Rebuilding an F-head Engine

Page 4: Starting the Crankshaft Installation

by Eric Lawson

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Crankshaft end
The crankshaft has been chromed. In case you're wondering if I'm building a "chrome monster," I'm not.   :-)   In this case, the chrome is to build up the worn surface where the front main bearing bears (indicated by the red line in the photo.) This area on this crank was worn about .05mm and I couldn't set the crankshaft end play correctly. The machine shop can plate up to .1mm or so and it is a good way to go as it doesn't warp the crank like welding will do.

I took it to another machine shop to get the wax (they use wax to prevent the chrome from plating on unwanted areas) off the crank. The wax is difficult to remove with just solvent and rags. The best and easiest way to get the wax off is to let a shop with a (very hot) "hot tank" clean it.

Bearing bore
In the center of this picture, in the main bearing bore, is the bearing dowel (see close-up view below). There is only one in the front main. In the remaining bearing bores, there are two--one in the block and one in the bearing cap. Don't lose them when you are disassembling the engine!

Bearing dowel

Bearing shells
Once all five of the bearing dowels have been reinstalled, the main bearings can be installed in the block. Be sure the oil hole in the bearing shell aligns with the oil passage in the block.

Four of the bearing shell halves are identical. These are placed in the middle and rear main bearing saddles in the block. The front main (closest in the picture) bearing is the only one that has the surfaces that extend over the edge of the machined surface on the engine block.

The surfaces that the bearing shells are placed into must be absolutely clean. The slightest bit of dirt behind the bearing will upset the alignment of the bearing and doom it to a quick failure.

Bearing half
The two front main bearing halves are different from one another. Only one will have the holes for the oil passage and bearing dowel.

Be careful not to install the front and middle bearing caps backwards. Take a look at the numbers cast into the bearing caps. They should all face the same direction as the numbers in the rear main bearing cap. Also, the alignment of the edges of the bearing caps will be slightly off if the bearing caps are installed backwards.

It is possible to incorrectly install the front main bearing shell in the bearing cap. To install it correctly, the grooves in both bearing shells should align when the bearing cap is correctly installed.

Do not install the bearing caps yet.

PlastigaugePlastigauge is used to check the bearing clearances. It is a wax "string" of a certain thickness. When the bearing cap is tightened, the string gets flattened. The more it gets flattened, the less the bearing clearance.

Check crank
A piece of the plastigauge is cut and placed on the crankshaft. The plastigauge is somewhat sticky. If you lightly press it onto the crankshaft, it will stay in place while you are installing the bearing cap.

Coat the bearing cap bolt threads with a small amount of oil and install the bolts. Torque one bolt a small amount and then the other an equal amount until you reach 40 ft/lbs (55 N-m) of torque.

Loosen each bolt in small amounts until they are loose and remove the bearing cap.

The width of the now flattened plastigauge can be compared with the scale printed on its packaging to determine the bearing clearance. Both inch and metric scales are printed on the plastigauge packaging.

Different Jeep service manuals recommend either .0003 to .0029 (.0076 to .0736mm) or .001 to .0025 inch of clearance for the main bearings. I'm not sure why there is such a difference in the recommendations. I think the .001 to .0025 inch values are more appropriate.

The places where the plastigauge was not flattened are the oil grooves in the bearing shell. Each of the places that the plastigauge is flattened should indicate the same clearance. If not, make sure there is no dirt behind the bearing shells or the mating surfaces of the bearing cap and block. If no dirt is found, then either the crankshaft or main bearings need to be examined.

This procedure should be performed on each main bearing. The plastigauge can be removed by scraping with your fingernail or a flat toothpick. -- Eric Lawson

Continue to Page 5: Finishing the Crankshaft Installation

Return to the Engine Rebuild Table of Contents.

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Last updated 9 May 1998 by Derek Redmond redmond@cj3b.info
All content not credited and previously copyright, is copyright Derek Redmond