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Should I Rebuild This Engine?

I revived my old pickup - 71 Dodge W100 4WD 225 ci (Slant-6). It sat unused for 5 or so years. The compression was good and after rebuilding the carb and putting in new ignition parts, it started. For an engine with 60,000 or so miles it runs nicely, except for a slight low-end knock. Maybe that’s why I stopped driving it several years ago - since my head injury my memory isn’t very good. I tore down the engine and discovered that the fit on the rod bearing on #3 piston was sloppy. I assume that’s causing the knock. A search on the internet and a search by the local big wrecking yard turned up no running used engines. If I had $2000 I could get a long block. But I don’t, so I can’t. So I’m thinking - 1) have the crank ground 2) have the machine shop provide the appropriate oversize bearings 3) put in new main seals 4) reassemble 5) get religion. There are probably several flaws in the plan, which is why I’m asking for help. A) What is the likelihood that the #3 rod (the one that knocks) is bent? B) How would I know if the rod was bent? C) If I have to replace the rod (or piston), should I also replace other pistons … I’m thinking maybe #4 piston D) is there a better low-cost solution. NOTE: this truck will be used mostly here on the farm - and occasional trips into town (16 mile round trip). It will not get any long trips and no high speed - other than the 50 MPH limit on the road to town. Or should I junk this and start saving my money to buy a tractor?

Have the machine shop check and resize the connecting rods and if the bores are in good shape hone them, re-install the pistons with new rings, rod and main bearings.

The first step might be to carefully inspect the #3 rod journal, get in there with a bright light and take a good look at it…Beg, borrow, buy a micrometer and carefully measure it…If it’s not damaged, I would install a set of rod bearings and button it up and see what happens…if the mileage is correct at 60K, then the rest of the engine should be in pretty good shape…

The begs the question: “Why did that #3 bearing fail??” Did something very bad happen to this engine? Like way over-reved or run out of oil? The 225 slant-six is a 250,000 mile engine…

I guess I miss spoke myself. And made an assumption, also. I jumped to the conclusion that the engine was out and being dismantled. Certainly, the crank needs to be sized and dressed as needed as Caddyman suggested.

I would pull that rod and have it resized. Then have the crank reground and put it back together. It will be fine. I once had a Ford 300 6 cyl. That would have good oil presser then it would then would go to 0 psi. I pulled the oil pump and found the relief valve would stick open. I then checked all bearings, main and rods. There was some copper showing on most of them. I checked the crank and it was with in speck. So new bearing and oil pump. I drove it for 2 years and sold it. 2 years later I seen it at my friends body shop. The guy I sold it to said he had put over 70,000 on it and sill was running fine.
I also bent 2 rods in my Jeep 4.0 L . I pulled the 2 pistons and rods put in 2 new rods with new bearing. Reused the pistons and rings. Its runs fine and uses no oil between changes.

I would want to know what those compression numbers are before offering much, if any, advice. The slant six is a great little motor and 60k miles is a bit young for one to be suffering from a rod bearing knock.
It would be hard to believe that a rod bearing would be the only thing prematurely worn.

When I first tried to start the engine after it sat for so long I squirted some oil into each cylinder and tried cranking it. It didn’t start, so I checked the compression. I used a schrader valve tester and checked it with cold-cranking. Given those non-optimum conditions I figured that 130-140 pounds was good enough. After some time fiddling with it it started and ran surprisingly well (not counting the knock). I pulled the engine and removed the oil pan. That’s where I found the sloppy rod bearing. All of the rods except #3 are tight when wiggled. All the rod bearings looked good except the upper half of #3. It has some pitting. The crankshaft doesn’t look abnormal. I don’t remember any incident that would damage the engine. But after an accident I was in I have a period in my life where I don’t remember much of anything.

The compression still sounds low to me; especially considering those numbers came from what was essentially a wet compression test due to the oil in the cylinders. If oil had not been added to the cylinders those numbers may have been far lower.

You might get away with just dropping a set of rod and main bearings in it and calling it good. If you don’t have micrometers to measure crank journals with, etc. you can use Plastigage for a rough approximation of how worn the journals are and how much oil clearance exists.
Plastigage won’t tell you if the journals are tapered or egged though.

If the engine were mine and since it’s out of the truck and with the pan off I’d have to pull the head for a look-see inside; followed by a new set of rings and possibly a valve job depending on how much, if any, leakage exists around the valves.