Case of the 3rd. case!

[wwwcad]

Hi All,
I took my engine case to the machine shop and the machinist had a question I couldn't answer. I also can't find the answer in Pellow's book.

We need to know the normal "look" of the back of the oil pump cavity. In other words: when you remove the oil pump cover and take out the 2 gears, is it normal for there to be two circles (reminds me of a figure 8) in the back of the case where the gears would rest or touch. About 1/2 to 1mm deep or so? Or should the back of the oil pump cavity be totally smooth with no indentation where the gears rest? I think it's supposed to be that way (since I have two 3rd. pieces and they are identical to the naked eye) and he thought maybe it was a sign of case wear from the gears turning over time with too much debris in the oil. He thinks this could potentially affect oil pressure when I reassemble if the indents don't belong there.

What do you experienced folks think about that?

Thanks,
Bill

[C. Clark]

Bill,
As I remember when the oil pump gears were removed from my 912, there were the circle indentations that I think you are refurring to, perhaps a mm or so...

The oil pressure is generated in part from the mesh of these gears, you can use a feeler guage to attain the factory spec. a VW friend says, have the gears pressed of the shafts and reversed and bingo you have a knew oil pump. That sounded like a cheap trick to me so I just put it all back together, it works fine.

I was concerned about the wear on the key at the end of the shaft though, the one that fits in the timing gear, but I was told a new shaft will show signs of wear after just a few thousand miles, one of those German things I guess. Pellow mentions that I believe, I hope this helps..... I think your ok, although I'm certainly no expert.
Best of luck with the engine,
Chas.

[wwwcad]

Hi Chas,

Thanks for the reply. Hopefully someone else who also knows something about this will chime in, just to make me feel comfortable with the whole thing!

Regards,
Bill

[Green 912]

Ding! Yes there are indentations in the bottom of the oil pump well in the 3rd piece. To check clearance for wear you put the pump gears in and put a straight edge across the top of the face of the housing over each gear. The gap should be from .0024 to, 0050 in or 0.06 to 0.128 mm. Clean the case face before measuring with a feeler gauge.

The oil pump is driven by a tang on one pump gear that fits into a slot in the center of the Cam. This is a point of failure as the tang and slot will wear against each other and get sloppy. This is also sometimes will make noise and will lead people to think that real bad things are happening in the motor. The cam can be welded up and the slot reground if it is damaged.

[wwwcad]

Hi,

Thanks! That was the answer I was looking for! Couldn't find it in any of my repair manuals.

Regards,
Bill