Hi all.
In most cases, the 'S' cams work timed to the 'dots', but the two I know of that needed to be retarded on the inlet cam by 1 tooth are the machine owned by Bill Harris and the machine I have here.
That said, I have been aware of the close proximity of the valves to the piston crown on TDC overlap on these machines since I first got involved with them and always stress that when fitting these cams, the engine should be turned over very gently by hand with the spark plug out, to check for interference between valves and piston.
My favourite way is to fit the exhaust cam alone to start with and test with the usual 'nil' valve clearance first. If all is well like this, I then unwind the adjuster [to lengthen the pushrod] by 6 flats of the adjuster, or 1 full turn. I repeat the test and if there is no sign of valve / piston interference, it is good to go, although 8 flats and clearance is a bonus, the 6 flats will do. I have not heard of an exhaust cam giving any clearance issues while timed 'on the dots' to date, but if the case were to arise, advancing its' timing would be called for.
Next, the inlet cam goes in 'on the dots' and again, the engine is turned over very carefully by hand with the valve clearance at 'nil' - a spanner on the worm nut hexagon is good for this. Either, the engine will make two full revolutions, or it may come to a stop as it just comes up on TDC overlap. If all is well, I lengthen the pushrod adjuster by 4 flats and if there is still no obstruction, it is good to go, although 6 flats clearing is a bonus. If I encounter an obstruction, either set at 'nil' clearance, or overadjusted, then I retard the inlet cam by 1 tooth, as I have done in the machine currently here, which has a 64 mm diameter, 2 mm deep 'raised' area in the centre of the piston crown [see below], but the valve faces left standard. On the machine I tuned a few years ago, I bevelled the faces of the valve heads and reduced their diameters very slightly, to give leeway for all the different cams and timings that I tried, so I had a little extra leeway with this machine.
In view of how well this machine is working [with 9.2:1 compression] with the inlet cam retarded by 1 tooth, perhaps I should recommend this as the default timing and 'on the dots' as the alternative that may not always be workable.
I will do some digging later, as I wrote up all the cams and cam timings tried in the AVL a few years back and the 'S' cams' data will be in with them. It has also been mentioned that the depth of the cuts on the valve seat faces can vary from the factory, so, some valves may be closer to a piston than others.
B.W.