Hello everyone, and welcome to the latest from The Toolbox. This post is on a Caterpillar C7 rebuild. They are known for problems with broken compression rings, resulting in massive blow by and oil slopping from the breather tube and even out the oil fill at high rpm. The C7 Acert is the latest version of the 3126, the HEUI injected motor found in many medium duty brands. This one was in a 2005 GM C6500 which sold at auction because of the cost of repair. The new owner bought it for $5000 before he asked or found out about potential problems. “Let the buyer beware” was never more meaningfully applied to a situation. He wasn't so happy with the sale when he found out it would cost another $5000 to repair the motor. Still, 10 grand for the truck was not a terrible deal, as it was equipped with a liftgate and was cosmetically and mechanically sound but for the engine.
This is the second time I have rebuilt one of these in situ, with the engine in place. The problem is that the cylinders are cast in place and are part of the block, instead of being replaceable as in the International DT 466. This means much greater cost because the block needs to be bored to accept a new liner, then the liner is bored to size and honed. I have seen other forums suggest that this process will reduce displacement, but that is the result of not understanding the process. The engine block has enough material to allow a liner of sufficient size to bore to the original size, so stock pistons are used for replacements. The real problem is that a service with a portable boring bar needs to be called in for machine work, and the machinist needs about 3 feet of headspace above the block for the boring tool. It a C6500, there isn’t enough space to get both hands on the engine, let alone get the head off or work on the deck. This meant the cab had to come off. Other repair alternatives are replacement with a reman engine or block or with a salvage engine. One could also remove the engine, strip it, and take it in for the machine work, but that would really add to labor cost, not to mention the extra time to outsource that work. The problem with those routes are that to pull the engine you would need to remove the hood and radiator assembly and still need to at least lift the cab off the frame and then change over a bunch of parts, so I don’t think you would save anything in labor. Besides, I am not impressed with the workmanship of any rebuild I have seen, and a salvage motor could have the same problem at any time.
Removing the cab may sound daunting, but it is done all the time at salvage yards and as this was a hydraulic brake truck, there were no airlines to deal with. I had the cab off and dropped onto a stack of pallets in 4 1/2 hours. A forklift and a 3 inch ratchet strap through both doors and another to the firewall support did the trick. The other 3126 I did was in an older Ford Louisville, where cylinders 2, 3, and 4 needed to be resleeved. By removing the front engine crossmember and taking the rear trans mount loose, I was able to tilt the engine forward enough to clear the firewall with the boring equipment.
Once I tore down the engine I found the top rings in cylinder 3 and 4 were broken and those cylinders badly scored. The others had absolutely no wear ridge at the top and no scoring, so just cleaning them up with a hone would save the customer considerable money. Note that on this engine the piston oilers need to be removed for any work to be done on the block. The oiler nozzles stick up into the bottom of the cylinder and they will be damaged by the bore head or the hone if they are left in place. If there is no ridge at the top and no scoring you can feel with a fingernail, you can just break the glaze with a hone and reassemble with a new piston and rings. The machinist came and relinered the two cylinders, then bored the new liners to size with a deck plate bolted to the block. Installation of the head must be simulated for the final bore sizing, or the pull of the head bolts will pull the cylinders out of round when the head is installed.
Before honing, I pushed a new top ring 1/3 of the way down each of the cylinders with a piston to check end gap, and all were within spec. Using a Flex-hone, (or berry bush) hone and a liberal amount of 10W-30, I honed each cylinder for 20 seconds at about 60 up and down cycles/minute. A finish hone for about 10 seconds at120 cycles/minute gave me a good 45 degree finish hone at the speed my air drill ran at. It is important not to overdo the honing, as all that is necessary is to have a good pattern for the rings to seat to. I was also using a brand new hone that cut pretty aggressively.
After all the machining swarf was cleaned off the inside of the engine and the crank it was reassembled as usual, keeping in mind that there are a couple of fussy things about assembling this motor. The piston oilers can be put in after the pistons and rods are in, but there is more room to work if it is done before the pistons are in place. You need to hold the oiler up tight to the block when you put the bolt in, or the end will bind in the block oil passage and it will bend, then the oil will leak out around the oiler instead of going through the nozzle. That will certainly cause the pistons to overheat or the wrist pins to seize. If the nozzles are in place when the pistons are installed, care must be used so the rods don’t hit them on installation.
