose they could have incorporated the adjuster part of the Alfa type into the Rover type setup, but given that you would still have to go through the 'inconvenience' of removing the camshaft for access, the 'convenience' of having a screw-up adjuster is negated. This coupled to the fact that threads tend to rotate, and shims don't squash so easily, means that the maintenance difficulties are offset against accuracy.
And if you had new hardened valve seats fitted, the maintenance aspect would be deleted altogether.
Probably not a very useful musing, but it kept me entertained on my commute home!
Michael,