Oldsmobile V8 technical information
Listed below is some helpful information about some commonly asked questions dealing with Olds V8's.
| HEADS/BLOCK OIL DRAIN BACK HOLE MODIFICATION: |
Increasing the size of the drain back holes as described below decreased the amount of time it took for 1-1/2 cups of 30W oil to drain from the cylinder heads to the pan by more than half.
Open up the holes in the front and back of both heads, and the block since the oil flows thru the block for that short area too. The rear hole is really the only important one, but I drilled out the front and back holes in case the heads were removed and put back on opposite.
The stock holes are 3/8 (.375"). The hole is dog legged - you have to drill half of it from the valve cover side of the head, and half from the block side of the head - It's the lower half of the hole that goes by the head bolt hole. I used a 7/16 (.438") bit for the upper (valve cover) half of the hole, and a 27/64 (.422") bit for the lower half.
They actually did break into the head bolt hole in the head that is in that area, but I figured this was not a problem as the oil would just fill up the area around the bolt, and not do any harm - it won't leak out anywhere. I did de-burr the broken out area though to make sure that no chips would break away later on.
Use a slightly smaller drill like a 13/32 (.406") bit if the break out bothers you.
3/8" diameter hole = .110 square inches area. 27/64" hole = .140 square
inches. This is a 27% increase in area.
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Filling the heat crossover passages
Do this before any machine work. The passages can be filled with aluminum from old pistons, or with ZA-12 (zinc w/ 12% aluminum) alloy purchased from speed shops. The melting point of the zinc is around 800 degrees F, aluminum's is around 1200F, so I prefer the aluminum. The disadvantage to using aluminum is that you need an acetylene torch and a cast iron skillet to melt it. The ZA-12 can be melted in a propane grille with a regular kitchen frying pan. Either ask the wife, or make sure she won't miss it!
Remember - this does not just keep the intake cooler, but also is "porting" the exhaust ports, and separating the center exhaust ports so your headers take full effect.
Click on the photo to enlarge.
| || Brazing the center divider on the siamesed exhaust ports |
With most Olds heads, the exhaust center divider does not come all the way out to meet the header surface. Widening and extending it as shown here aids in exhaust flow and header scavenging. This is the kind of thing that will gain you horsepower and torque across the RPM band, without hurting fuel economy.
The finished product is shown here, after the port has been shaped, and the exhaust header surface milled flat. If having this work done, plan on having to mill the exhaust surface as well as the deck surface as they will warp. The heads should be heated prior to brazing to reduce thermal stress on the cast iron and minimize the chance of cracking the heads.
This step should also be done before any machine work. The heating could in some cases make any pressed in exhaust seats fall out.
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Correct rocker arm geometry
1. Base Circle - roller tip on intake side of valve tip
2. Mid Lift - roller tip center of valve tip
3. Max Lift - roller tip on exhaust side of valve tip
- Too long a pushrod will bias the rocker tip's sweep across the valve to the exhaust side.
- Too short a pushrod will bias the rocker tip's sweep to the intake side.
- Too short a pushrod can also cause the rocker arm to interfere with the radius on the rocker arm stud, and the rocker arm to interfere with the valve spring retainer at low lifts.
Milled heads, milled blocks, cut valve seats, different length valves, different brand lifters, and varying camshaft base circle sizes all change the ideal pushrod length that will give you correct rocker arm geometry.