E: Engine Series ‘C’ Comet Camshafts

[Bill Thomas]

From Paul Richardson book.
Mk3 Cams were introduced early in 1952, In the interest of mechanical quietness.
Normally the number 3 is stamped on the end of the Camshaft.
The Valve timing obtained with these Cams is Identical to the earlier pattern, And they give no loss of performance in any way.
Due to the quietening ramps, however, The exact opening and closing points of the valves cannot be ascertained, and for this reason the present method is to measure the lift at top and bottom of dead centre with a tolerance of plus or minus three degrees.

There is a bit more, But it will only mess with your head !!, All the lads agree the overlap way is the best, With what we know now.
Looks to me, You have a mk 3 Cam , I think it will be OK. Cheers Bill.

 

[aciera]

Dear Bill, I tried your method, it shifts a tooth on the pinion, it approaches the measures you gave me, about 40 degree Injet opening and it improves the closing valve exhaust a little faster.

For measurements every ten degrees, I'll try, but I'm not sure I understand the procedure to follow.

 

[timetraveller]

If you have two dial gauges use them both, one on the inlet and one on the exhaust valve. Ideally this means removing the oil tank. If you only have one dial gauge then do one valve after another but ensure that the degree plate cannot move while you are moving the gauge from one valve to the other. Find top dead centre by putting a stop down the plug hole and GENTLY turn the engine forwards and backwards taking a reading on the degree plate. Add the two values together and divide by two, re-centre the degree plate and then go back and check that you have top dead centre correctly indexed on the degree plate. If not, rotate the degree plate forwards or backwards a degree of two until you have it correct. Now start and take a measurement of the reading on the dial gauge every ten degrees of engine rotation. Before you do this nip up each tappet adjuster so that the dial gauge(s) move five to ten thou, lock up the tappet adjusters and re-zero the gauges. Write down on a piece of paper the engine rotation position and the dial gauge reading of the valve lift. Do this for several degrees before and after the cam lift has started and ended and at some stage watch the dial gauge while you rotate the engine so that the cam follower is tracing round the base circle. There should be no deflection larger than about one thou.
When you have finished this you will have a table with three columns. The first is the engine rotation position and the second is the lift for one of the valves. The third column is the lift of the other valve, inlet or exhaust. You can now type these figures into your spread sheet or send them to me and I will do it for you. It has taken longer to type this out than it takes to do it. If you do not wish to remove the oil tank then you can make these measurement from the top of the tappet adjuster but my advice is to remove the oil tank to do the job and while it is off the bike give it a really good clean inside. Unless it has been cleaned recently it might well have nearly seventy years of dirt and metallic particle in it which will need to be removed. You can make an inspection cap to fit into the top of the oil tank which makes the cleaning much easier but that is another project.

 

[Bill Thomas]

I would put it together as you have done it in letter 22.
But if you wanted to play, There are 5 keyways in the half time pinion, On the crankshaft, Which you can move to another slot to adjust the Cam timing by a small part of a tooth.
Sorry if you already knew that.
Another thing, Make sure you have a lot of end float on the Breather pinion, The alloy centre expands a lot and can give trouble. Cheers Bill.

 

[davidd]

The Mk1 or Mk3 are perfectly easy to measure. You just can't use the mostly useless Vincent method. The Megacycle Mk1 times at 0.040"valve lift as:

IO 30
IC 44
EO 44
EC 30

I don't know the valve height at which Patrick measured to get his figures.

Below is a graph of a Megacycle Mk1 with measurements taken every 10 degrees.

Vincent found on the dyno that the engine tended to produce more power when equal lift was moved to 4 degrees. It may have little relevance today as most engines and the fuels they use vary quite a bit from what was used in the late 1940's. However, all the literature and diagrams show the four degrees.

David

 

[stu spalding]

Bear in mind that when fitting a Mk 2 cam, as well as cutting the lower valve guides, it is sometimes necessary to relieve the timing chest wall behind the rear inlet cam follower. This was required on my late "C" cases. Cheers, Stu.

 

[davidd]

Bear in mind that when fitting a Mk 2 cam, as well as cutting the lower valve guides, it is sometimes necessary to relieve the timing chest wall behind the rear inlet cam follower. This was required on my late "C" cases. Cheers, Stu.

Yes. On the Comet, this is the area that needs attention because the inlet lobe of the cam will hit the boss that has been carved away in the photo. The boss supports the decompressor stud.

David

 

[Oldhaven]

Thanks for that advice about the cam lobe interference and the explanatory picture, as that is something I have not checked yet with my TP cam while setting up the timing gear, though I have not noticed any clicking or stoppage while doing the many rotations fo the gears I have done so far. Things do get very tight in there, I know. The picture also shows the valve lifter tunnel blocked off, something I will have to do if I don't put a breather of some sort there.

Ron

 

[TouringGodet]

I updated the thread title, to Camshafts.

 

[davidd]

Ron,

A 3/8" bolt for the decompressor hole.

David