Balanced Wrist Pin

[Steve G]

I wonder if someone can shed some light here?

Rebuilding a 3.1 scl and I'm about to mount the pistons and cylinders.
The wrist pin has an extra portion at the end of it that looks strange when installed.
It's a very long time since it was stripped and its been messed with before so I don't know how reliable my memory is!
The pistons were manufactured by mahle and mounted on 97mm turbo barrels. They are high compression compared to the turbo. Is the extra portion a balance weight for the dome of the piston? Should it be fitted on the opposite side of the dome?
Obviously no manuals or drawings for this one, sometimes it's a bit of a guessing game!!
Looking at the ware pattern it would suggest they should be fitted that way
Photos below...

[Steve G]

[Steve G]

[Steve G]

Looks like it should be this way, just seems a bit strange......

[PeteKz]

Yeah, I haven't seen that before. It seems that the keeper circlip would not work well with the tapered end of the pin. One of the old guys here might know.

[porschedude996]

Very interesting. Leave it to the Germans. Looks to me that the extra material should be opposite of the dome. I bet that if the circlip was installed on the extra end before the pin goes in, the pin shoulder hits right where should if it were a standard pin. Which means the pin has two circlips. I would not think the circlip could be installed or removed with the pin in the home position.

[Steve G]

Originally Posted by porschedude996

Very interesting. Leave it to the Germans. Looks to me that the extra material should be opposite of the dome. I bet that if the circlip was installed on the extra end before the pin goes in, the pin shoulder hits right where should if it were a standard pin. Which means the pin has two circlips. I would not think the circlip could be installed or removed with the pin in the home position.

You're absolutely right, there are two circlips per piston and its only possible to remove 1 with the pin installed.
The more i think about it the more it makes sense to install the extra length opposite to the dome.
What is also interesting is that the valve pockets are both the same size as the intake valve!!
I've never seen anyone post a picture of a 3.1L stripped down so its reasoning and guesswork all the way!!

Here's another........with turbo barrels there was a metal sealing ring / head gasket....ive been told it shouldn't be there but they were fitted when i stripped the motor ( this much i remember )

[reclino]

The only thing I can think this does is allow for easy balancing of a piston and wrist pin combo. Take a little off the long side of the pin to bring the pair to the desired weight. This technique would save manufacturing time, but the end product is heavier than it needs to be.
So I see this as an interesting bit of cost saving engineering, not something for a hand built race engine.
Just my thoughts, as an engineer. Only assembled one of these engines so I am far from an expert.
David