Conrod nuts. XZN? 12 spline flange? Double Hex?

Am stripping down a couple of V12s. Have got to the conrod nuts. Need a suitable key but not sure what system it is.

Anyone know?


what socket do I need for these nuts?

Can’t remember the size but it’s just a normal 12 point socket.

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Thanks baxtor

I did a bit of Googling and yes it appears you just use a 1/2"AF 12-point socket.

Seems a bit strange that there isn’t a dedicated key for a special type of fastener but then again it’s not often there is a simple answer to a problem :slightly_smiling_face:

Twelve point are fairly common. At least some of the old York A/C compressors used them on the cylinder head. They predate the fancy types that require special wrenches, like Torx. BTW, just as there are both external and internal versions of Torx, there is an internal version of 12 point fasteners. They’re called “triple square” and used mostly on German cars.

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Military aircraft engines use 12-point nuts and bolts exclusively. Mostly tiny stuff, like 1/4". Finding 12-point 1/4" sockets can be a little tricky, and the mechs that work on those engines every day will wear out a socket in a month or two.

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You can get a twelve point 1/2 AF socket from Snap on. They’ve just replaced mine FOC! I’ve only had it 43 years! It was bloody expensive in 1975!

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Funny you should mention that Nigelplug cos I saw on an MG forum last night that the tool needed is Snap-On F161. I made a mental note of that but was intending just to use any 12-point 1/2"AF socket I might find.

Was raking thru my sockets tonight and turns out I have that exact socket. Don’t think I’ve ever used it before lol.

It worked fine tonight. I have the first 6 pistons out

That engine was well-taken care of, oil-wise: very clean crankcase.

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Hi Wiggles.

Yes. It will be interesting to see if my original engine is as clean inside.

This engine seems to have suffered some external trauma before I got it tho…


looks like this engine suffered heavy impact to sump


wonder why the casting is damaged on both sides

When synthetic oil is used, you expect to find the engine this clean.

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hard collision, engine breaks the mounts and moves forward?? guessing.

That actually makes real sense ronbros

I assumed it was an impatient mechanic separating the box from the engine or perhaps someone trying to take the engine out the car but leave the box there