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Rod & Main Bearing Clearance


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Here it is. Arriving at the proper rod and main bearing clearances is easier than you might think. The problem that most engine shops seem to have, as far as I'm concerned, is laziness. Another problem is: Pontiacs do not have bearings available in .001 or .002 under sizes past .010 (ten thousandths) So, you cannot "chase" clearances like you can with a Chevy.

~ Another BIG problem is the fact that every factory Pontiac shop manual has been wrong since 1955. Depending on what year you're looking at, most of them say the upper allowable clearance is .0035. Don't you EVER build one with those clearances unless you want your oil pressure guage to move like a tachometer. This is exactly where the old saying, "Pontiac's are known for low oil pressure," comes from.

Here are the steps to follow in order to blueprint the clearances correctly. and remember this, YOU ALWAYS CUT THE CRANK LAST

The main bores must be inspected after the block is cleaned. You check for straightness and diameter size. The straightness is brought in by properly clipping the main caps, and then a skilled hone operator must hone the main bores carefully. We find that the housing bore size should be kept to the middle or to the minimum to maintain good bearing crush. NEVER PAST THE MIDDLE. Loose housing bores are the main culprit behind spun bearings. So, on a three inch main bore the factory tolerance is; 3.1880 to 3.1890 This is the size of the hole without the bearing in place. Our tolerance is 3.1880 to 3.1885. On a 3.250 main bore, (421/428/455) the factory says; 3.4380 - 3.4390. Again, we use 3.4380 - 3.4385. The checking clearance is taken from the twelve-to-six o'clock position using a calibrated dial bore gauge.

Doing this maintains the proper crush on the bearings. This is crucial. The same theory applies to the rod bores. All Pontiac rod bores are 2.3745 - 2.3750. Again we use the small to the middle 2.3745 to 2.3748. Always preferring the smaller dimension. The bores are OK? Proceed.

You will now measure the crank and buy the appropriate rod and main bearings. If the crank is a wore out .010, you're buying .020's etc.
It doesn't matter how far under you must go, IT IS THE CLEARANCES THAT COUNT, NOT THE undersize.

Take the bearings, install them and torque them to factory specs. Make sure the bolt holes and threads are super clean and use a very light coat of engine oil. Brush it on, then wipe it off. This leaves a proper VERY THIN COAT. Same goes if you are using the moly lube supplied by the bolt manufacturer. Do not use too much. Also make sure that there isn't a burr, or raised metal at the edges of the housing bores where the bearings used to sit. Use your fingernail to feel all edges. Use a small round file to completely debur, Then clean again.

After torquing, accurately measure with a dial-bore gauge the dimension of the holes with the bearings in place, at twelve and six o'clock. The 12 - 6 location is where all rod and main bearing clearances are measured. Label and write these dimensions down.

You now have the dimensions that exists with your parts. Not a nominal figure taken from a book, but a real number.

You'll take these numbers, along with your crank, to the crank grinder, and say, "Grind my crank .0020 (Two thousandths with a zero, it's not "twenty") away from these dimensions."

Any crank machine that's in good working condition can hold these tolerances. You have to hope that the operator does his part and knows how to run the machine properly. Of course you can check his work with a micrometer.

Our personal tolerances are .0015 - .0022 on both the rods and mains. Under no circumstances will you use plasti-gage to measure anything. Maybe the space between aunt Edna's teeth. It's totally unreliable and worthless in my book.

Follow these steps and you'll have great oil pressure warmed up at idle using the standard 60 lb. oil pump. (R.A.4) If all of your rod and main bearing clearances are .0025 or more, your oil pressure at idle will be 15 - 20 lbs, if you're lucky.

Do it right and engine life will be greatly increased. Remember, it's your parts. You're the paying customer. Don't be intimidated by arrogant shop owners, and don't be afraid to ask the "best shop in town" if they will follow these simple blueprinting procedures. If some fuzzball wants to argue that "factory book tolerances" should be acceptable, just look at a stock exhaust manifold. No thank you.

Never take it for granted that a recommended shop will do the procedure as I have outlined. Communicate. Let them know you'll be inspecting their work.

Good luck.
Bruce Fulper