LOL...That bolt came right out for me. Now I am battling the three hex bolts. I've been wrestling with the top rear one and it is SOME kind of tight! Haven't even touched the other two yet. Using 1/4 U-joint like here: water pump tools

I'm tempted to order up that snap-on tool about right now!

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2012 E350
2006 Callaway SC560

I just replaced mine 2 nights ago, and it wasn't that bad. Took me about 5 hours and I was taking my time. Everything on this car comes apart great! Except for the actual pump. The 2 rear bolts weren't the problem for me, neither was the top allen bolt. The tough one for me was the lower front bolt, obscured by the tensioner. I just removed the bolts from the tensioner but did not remove it completely - it was just enough to move it aside and get to that bolt. I didn't use the Snap-On FLX13, just a 13 mm socket and a combination of different length extensions, a wobble, and a U-joint. To hold bolts in the socket I used a piece of paper towel. I also removed the oil dipstick tube in order to reach the upper rear bolt with my hand and secure the socket / support the u-joint. I strongly recommend using a 6-point 13mm socket - with some of the weird angles, the socket isn't always perfectly seated and the 12 point socket might round off the bolt head. Then you'll have real problems getting it out!

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

That is what happened to me (1992 300TE, waterpump changed before by dealership in 2001). And I stripped the one on the lower right side... Fortunately it came out with a reverse drill. The FLXM13 still came handy when I installed the new 13mm hex bolts.
Regards, Bruno

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1992 300TE 160 kmiles