Rebuilding an F-head Engine

Page 6: Rods and Pistons

by Eric Lawson

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It is finally getting cool enough in the garage for me to get back to work on the engine.

Finding a set of rods for this engine was quite an ordeal. One of the original rods had been damaged beyond repair and I couldn't find a single rod that would properly replace it. I ended up buying another set of rods, but a couple of them wouldn't fit correctly onto the crankshaft that I had already installed in the block. I was given a second set of rods and these were the ones that I finally used. The first set of replacement rods fit a crankshaft that will be going into an 4L-134 engine, so I kept them for that project.

The second set of rods were sent to the machine shop to be reconditioned. The reconditioning consisted of checking for cracks, resizing the bearing bores and balancing them. I believe the crack check and resizing operations are required operations. The balancing of the rods' weights is only needed if you are unsure if the rods came from a matched set.

Note: John McCasland has written to say, "One item of importance that I've discovered (the expensive way) is that if you have the crankshaft turned on engines with long throws on the crank, it is IMPERATIVE that you also have the rods reconditioned meaning that the bearing end must be machined so that it is once again perfectly round, unless you enjoy spinning rod bearings."

Replacing Connecting Rods

If you have to replace any connecting rods here are some things to check when selecting them.

First on the list is whether the rod is odd or even. Next is checking if the amount of side play the rod will have, when installed on the crankshaft, is within specification. Finally is whether the weight of the rod is close enough to the others for a machine shop to be able to make the rods' weight match.

Odd and even refer to the cylinder number in which the rods will be used. The cylinders are numbered 1 through 4, with number 1 being closest to the waterpump. Usually there is a number stamped into the rod that indicates the cylinder where the rod was used. This number is on the side of the rod as shown. The rods that I used for the picture did not have numbers stamped on them. I added the numbers to the image to show where to expect to find the numbers. Rods with even numbers can be interchanged with other even numbered rods. Rods with odd numbers can also be interchanged with other odd numbered rods.

If the rods don't have numbers stamped in them, the offset in the rod bearing bores can be used to determine if you have an odd or even rod. Place the rod as shown and with the oil spray holes facing upward. The yellow lines in the picture show the offset, and the red lines show the location of the oil spray holes. The rod on the left is an even rod. The rod on the right is an odd rod.

To check the side play, an old set of bearings can be used. Install the bearings in the rod and install the rod onto the crankshaft. Tighten the bearing cap nuts to 10 to 15 ft/lbs torque. Slide the rod over to one side of the rod journal and measure the clearance on the other side of the rod. Using a feeler gauge, the clearance should measure .004 to .010 inch (0.1 to .25mm).

If the diference between the lightest and heaviest rod is within 1/8 ounce (3.5 grams) the rods do not need balancing. A machine shop should be able to balance the rods if their weights are within 1 ounce (29 grams).

Any scale that has the required resolution and a capacity of at least 2 pounds (1Kg) can be used to weigh the rods. A machine shop can do this for you. Before my workplace got a postal scale, I used to take the clean rods to the post office, lay a piece of paper on the scale and weigh them there.

Checking the Rings and Pistons

There is an official procedure to check the clearance between the piston and the cylinder bore. It specifies that the measurements be done at at 70 degrees F (20C). It has not been that cool in my garage for five months, so I don't bother with the official method. I ask the machine shop to hone the cylinders to size and, if needed, to mark each piston so I know which cylinder it is mated to. To check their work I simply slide the upside down piston into the cylinder bore. If the piston moves in the cylinder without having to push it AND there is only a slight amount of side to side play, I assume the machine shop has done the job correctly.

The rings should be checked to ensure you didn't accidently get an incorrect one. As you are working with the rings, be sure to not mix the different rings. Some ring sets have different 1st and 2nd compression rings and they must go in the position indicated.

Take a compression ring and push it into the bore of the cylinder where it will finally be installed. Use a piston to align the ring in the cylinder. Then use a feeler gauge to measure the gap between the ends of the ring. Do this for all eight compression rings. The ring gap should be between .007 and .017 inch (0.18 and .45mm). Some books say the ring gap can be as large as .045 inch (1.15mm). You don't need to check the oil rings. The oil rings are three piece units with one piece that looks somewhat like a spring and the other pieces being very thin rings.

Each ring set should come with a set of instructions. If they are available, use them. If you don't have the instructions, here is a generic set of instructions:

Installing Rings

First install the oil ring expander into the bottom groove of the piston. The expander is the unusual looking ring that looks somewhat like a spring. Make sure the ends of the expander touch one another, but do not overlap. On my ring set, the ends of the ring were painted red and yellow which made this step easy. Most of the time you will have to check very carefully.

Next take one of the oil rings and install it above the expander. The oil ring is the thin ring. Place the ring atop the piston and push one end of the ring down until it hooks under the ring groove. Then push the free end of the ring with your finger so that it will expand the ring and work the rest of the ring into position.

The same procedure is used to install the other oil ring below the oil ring expander.

