Tech: What About the Valve Springs?

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Valve spring Tech

I bought a cam for a 1960s race engine that I am building for a motorcycle project. This made it necessary to find some valve springs that would be within the specifications required by this NOS vintage race cam. The problem is that the spring kits recommended for this cam have not been available for many years. It was possible to find others that should work by dimensions, but I wanted to be sure that they were correct relative to the original specs. So I purchased three brands of valve springs from other companies that still make them available today (photo 1), but none of the companies could give me any data as to what the spring pressure would be at my valve stack height or the compressed spring stack height.

Valve springs need enough pressure to keep the valves from floating at high RPM so that there will be no loss of power from valve float or, worse yet, the floating valve hitting the piston and causing engine failure. This engine will rev up to 11,000 RPM and needs to pump out some power, and more importantly, be reliable. If the valve spring has excessive pressure, it will cause premature wear of the cam and rockers or cause undue pressure on the cam bearings, resulting in permanent damage. Over the years, I have seen springs with so much excessive pressure that it caused a valve to fail, which did extensive damage to the engine. This is why I needed to be sure that the springs I chose to use would be correct for this cam application.

The first thing you need to check with new springs or changing the spring stack height is that the valve spring will not bind or bottom out. Most cams or engines will spec an installed valve stack height. Be sure when checking the valve spring heights that the spring caps are in place. This engine had a recommended valve spring stack height of 1.00”. In photo 2 you can see I placed the valve springs in a drill press vise at the 1.00” suggested installed height. The total valve lift from the new cam is actually .250”, meaning that the compressed height would be .750” (1.00” minus .250”). I checked the spring at .750” height and all is well with no binding (photo 3). I then checked to see when the spring would actually start to bind and that was at .676” (photo 4). I felt good about these springs so that no binding would happen, so it was time to see which springs would best match what is recommended.

I live out in the country and I checked with some engine machine shops to see if they could check the spring pressure for me, and everyone that could didn’t have the time available for a few days. I decided why not check the springs myself? I used my Bridgeport mill to do this, but this could easily be done with a drill press or an arbor press. First, placing the electronic scale on the mill table and next placing an aluminum block on top of the scale to help ease in checking the valve spring height. I set the scale to 0 lbs. after the block was placed on it so the block weight would not alter the actual spring pressure rates.

I used a 1.00” thick piece of aluminum as a gauge for the installed spec valve spring height (photo 5) to check the installed spring pressure. The cam called for a spec that was for the installed height pressure to be between 18.0-21.0 lbs. When I tried the three sets of springs, the first set measured at 19.6 lbs. (photo 6) which was perfect. But would these springs be within spec at the compressed length? Compressing the valve spring stack to .750” (photo 7) and then checking the spring pressure, it measured at 76.0 lbs. (photo 8). The specs from the cam company were that the springs needed to be 68.0-81.0lbs. This placed this set of valve springs between the numbers the cam needed and I was pleased to get springs that should work out great and not cause any engine loss of power or damage.

Since I had three sets of springs, I measured the other two and found that both were out of specification. Thankfully, the first set was right on.

Now that you know how to check valve springs, when you are doing a valve job, you can now see if any springs are starting to get weak before they could cause engine damage. If you are installing a new cam and springs, you can be assured the springs are correct for your application. It’s always a good idea to check for yourself, even if the springs are supposed to be the correct ones.

By Steve “Brewdude” Garn

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