ET: Engine (Twin) Identifying cams

[jim burgess]

I have an rear cam from a post war twin that has been welded to its wheel and and reprofiled after being built up with stelite. It has a number 3 stamped on the inboard end. Because it has been rebuilt I am not sure if I can rely on the 3 as a true identification, because it could be anything... The front cam has received the same treatment and no identifying number can be found anywhere.
So the first question is, how can I identify a cam?
When it was welded on, the wheel was 10 thou out so that as you spin the cam the wheel wobbles by 10 thou. This concerns me regarding meshing with the large idler.
Am I being over cautious?
What does the team think?
Best wishes and many thanks

 

[Peter Holmes]

I am not an engineer, but everything tells me 10 thou is too great an inaccuracy to use, I am sure it would function if you set it at zero backlash at the tightest meshing position, with the knowledge that you will have 10 thou backlash at the loosest meshing point when cold, I don't think that would be acceptable.

 

[Chris Launders]

Definitely not acceptable.

 

[davidd]

Jim,

The standard for comparing cams requires measuring the four valve events at either 0.040" lift or 0.050" inch lift. Thus, your timing card should look like this:

At 0.050" lift:
IO 26 degrees (Intake Opens)
IC 78.5 degrees (Intake Closes)
EO 48 degrees (Exhaust Opens)
EC 35 degrees (Exhaust Closes)


This cam is a factory Mk2 cam, Equal Lift is at 0.1695". The base circle drops 0.002" just before it opens. This is not a good thing, but it is common. It causes a lot of acceleration as the valve opens.

If you take readings from your dial indicator at the degrees mentioned above and the results are close, the cam profile is a Mk2.

Welding the pinion on the camshaft was done to prevent the cam from shifting in the pinion. The practice has never worked with consistent results. It is difficult to weld the hardened steel pinion and shaft with decent penetration and the weld tends to break, in practice. I use a small roller bearing to pin the two parts together and it has proven very reliable.

Otherwise:

David

 

[nobby]

found these under the bench. one is a mk3, but what would the other one be?
The mk3 has a weight of 454 grams, the "lightened" one weighs 347 grams

 

[vibrac]

I had some discussions on this back in January https://www.vincentownersclub.co.uk/threads/cams-looking-end-on.19960/
I still have not resolved what cams in my collection I should chose bearing in mind I am looking for a set of mild cams (for once!)
I suppose the only way is to measure them in an engine (not an option yet)
But my inclination is that the most symmetrical are the mildest and I should test them first
it will be novel for me an old 4deg BTDC man to actually measure opening and closing points!

 

[davidd]

Nobby,

I would presume the lightened cam is a Mk2, or at a minimum, a larger profile cam. As most owners know, you have to modify the decompressor spindle boss in the timing chest to run a large lift cam like the Mk2. If that work has been done, it is likely a Mk2.

I should note that lightning timing chest gears is always a serious mistake. The engine will produce less power at the higher RPMs. This is because Phil Irving calculated the timing gears, particularly the cam pinion, to have precisely the correct mass to damp vibrations caused by the reciprocating valve action. If the pinions are lightened the valves tend to move erratically as they speed up and sap power from the engine. These results have been proven on the Spintron by top-name sprinters.

Save your money and time and stick to buying a nice cam. The Megacycle Mk2 is a very nice cam. It was originally designed by Andrews and purchased by Megacycle. Megacycle may have modified it some, but it produces about 40 HP per cylinder with 10:1 compression, and is a relatively low lift (it does need the Mk2 clearance in the timing chest). It has proven to be very reliable.

At 0.050" lift:

IO 37
IC 51
EO 51
EC 37

David

 

[nobby]

Nobby,

I would presume the lightened cam is a Mk2, or at a minimum, a larger profile cam. As most owners know, you have to modify the decompressor spindle boss in the timing chest to run a large lift cam like the Mk2. If that work has been done, it is likely a Mk2.

I should note that lightning timing chest gears is always a serious mistake. The engine will produce less power at the higher RPMs. This is because Phil Irving calculated the timing gears, particularly the cam pinion, to have precisely the correct mass to damp vibrations caused by the reciprocating valve action. If the pinions are lightened the valves tend to move erratically as they speed up and sap power from the engine. These results have been proven on the Spintron by top-name sprinters.

Save your money and time and stick to buying a nice cam. The Megacycle Mk2 is a very nice cam. It was originally designed by Andrews and purchased by Megacycle. Megacycle may have modified it some, but it produces about 40 HP per cylinder with 10:1 compression, and is a relatively low lift (it does need the Mk2 clearance in the timing chest). It has proven to be very reliable.

At 0.050" lift:

IO 37
IC 51
EO 51
EC 37

David

As always, a good advice!

 

[Monkeypants]

Nobby,

I would presume the lightened cam is a Mk2, or at a minimum, a larger profile cam. As most owners know, you have to modify the decompressor spindle boss in the timing chest to run a large lift cam like the Mk2. If that work has been done, it is likely a Mk2.

I should note that lightning timing chest gears is always a serious mistake. The engine will produce less power at the higher RPMs. This is because Phil Irving calculated the timing gears, particularly the cam pinion, to have precisely the correct mass to damp vibrations caused by the reciprocating valve action. If the pinions are lightened the valves tend to move erratically as they speed up and sap power from the engine. These results have been proven on the Spintron by top-name sprinters.

Save your money and time and stick to buying a nice cam. The Megacycle Mk2 is a very nice cam. It was originally designed by Andrews and purchased by Megacycle. Megacycle may have modified it some, but it produces about 40 HP per cylinder with 10:1 compression, and is a relatively low lift (it does need the Mk2 clearance in the timing chest). It has proven to be very reliable.

At 0.050" lift:

IO 37
IC 51
EO 51
EC 37

David

Wow, I did not know this and I'm guessing neither did Terry Prince. I have all of his special timing chest parts in the 1360 and they are all lightened. The pushrods are also lighter than stock, so that goes into the Irving equation.
It all seems to work rather well in lightened form, but it's possible that it could work even better if the timing gears were heavier.

Glen

 

[davidd]

It all seems to work rather well in lightened form, but it's possible that it could work even better if the timing gears were heavier.

There is little to no dyno work on this subject, but four strokes suffer inherently from the "bang and glide" problem because there is only one power stroke out of the four strokes. The solution has been to use dampers on clutches, ESAs, and rear wheels. The sprinters were the first to notice the problem with valves bouncing around and failing to seat properly. Stuart Hooper used to pack his flywheels full of tungsten slugs and would often say that he could not get his flywheels heavy enough to hit the speeds he was shooting for with his LSR attempts.

Marty Dickerson used to tell the story of handing a lightened cam pinion to Phil Irving at one of the Vincent rallies. Phil examined it carefully, then threw it on the floor and said "I spent hours calculating exactly the correct weight of the pinion to damp the valve opening and closing and you go and ruin it!"

I don't see anything wrong with lightening the reciprocating parts, but the parts that are rotating and doing the damping should be approached with care. Racers who enjoy lightening things rarely worry about reliability and performance.

David