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Oil Clearance Is Not A Myth It Is A Calculation

Posted by Carl Amundsen on 8th Oct 2014

TUESDAY, FEBRUARY 8, 2000

OIL CLEARANCE IS NOT A MYTH IT IS A CALCULATION

By: Carl Amundsen Date:2/8/2000

Over the years I have had many opportunities to discuss oil clearance with engine builders. Some are building for their own use, some are small shops while some are large. In any event when the question is posed "HOW DO YOU KNOW WHAT YOUR OIL CLEARANCE IS?" the responses vary. Some can give a good mathematical response, some don't have a clue. The common denominator here is that everyone knows they have oil clearance.
A common phenomenon that occurs when a motor fails is that no one looks in the mirror for answers. The vast majority of the time it is blamed on an engine part failure. The connecting rod too many times is the favorite fugitive.
A LESSON IN MEASUREMENTS
A very dangerous but commonly used method of measuring is a dial caliper. The average dial caliper is super for measuring the thickness, inside diameter, and outside of an empty toilet paper roll. A SHOCKING STATEMENT , NO DOUBT! If you look through some catalogs you will find a dial caliper for $40.00 that has an advertised accuracy of +/- .008 . That is an error factor of 16 thousandths. Pay $200.00 for a digital caliper with advertised accuracy of +/- .0015, the margin of error is now 3 thousandths. An error factor of 1 thousandths can cost you a motor. Calipers are great tools for some things but not for the task at hand. WE MUST BE ABLE TO READ IN TENTHS OF THOUSANDTHS(.0001)
ON WITH THE LESSON
If you were to take a good 0 to 1 inch micrometer and measure the thickness of a page in the ARC catalog, you will find it to be five thousandths (.005) thick. This is very important to remember as you read on. Now feel the paper with your fingers and imagine somehow splitting one of these pages 50 times. One single sheet would now be 1 tenth of 1 thousandth of an inch thick (.0001). I hope I have your attention, because this is getting down so that you can't even feel the thickness. Just incase I am going to be confusing you with decimals please remember the following:
1.0000 = 1 inch0.1000 = 100 thousandths 0.0100 = 10 thousandths0.0010 = 1 thousandth0.0001 = 1/10 thousandth

It makes no difference who you buy your parts from, nor does it matter what brand they are, ARC included,they need to be checked and double checked. NOBODY IS PERFECT.
For example let's build a stock stroke Briggs racing engine. The following is the measurement specifications on the typical parts to be used:
Stock Briggs crank rod journal size......................... .998 +/- .0015Aftermarket Connecting Rod................................... 1.150 +/- .005Aftermarket Rod Bearing thickness........................ .075 +/- .003
ALL THE PARTS WE ARE USING ARE WITHIN THE MANUFACTURERS TOLERANCES.
You are going to use a medium weight oil and you are shooting for .0025
The Connecting Rod is on the small side................. 1.1495The Rod Bearing is on the big side.......................... .0753The Crankshaft is on the big side.............................. .9985
Let's put this motor together and go racing.
The Connecting Rod is small by .0005...................... 1.1495The Thickness of the Rod Bearingare big by .003(2 bearings) =.1506............................. - .1506Net Rod Bore size with Bearingsinstalled................................................................... . 9989The Crank Rod Journal is big by .0005..................... - .9985The Calculated oil Clearance = ............................... .0004We were shooting for ............................................... .0025
With a little luck this motor will crank up and run, as long as the motor is running at no load and a low RPM it may be OK for a while. The minute you go racing the lack of oil flow between the bearing and the crank journal will cause heat build up. The bearing will seize on the crank journal, break the rod and just make a mess of everything. BAD PARTS RIGHT ? WRONG !
Now let's build another motor and go the opposite way, still trying to achieve an oil clearance of .0025
The Connecting Rod is on the big side........................ 1.1505The Rod Bearing is on the small side.......................... .0747The Crankshaft is on the small side.............................. .9965
The Connecting Rod is big by .0005............................ 1.1505The Thickness of the Rod Bearingare small by .003(2 bearings) =.1494............................. - .1494Net Rod Bore size with Bearingsinstalled...................................................................... 1.0011The Crank Rod Journal is small by .0015..................... - .9965The Calculated oil Clearance = .................................. .0046We were shooting for .................................................. .0025
This motor is going to run, but what is going to happen here is: The bearing is going to get pounded at the top and bottom of the rod bore, because there is an air gap between the two surfaces. The oil is not thick enough to prevent this from happening. This is going to convert the rod bore into the shape of an egg standing on end. The crankshaft will now start to loose it's round shape and wear.
EVERY MINUTE IT RUNS, THESE PARTS WILL INCREASE THE OIL CLEARANCE UNTIL IT EXPLODES. HOW LONG WILL IT LAST? I CAN'T SAY EXACTLY, BUT NOT TO LONG!
The crying shame here is that everyone will point their fingers at the connecting rod, bearings and/or crankshaft as the culprit.
THE MOST OVERLOOKED AREA IN ENGINE BUILDING IS AS FOLLOWS:You have a motor that has run for many months , but you notice you're getting a little blow-by and it could probably use a set of rings. It needs freshening up so we tear it down. Every thing looks great so we touch the bore with a hone and put a new set of rings in. Back to the races. If it ain't broke don't fix it. STOP! Under the most ideal conditions engine parts will change with use. Bearings can look good but will wear, connecting rod bores will change shapes and crank journals will wear and rod bolts will stretch and fatigue. It is just as important now as ever to check the dimensions of the parts. Don't go to sleep.
Some may think there is some deep mystery behind the term oil clearance. The reason is, everyone has a different opinion as to what it should be. As a rule of thumb, it can be anywhere between .0015 and .0035 and be in the ball park. If the clearance is on the low side, use thin oil, heavy oil will not work. If clearance is on the high side , use a heavier oil, thin oil will not work. How can you make a judgment on the oil to use unless you know what your oil clearance is? THERE IS NO MAGIC, YOU CANNOT KNOW WHAT YOUR OIL CLEARANCE IS UNLESS YOU MEASURE THE PARTS THAT AFFECT IT.
DEFINITION OF WHAT OIL CLEARANCE IS:
"The distance between two very smooth moving surfaces that will allow oil to be present at all times, coating both parts with a film of oil so that metal on metal contact never happens. It must be small enough to retain enough oil and large enough to allow a fresh cool supply to move through every microsecond."
If you do not check your parts and build 4 motors and 3 of them seem to have a pretty good life span you are lucky. If one fails out of the gate or shortly thereafter, shame on you. It takes less than 15 minutes to check and measure all the parts in a motor and the benefits are fantastic. It is possible to build an engine that will run until the cows come home, or something like that. There are many engine builders out there that do a fantastic job in this area, but on the other hand there are more that don't.
ASSUME NOTHING, BELIEVE NOTHING AND LAST BUT NOT LEAST, CHECK EVERYTHING.
I hope we have given you an insight to one reason for engine failures. If you need us, give us a call, it's all free. 1-800-521-3560