shimming valve springs - how accurate does it need to be?

[Peroni]

Working on installing a set of Comp 910 springs, first time I've done this job. I'm using a Comp Cams 4929 spring mic. Per the spring mic I'm getting a 1.906 measurement. However when I measure across the spring mic with my calipers I get 1.873. Confirmed this with a drop indicator. Obviously the spring mic is not very accurate nor did I expect it to be given the way it's made. But a .033 difference is pretty darn big!

So which number do I go with? Seems one could end up with a much higher seat pressure than intended very easily.

 

[mandkole]

If your calipers are zeroed and a decent digital set (like Mitutoyos), Id use them. Are you using a compressor to verify the rate at that height?

 

[Peroni]

Just to update this I did find the problem. The graduated barrel of the spring mic was only attached to the thimble with heavy grease and was turning on me. I cleaned it up good then affixed the barrel with some sleeve retainer making sure the calibrate it with a gauge block stack before drying. Now it seems good to +- .002 which is much better.