It is worthwhile to spend a little extra time making sure everything is hooked up correctly and well assembled before the cab is set back down. There is very little room in this engine compartment to work on anything once the cab is back on. Luckily this engine ran and survived a 10 mile test drive with no problems, so I didn't need to go back and tweak anything.
This is the second time I have rebuilt one of these in situ, with the engine in place. The problem is that the cylinders are cast in place and are part of the block, instead of being replaceable as in the International DT 466. This means much greater cost because the block needs to be bored to accept a new liner, then the liner is bored to size and honed. I have seen other forums suggest that this process will reduce displacement, but that is the result of not understanding the process. The engine block has enough material to allow a liner of sufficient size to bore to the original size, so stock pistons are used for replacements. The real problem is that a service with a portable boring bar needs to be called in for machine work, and the machinist needs about 3 feet of headspace above the block for the boring tool. It a C6500, there isn’t enough space to get both hands on the engine, let alone get the head off or work on the deck. This meant the cab had to come off. Other repair alternatives are replacement with a reman engine or block or with a salvage engine. One could also remove the engine, strip it, and take it in for the machine work, but that would really add to labor cost, not to mention the extra time to outsource that work. The problem with those routes are that to pull the engine you would need to remove the hood and radiator assembly and still need to at least lift the cab off the frame and then change over a bunch of parts, so I don’t think you would save anything in labor. Besides, I am not impressed with the workmanship of any rebuild I have seen, and a salvage motor could have the same problem at any time.
Removing the cab may sound daunting, but it is done all the time at salvage yards and as this was a hydraulic brake truck, there were no airlines to deal with. I had the cab off and dropped onto a stack of pallets in 4 1/2 hours. A forklift and a 3 inch ratchet strap through both doors and another to the firewall support did the trick. The other 3126 I did was in an older Ford Louisville, where cylinders 2, 3, and 4 needed to be resleeved. By removing the front engine crossmember and taking the rear trans mount loose, I was able to tilt the engine forward enough to clear the firewall with the boring equipment.
Once I tore down the engine I found the top rings in cylinder 3 and 4 were broken and those cylinders badly scored. The others had absolutely no wear ridge at the top and no scoring, so just cleaning them up with a hone would save the customer considerable money. Note that on this engine the piston oilers need to be removed for any work to be done on the block. The oiler nozzles stick up into the bottom of the cylinder and they will be damaged by the bore head or the hone if they are left in place. If there is no ridge at the top and no scoring you can feel with a fingernail, you can just break the glaze with a hone and reassemble with a new piston and rings. The machinist came and relinered the two cylinders, then bored the new liners to size with a deck plate bolted to the block. Installation of the head must be simulated for the final bore sizing, or the pull of the head bolts will pull the cylinders out of round when the head is installed.
Before honing, I pushed a new top ring 1/3 of the way down each of the cylinders with a piston to check end gap, and all were within spec. Using a Flex-hone, (or berry bush) hone and a liberal amount of 10W-30, I honed each cylinder for 20 seconds at about 60 up and down cycles/minute. A finish hone for about 10 seconds at120 cycles/minute gave me a good 45 degree finish hone at the speed my air drill ran at. It is important not to overdo the honing, as all that is necessary is to have a good pattern for the rings to seat to. I was also using a brand new hone that cut pretty aggressively.
After all the machining swarf was cleaned off the inside of the engine and the crank it was reassembled as usual, keeping in mind that there are a couple of fussy things about assembling this motor. The piston oilers can be put in after the pistons and rods are in, but there is more room to work if it is done before the pistons are in place. You need to hold the oiler up tight to the block when you put the bolt in, or the end will bind in the block oil passage and it will bend, then the oil will leak out around the oiler instead of going through the nozzle. That will certainly cause the pistons to overheat or the wrist pins to seize. If the nozzles are in place when the pistons are installed, care must be used so the rods don’t hit them on installation.
It is worthwhile to spend a little extra time making sure everything is hooked up correctly and well assembled before the cab is set back down. There is very little room in this engine compartment to work on anything once the cab is back on. Luckily this engine ran and survived a 10 mile test drive with no problems, so I didn't need to go back and tweak anything.
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