Spring pressure

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comnoz

comnoz

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Here is what happens to the cam when the valve spring pressure is too low over the nose of the cam. This was caused when my exhaust valves eroded the seats and the valve spring installed height increased too much.

Note the damage is on the closing side of the cam lobes and just on both exhaust lobes.
It's from the hammering that happens when the follower can not follow the lobe.

Spring pressure
Spring pressure
Spring pressure
 
Jim,
Is that galling caused by loss of oil film or is it hammered from tappet bounce ?
You had mentioned in another thread about older norris cams being soft, can these cams be rehardened if their still in good shape?
 
What was the calculated spring pressure due to the change in height that caused this?
I'm running 59lbs calculated and have done for 16K miles. When I changed the leaking base gasket at 11K all was still very good. The tappet gaps have not changed either.
 
RatedSpeed said:
Jim,
Is that galling caused by loss of oil film or is it hammered from tappet bounce ?
You had mentioned in another thread about older norris cams being soft, can these cams be rehardened if their still in good shape?
Click to expand...

The metal is flaking due to valve train separation. The parts -pushrod, follower, rocker arm and valve all separate from one another by a few thousandths as the valve goes over the peak lift and then come back together when the spring regains control -with a crash. Only a few thousandths but with a lot of pressure and momentum. Hammer on a piece of hard metal enough times and it begins to flake. The followers still looked perfect -so no oil film problems.

Spring pressure


The only way to make the cam harder is by sending it to a shop to have it welded up and reground.
But, if it has more than 10,000 miles on it and it still checks good, it is not likely to be soft.
 
Madnorton said:
What was the calculated spring pressure due to the change in height that caused this?
I'm running 59lbs calculated and have done for 16K miles. When I changed the leaking base gasket at 11K all was still very good. The tappet gaps have not changed either.
Click to expand...

They were down to about 70lbs on the seat. I would guess my use and operating conditions are a bit tougher than yours.
 
Is there a target seat poundage on a stock cam, stock FA head with Kibblewhite springs ?
 
Time Warp said:
Is there a target seat poundage on a stock cam, stock FA head with Kibblewhite springs ?
Click to expand...

With new springs I shoot for 100 to 110 lbs on the seat -with a stock cam.

Quality springs will loose about 10 lbs in the first few hundred miles and then level out.

I discard them when they get down to 80.

Less than 80 usually starts showing damage if you travel at interstate speeds.
 
comnoz said:
With new springs I shoot for 100 to 110 lbs on the seat -with a stock cam.

Quality springs will loose about 10 lbs in the first few hundred miles and then level out.

I discard them when they get down to 80.

Less than 80 usually starts showing damage if you travel at interstate speeds.
Click to expand...
Hi Jim,
How do you measure seat pressure? What tool/set up do you use?

Thanks,
Ed
 
It's pretty much something I built just for Norton heads.

It is a bench mounted arbor press that is converted to a valve spring compressor.

It has a load cell and travel indicator mounted to the arbor with a digital readout so I can see the spring pressure on every head I assemble.

Spring pressure
 
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comnoz said:
They were down to about 70lbs on the seat. I would guess my use and operating conditions are a bit tougher than yours.
Click to expand...
I'm using 150mm radiused followers with a cam that has 10.5mm on lift on the lobe. Also using a FA head. Most days I have to filter in traffic for 3 or 4 miles so not ideal, though temperature here is mainly in the 20's and below at see level.
It will be stripped down soon to fit one piece tappets and test some other new parts. I'll take some photos.
 
Sorry for the bad luck Jim, but thanks for posting the thread. Looking forward to the rebuild progress and pics. Cj
 
cjandme said:
Sorry for the bad luck Jim, but thanks for posting the thread. Looking forward to the rebuild progress and pics. Cj
Click to expand...

Well, the rebuild process is not much. It is a quick change cam so it has already been replaced.

The damaged cam was a full race grind and I was not happy with it for my use -but it did have a good top end rush.

So now I am back to a standard Commando grind.

I plan to have the top end back on the motor today. Jim
 
comnoz said:
Well, the rebuild process is not much. It is a quick change cam....

I plan to have the top end back on the motor today. Jim
Click to expand...

To this laymen wrench turner, that’s funny right there.....
 
Some cams are ground with a closing rate which is gentler than the opening rate, to provide a softer landing for the valve train. I think Commando cams are usually symmetrical.
 
Jim, I know this is old ground to cover, but for those of us with stock motors, what kind of spring pressures should we begin to be worried about? I spend most of my time in the mid-range but hate to think that I'm hammering my cam when I decide to open it up.

Russ
 
The spring pressure you need is relative to the cam you are using. If you build a top end motor, it becomes expensive. Hot cams usually move the power band further up the rev range, so then the bottom end of the motor becomes less reliable, and you need the billet crank and thicker cases. It would be easy to build a Commando engine with a power band from 5 to 8, with heaps of power. - All you need to do is fit cams with much longer duration and radical timings, but with much slower lift and closing rates. It would probably last only a few minutes. Every time you improve the performance of your motor, there are usually trade-offs. Nobody in their right mind would use megaphones on a Commando with a hot cam.
For racing, a Commando engine is very good as long as everything you do to it, is about improving torque rather than moving the power band upwards. Theoretically, a radical cam gives more power, but what works in theory is often useless in practice.
 
rvich said:
Jim, I know this is old ground to cover, but for those of us with stock motors, what kind of spring pressures should we begin to be worried about? I spend most of my time in the mid-range but hate to think that I'm hammering my cam when I decide to open it up.

Russ
Click to expand...

For a stock cam see post #7
 
gortnipper said:
By that do you mean a stock cam, or the 312a you used to run?
Click to expand...

I installed a NOS Norton cam that I had tucked away. It was actually a cam for a magneto engine but since any cam is going to need some machine work to fit in my motor it worked out well. It was known as an SS grind which is the same as the stock Commando cam.
If I don't like it I can always go for a 312a but I suspect it will work just fine.

Once I get the new gearbox installed I still plan on switching the cam for a twingle cam. Just for grins.
 
My 500cc short stroke Triumph engine had all the good stuff done to it, however it only had a 4 speed CR gearbox. It's power band was from about 6000 RPM to 9,500 RPM. You could choose where you wanted to lose a race. Gear it low and you could out-ride everybody on the tight stuff, but get done-over on the straights. Gear it high and you were not fast enough on the tight parts of the circuit. With 6 speeds close ratio, it might have been very good. The Seeley 850 is a world apart. 'Torque wins races' ?
 
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