ET: Engine (Twin) Cam Identification

[LoneStar]

I've acquired a Rapide basket case, and my first task is to take inventory. I'd guess there's about 70% of a complete bike. Lots of parts are missing, but in other cases there are extras; in particular there are 3 front cams and 5 rear ones. Questions -

- Two of them have '1' stamped in the notch at the end - does this indicate Mark I grind?

- The rest have no stamping. Is there a relatively simple way, short of installing and mapping the lift curves, to distinguish between the Mark I and Mark II grinds? Maximum lift specification, perhaps?

Thanks!

 

[chankly bore]

Mark 1 and Mark 3 cams are roughly symmetrical about the point of maximum lift when viewed end on, whereas Mark 2 cams have a hump on one flank. They also have a greater lift which is why you need to shorten the lower valve guides before fitting them. Figures for lift at T.D.C. for Mark 3 cams are given in "Richardson", page 117, first edition. After saying all that, your profiles could be anything after 70 years, so choose the ones that suit your riding best, check your clearances, including the exhaust lifters and lower guides, time them to equal lift at 4° B.T.D.C. and "suck it and see", as my apple-cheeked old Welsh mother used to say! Input from real engineers welcome.

 

[greg brillus]

First off, check the condition of the lobes........If they are badly pitted or worn, then don't go any further with them. Once you are able to match them as a set, check the diameter of the base circle to see if they are very close to one another, then check the lift height of the lobe and again compare. If the cams are obviously second hand, check the diameter where it presses into the cam pinion gear........These need to have an interference fit, if they come loose they will spin in the gear itself. These pre-checks will tell you if any of what you have is usable or not........I had a set of cams the other day where the base circles were out by about 2 mm from one to the other........this caused the valve lifter set up to work on one cylinder but not the other. A standard Mk 1 or 3 cam will have about 0.290" lift at the lobe, and a Mk 2 will be around 40 to 50 thou more........These figures are actually more than that at the valve itself........around 0.330 and 0.380 at the inlet valve.

 

[timetraveller]

Are you sure about that Greg? The fact that the cam nose slides along a lever follower in such a way that the cup at the end of the follwer moves by a variable ratio compared with the cam eccentricity always makes me think that there was more valve lift than cam eccentricity.

 

[greg brillus]

These are approximate figures from the myriad of cams i have checked over time, the lift at the valve is always more than at the lobe itself. It is just a rough guide for the owner to see if his cams are of any use.

 

[LoneStar]

Greg,

Thanks, very useful info!

On the base circle diameters, I'm not clear on the importance of a close match between the two cams. Can't a modest difference be compensated for, either with pushrod length or lifter adjusters?

 

[timetraveller]

Oh Dear!

 

[roy the mechanic]

Judging by the pictures, I would class them all as "old tat". Unless you are intending to build an auction special I would reccomend buying some new ones from a reliable source. Then the spec sheet will tell you what is what. Ok I am pickey, but Iike to be able to put my name on my work. Roy.