Below is a side-by-side shot of a plastic cover from a 1997 Vortec engine and to the right of it is a '80's non-roller steel timing cover for comparison.
Some differences are:
- The plastic cover has more room around the crank gear. This cover was equipped with a crank sensor, so the additional room may be to make room for the crank position sensor rotor.
- The steel cover is deeper, from inside the front to the sealing surface.
- Generally, there seems to be more room around the inside of the steel cover, even though the photos don't look that way due to how the covers are shaped. This is except for the area around the crank gear.
- The plastic cover has fewer bolt holes at the top- an older steel cover would have holes that are not used on the Vortec-type block. The Vortec-specific holes could be put into the older block as long as care was used to position them correctly using a transfer punch.
- The plastic cover has male pins or dowels to locate the cover onto the face of the block; the steel cover has holes in the same position for the dowels that are used in the older blocks.
- The plastic cover's bolts are shouldered to prevent over-tightening the cover.
As seen below, the older block used dowel pins (male pins in block, holes in tin cover), while the Vortec plastic cover has the male pins (larger diameter than older blocks dowel pins) in the same location.
This 880 block has all the holes present and tapped; the two top holes are not correct for the tin cover, though. The dowels are absent, instead there are holes.
The dowel holes in the 880 block (used as locator holes for the "pins" on the plastic timing chain cover) are >15/64" and <1/4" (right for a press-fit 1/4" dowel pin, I'd guess). I had thought these holes were a larger diameter, but it was the chamfer at the mouth of the holes that made them look a bit larger.
The hole in a tin timing cover is ~17/64- so there's ~0.015" "play" in there, anyway.
The p/n for plastic cover w/sensor
hole is: p/n10244600.
The number for plastic w/o sensor
hole is: p/n12552557.