Timing marks

[sidecar willy]

Looking at assembling my Comet 250cc engine..and in particular the cams. There are two lines/timing marks on the cams...why are there two? Which mark/line to I use to mate up the centre pinion?

Thanks

[Alan Cun]

Hello Willy,   Some Doug cams were so marked as to be used for both intake and exhaust situations. If you set your pistons at top dead centre, your shaft half time pinion should have a mark top and bottom. Now you should position the cam lobes pointing slightly forward, that is the intake (top) pointing down and the exhaust pointing up. In this position the top cam should be just opening the intake valve and the bottom cam should have just closed the exhaust valve. The rear cylinder should now be on compression both valves being closed. Try this and the marks should be right. Al

[Doug]

What Alan just outlined should be clear looking at this typical timing diagram that Douglas included in their manuals. This is from a 1926-30 350cc handbook, but it would apply equally well to your Comet.



The factory would have only used one timing mark per gear/cam, but as Alan mentioned, someone may have added an additional mark to and inlet or exhaust cam to use it in the other position. Other than the orientation of the timing mark in relation to the lobe, the inlet and exhaust cams are identical.

Below is a picture of a set of cams from a 1935 350cc Cotswold engine, which would be identical to that used in your Comet. Both wheels are stamped with the letter A, just the one is stamped on the reverse. Note in both cases the timing mark is four teeth over from the lobe.




-Doug

[sidecar willy]

Alan/Doug..thanks for the speedy feedback. Much appreciated.