Block Drainback Modifications & Deburring

HIGH FLOW DYNAMICS Technical Pages

Block Drainback Modifications & Deburring

Rear Lifter Valley Drainbacks. The photo below shows the factory oil drainback holes at the rear of the lifter valley. The one on the left (pass side) has not been touched, the one on the right is completed:

Here is the pass side before:

And After:

Adding Front Drainbacks. Adding front drainback holes in the lifter valley is a great mod for a v-drive boat (since the motor sits backwards) or for a drag car (sudden decelleration with parachute deployment) or any max effort oil return situation.

The first photo shows the front of a lifter valley on a block with the added drainback holes:

The passenger side drainback is a no-brainer and easy mod...just eyeball the starting point from the OUTSIDE of the block and keep stepping up drill bits until you're at about 1/2-inch drill and on plane with the oil channel in the valley, then use the carbide bit to smooth it out:

View from the outside:

Notice in the above photo how the lifter bosses are visible through the new front drainback hole...this side is easy and can be very effective for oil drainback in the above noted applications.

The driver's side will also be effective but positioning the drainbback hole is critical. We are working very much near water jackets and oil galleries when doing the driver's side. Must be angled downward and a pointed a little toward the center of the block. In the photo below, study the angle of the driver's side drainback hole (left) compared to the passenger side hole (right):

Note (above) that the hole on the driver's side is very close to the cylinder bank's water jacket and lifter's oil gallery. The drainback hole is a little offset from the drainback channel, but it is still on the same level as the channel, so oil will drain, no problem. This hole does not need to be 1/2-inch to work (1/2-inch increases risk of breaking though into the wrong place). Look at the photo below:

You can use your finger in the waterpump hole as a guide/help you feel how much wall you have to work with and also help to angle the drill accurately. Additionally, a flashlight in the cylinder deck's waterjacket holes can be helpful too. Note the proximity of head's oil drainback hole next to our new 1/2-inch hole. There is certainly enough material to do this, but be careful because of the several walls where you may break through and really open a can of worms.

Casting Flash Deburring. This is straightforward common sense deburring. Excercise this treatment wherever it is fit to do so. In the example below, the ventilation hole on the left is as cast and the hole on the right has had the casting flash ground off and been smoothed. Smooth all of them like the one on the right, and wherever there may be flash.

Incidentally, the C8VE-B and C9VE-B blocks (like above) have larger ventilation holes in the lifter valley (note the protrusion of the lifter bosses into the larger diameter vent holes) whereas the ventilation holes in the later blocks are of a smaller diameter.

Matching of the top end drainback passages (from the cylinder heads to the block) should be checked for alignment and corrected. Visual inspection of this alignment may be performed while the head is seated on the block, or by using the head gasket for inspection of hole alignment while placed against each of the two parts. Grind as necessary to match holes in the mating surfaces between the block & heads. When chamfering cylinder head drainback holes in the valve train area, be careful not to grind into the right-angled part of the casting (right next to the drainback holes), as there is a water jacket directly beneath it.

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