While you are installing the oil rings, be careful so you don't nick the piston. If you do manage to nick the piston, a piece of emery cloth can be used to smooth the nick.

Next, install the 2nd compression ring. You will need a ring expander to do this. Expand the ring enough to slip it over the piston and position it into the groove above the oil ring set. The rings have a top and bottom side. Look on the part of the ring that will get hidden by the piston for a dot or the letter T. This will be the side of the ring that will face toward the top of the piston.

Crankshaft shimsFinally do the same with the 1st compression ring. This ring also has a top side and will be marked just like the 2nd compression ring.

When you are all finished, the rings will be as shown in the picture. Note there is a thin groove between the top of the piston and the 1st compression ring. This groove is to slow down the transfer of heat from the top of the piston.

The rings will need to be positioned correctly for the best engine performance. Rotate the rings so that their gaps do not occur over the T slot or the piston pin holes. Also, the ring gaps should be staggered so they don't align with one another.

Assembling the Rods to the Pistons

Assembling the rods onto the pistons is fairly simple. The oil spray hole on the connecting rod faces away from the T shaped slot in the side of the piston and the specified torque for the piston pin lockbolt is 45ft/lbs. Here are a few hints that will make the job easier:

Use a tap to clean the threads in the connecting rod's piston pin lock bolt hole so that you can turn the bolt all the way into the rod by hand.

Use new split lockwashers to secure the piston pin lock bolts. Get hardened lockwashers rated for use with grade 8 bolts. Do not use standard lockwashers as they will not work well in this application. You will probably need to go to an industrial hardware supplier to find these lockwashers.

There is a groove in the piston pin. The lock bolt slides along the groove and prevents the piston pin from moving. Move the piston pin as needed so that you can, using your fingers, tighten the lock bolt until the lockwasher prevents further tightening. If you use a wrench too soon in the process, you risk stripping the lock bolt's threads.

Usually the piston pin will not easily slide through the end of the rod. I tap a wedge shaped piece of hard plastic into the slot cut into the rod until the piston pin moves easily through the rod. If you have to use a screwdriver as a wedge, be careful not to gouge or nick the rod. Gouges and nicks can cause cracks to form in the rod.

It takes a special tool on a torque wrench and some calculations to get the correct torque on the lock bolt. I don't have the necessary tool, so I tighten the bolt with a regular wrench until my hands hurt from the efort. This is unscientific, but so far I haven't stripped any bolts or had any bolts work loose.

When the piston is correctly installed onto the piston, it will look like this. The red line points to the oil spray hole. The yellow line points to the T shaped slot in the piston.

Installing the Pistons in the Cylinders

Rotate the crankshaft so that it is at bottom dead center at the cylinder you will be working in.

Remove the bearing cap from the rod. Now is a good time to ensure the oil spray hole in the rod is not clogged.

With the engine block in its normal upright position, lower the rod into the proper cylinder. Make sure the oil spray hole is facing away from the camshaft. If everything is correct, the mark in the top of the piston should be facing toward the front of the engine. Also, the longer offset in the rod bearing should be facing away from the nearest main bearing.

I lower the rod into the cylinder and set the skirt of the piston onto the side of the cylinder while I'm getting the ring compressor adjusted.

Thrust washer
Put the ring compressor onto the piston and tighten the compressor as much as possible. The ring compressor does not need to cover the entire piston. Lower the piston into the cylinder and hold the compressor tight against the engine block. Use a hammer handle to tap the piston into cylinder. Each tap should move the piston slightly. If it seems like the piston is not moving, stop, reset the ring compressor and try it again.

Turn the block upside down and pull the rod up enough so you can install the rod bearing. Before actually installing the bearing, make sure the bearing area in the rod, the cap and both sides of both bearings are clean. After the bearing is installed in the rod, pull the rod up against the crank.

Install the bearing cap onto the rod. Tighten each bearing cap nut to 10 ft/lbs of torque. The rod should move side to side on the crankshaft. If you push the rod against one side of the crank journal, the end play (side to side movement) should be .004 to .010 inch (.1 to .25mm).

Remove the bearing cap, place a piece of Plastigauge on the crank and reinstall the bearing cap. Tighten each bearing cap nut, a little at a time, until the torque reaches 35 ft/lbs. Remove the bearing cap again and check the bearing clearance.

The clearances are the same as the crankshaft main bearings. Should the Plastigauge show an uneven clearnace, check behind the bearing for dirt or oil.

Once you decide the bearing clearance is correct, push the rod away from the cranksahft and apply engine assembly lube to the bearing surfaces. Reinstall the bearing cap and tighten the bearing cap nuts to a torque of 35-45 ft/lbs. Rotate the crankshaft, loosen the nuts 1 turn and retorque them. As with the main bearings, this ensures that the assembly lube doesn't cause incorrect torque readings.

Repeat this for the other three cylinders. -- Eric Lawson

Continue to Page 7: Preventing Oil Leaks

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Last updated 14 August 1999 by Derek Redmond redmond@cj3b.info
All content not credited and previously copyright, is copyright Derek Redmond