if yours is like mine, the angle that the feeler gauge is at makes the measurement a little tougher than it should be. i will say this... one size bigger on the gauges should not be able to be pushed through the entire gap, but will kinda stick in there a little. i know it sounds like a lot of crap to do all that gauge switching, but how often do you plan on doing this crap?

so when you think you've got it, whip out the next gauge size up and try to get it through. you should be able to get it started, but NOT able to get it all the way through. hope this helps. worked for me.