Valve Springs

November 17, 2009 by mike.cox

Valve springs and installed height.
All those with two-strokes skip one page, but don’t smirk, check your reeds and clean your exhausts out with a small tissue.

We all need to replace valve springs on an ongoing basis if we want best performance.
Failing to replace springs when it becomes necessary can allow valve bounce to occur before maximum revs are reached, and in extreme conditions, big damage.
 
So when should you replace valve springs?
All workshop manuals give measurements of new springs, easy to do a comparison whilst the head/s is/are off and everything is stripped. Keeping a record of kilometres travelled to measurements taken from new can give you a general replacement guide.
There is a piece of equipment that gives poundage readings when the spring is compressed. Some members have one and it’s interesting to see what the difference is between a new spring and an old one.
 
Saying all of the above, the valve springs are made to exceed the onset of valve bounce at maximum revs when new and even when they are 1,000’s of Km’s into hard use, they still would minimise valve bounce for normal road use.
 
Take the latest Jap bikes for instance. Many rev to 14,000 and above and springs close the valves.
These valves are small though, stems are thin and the overall mass is far less than a 1960’s machines valves.
 
For those who are not quite sure what valve bounce is, it’s fairly simple. The valve on all but the desmo bikes need a spring to close the valve. If the spring is too weak, the valve train will not follow the heel of the cam exactly as engine speed gets close to maximum.
(A valve train can consist of; valve, spring/s, upper spring retainer, collets, rocker arm and all its bits, pushrod and cam follower). Overhead camshaft bikes don’t have pushrods.
 
In fact being very weak, the valve can be pushed off the top of the cam as well.
In this case we have a valve that should be following the shape of the cam both on the rising and falling profiles but because it’s weak cannot follow the cam contour on the closing heel and floats behind the profile at high R.M.P.
If the spring was strong enough the cam follower would press against the cam and the valve would be in the correct position / distance to the valve seat as the piston rises, but it’s not and in extreme cases the valve being that much further open than it should be, the edge claps the piston or the other valve. If it touches the piston, the collets could jump out and the valve drops into the bore. Less dramatic, the valve gets bent and cannot seat properly.
Result. Bent valve, broken guide and in the worst case, the bent valve jams between the combustion chamber top and the rising piston. Ouch!!
 
Replacing valve springs with higher performance ones, or from another manufacturer if the engine has been tuned requires additional checking to make sure clearances are within spec.
 
Installed spring height.
This is the distance between the lower spring cup and the underside of the spring retainer.
Fit the valve; install the inner spring only or a very weak spring. Fit the collars and all the other bits required. Now measure the distance from the lower bottom cup and the underneath of the spring retainer where the outer spring would go. This can be done with the head on the bench.
If the clearance is too much, fit shims under the lower cup to get the correct clearance.
Once this has been done and shims fitted, all parts relating to this valve must stay together.
Simple hey?
 
If you don’t check spring height, then the pressure on the valve as it becomes close to the seat, could be far too weak and could cause other problems. The least maybe gas leakage passed the seat and the valve. And on an exhaust that could mean a burnt valve.
 
If the installed spring gap is far far too much, maybe the seat has been cut back many times and a new seat and or valve is required?
 
If there’s insufficient spring height then you have a much bigger problem. Either the valve seat is too proud in the combustion chamber, or the valve is too short (used a modified valve from another make). The possibility of coil binding can occur, which will bend pushrods.
To rectify this requires machining of the seat and or replacing the valve. Do not remove the lower spring cup or machine the surface under the lower spring cup unless you absolutely know what you are doing.
 
Valve springs that are too strong.
These springs will wear out the camshaft lobes, bearing faces and associated metal rubbing surfaces in very short order. There is absolutely nothing to be gained and everything to lose by fitting a spring that is too strong.
Many bikes have double valve springs. Having an extra inner spring is not there primarily to assist the stronger one, its there to minimise and damp out resonance / harmonics that may cause catastrophic failure of the primary spring.
Much like an opera singer breaking a glass through resonant harmonics generated through sound waves.
 
Finally there comes the check of clearances when the valve is fully compressed.
You can only do this when the head is back on the barrel/s and fully operational.
When the valve spring is fully compressed, (the valve is fully open) the clearance between the collets and the guide must be 30thou at least. If not machine the guide down until it is. Also this is the time to check for coil binding.
 
Hopefully the valve spring is not coil bound. But this cannot be if installed spring height is correct and you haven’t fitted higher lift cams, different rocker ratios etc.
 
Finally a final check to make sure the rocker arm/s doesn’t touch any part of the upper spring retainer/s in the relaxed position.