Idler Pulley Bolt Help

ok, this has been beat to hell, but I'm stumped. I'm cross posting from my other thread about water pump replacement.

Regarding the idler pulley trilobular 10.9 bolt:

I just bought a reconditioned pump from Mercedes Benz. There is a smooth bore where the idler pulley screws into. The idler pulley I pulled off the old engine is NOT a trilobular bolt. It's just a straight thread. I can't seem to find the part number for that bolt. Mercedes Benz of Georgetown couldn't find it.

Can someone provide the part number for that bolt?
Either that or I go after market water pump from Autozone that has a threaded hole
I try tapping that bolt.

For reference, pics of existing bolt attached.

Any help is appreciated. Sorry if this is listed somewhere. I have searched like crazy and can't find it.


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2004 140” HRC 2500 (Crewed)
If you do decide to tap the hole, I suggest mounting the pump and tap on a drill press to keep it square, then turn the chuck by hand. It would be a shame to tap a crooked hole on a new pump...

Who ever moved this, thank you!

I ended up going with a non- Mercedes Benz water pump. Here is my reasoning.
I'M A GIANT WEENY. Aqua Puttana/Vic said it looked right, but I didn't want to try something new when I was leaving in a couple days.
Also, it didn't have the triangular look to it that was mentioned. I didn't know how many times it had "cut" threads before.
The non-MB pump was already threaded and the bolt fit the threads. I used blue locktite.

As hkpierce said, I'm ordering a new bolt from Europarts-SD just so I can see a new one and hopefully learn a little more.

But in the end, I just didn't want to try something new and have my ignorant a$$ mess up my trip- 3 friends and motorcycles out in Terlingua.



Well-known member
Well if you do use aftermarket , just make sure it has a steel gasket not paper or cardboard one
If yours is a 2002 /2003 do make sure the fuel pipe retentions clips are installed or replaced. Read important.
Definitely used a metal gasket.

Mine is a 2005, so not sure if your comment about the "fuel pipe retention clips" is applicable. For a newbie, what are these and where are they located? When I replaced the HPFP (high pressure fuel pump) I used new hose clamps on the high pressure fuel line and the fuel return line.

What do fuel pipes have to do with the water pump R&R?


Well-known member
On a 2002 /2003 the HPOP fuel pipes run very close to water pump pulley.
New retention clips are essential being attached to the water pump.
In your case no worries.

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