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I am in the process of assembling M90 engine and was checking the oil pump and noticed this piston with a compression spring. I noticed that there was no tension. For some reason my pump doesn't have item 22 i only have in the diagram items 20, 21,23,24.
Has anyone had experience in changing out this piston?

I have not worked on this system, but item 22 looks VERY important.

I've shared what the blue service manual has on the oil pump. Note that item 22 is present.

very interesting Keehn...thanks for the reference of the manual.
I have another M30B28 motor and removed the oil pan and i have the same type of oil pump. no part the looks like item 22 in the diagram.

I believe the plug (#22) was for old production pumps. close up look there is no thread to screw on the plug. the clip hold a spring pocket to allow the flow of oil. Perhaps

Joe,

This looks to be a "technical update" (TU) to the oil pump design. Clearly, your engine must work fine, so I wonder if the exploded diagram simply shows the pre/post TU by design? It's a head-scratcher to me.

Regardless, I'll be doing a complete top and bottom rebuild of my 76 M30B30 this winter and I'll make sure to document what I find.

I took a look at the oil pump I removed from the M30B35 I had, and it's identical to the picture Joe posted. So you're right it's probably a technical update and your book is showing early pumps.