AMC 401 and 304 parts compatibility:

Hi, I recently purchased a "74" CJ 5 that I was told had a 304 in it. I ordered a complete rebuild kit, timing chain and gear kit, cam, rocker, lifters, howell TBI, and several other parts. Then, in the tear down, I find the 401 casting mark on the side of the engine and thought "Cool, but oh sh!t" Clearly the new pistons, bearings, rings, and seal kit are a no go. What about the cam, timing chain, rockers, lifters and such? What parts are compatible?

 

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The AMC V8's have the same block exterior dimensions. Most of the external stuff will bolt on. However, that does not mean they will work together. I will tell you if I found a 401 in a CJ, I'd do back flips in the driveway! That is the holy grail in a CJ. However, they are way more motor than that jeep needs. You have to be careful.

That being said, I'm finishing up building one for torque so my research is fairly fresh in my mind. You need to decide what you want the motor to do. Torque? HP? Stock rebuild? Tell us what you want and we can make better suggestions.

Don't let them bore the motor any more than absolutely necessary. Some people say that .030 over is the max. You can have it sonic checked and decide if you want to go more. Mine is at .040 and the bores are almost centered so I can go to .060 with no problems. I know some people have successfully ran it at .080 and then some.

But depending on what you want the motor to do, start with the cam. I doubt a cam for a 304 will do anything but kill a 401. The heads are interchangeable to a point. They will bolt on but some will not work. You will need new valves. The rockers and lifters might work.

But were it me, I'd keep the timing chain and gears, TBI, and other external parts and sell the rest. There are a lot of people rebuilding a 304.

And picts would be great. We like picts.

 

The main bearings are the same - a 401 crank will go in a 304 block. But the rods and rod bearings are different. And as you noted, the pistons are different, so the pistons and rings must be for a 401. So you can't use the head gaskets, pistons, rings and rod bearings. Most everything else will fit. As pointed out, a 304 cam will fit but it would be way too mild for a 401. The heads are not an issue, since you almost certainly have 360/401 heads on a 401 block, and the parts kit does not affect the heads.

As aallison pointed out, the 401 cylinder walls are comparatively thin from the factory, and can't stand a lot of overbore. If you bore too much, the cylinders will have hot spots and the engine will overheat. Does your engine run now? I suggest you cut the ridge, hone the bores, and put it back together with your pistons and new rings. There will be some taper, but the 401 block has a lot of nickle, and the bores don't wear much. A virgin block can go 30 over with no trouble, but beyond that, you want to sonic check the bores for core shift.

What's the condition of the engine now? Why do you want to rebuild?

 

Thank you for the replies. The 401 does run fairly well, I just want to add reliability to it by doing a rebuild. It leaks oil from almost every seal, so I figured if I have to tear it down then I would rebuild it complete and give me confidence to take it to the mountains and such. My thought is to pretty much keep it at stock and if I get a little torque bump out of it then that would be great.

Oh, and a picture....



Or two...

 

Looks like a great Jeep! And lucky you, the 401 is a great motor. It's very surprising that the previous owner would not have touted the 401 and boosted the price of the Jeep accordingly.

Don't mean to lecture you, but if the engine runs ok now, it would be a big mistake to rebuild it. A seasoned engine with good vitals is just as reliable as a newly rebuilt engine. And you are using up the engine's lifetime by doing a rebuild for no good reason.

You can fix your oil leaks with the engine in the chassis. How experienced are you with engines?

Does it overheat? What's your hot idle oil pressure? Have you done a compression check yet? I suggest you do some more research and learn a little more about the engine's condition before you cross that bridge.

 

All good advice so far. Thank you! The engine is prone to overheating. Bad radiator and no shroud. Both problems that are going to be fixed. Unknown oil pressure as the gauge was not working, I haven't done a compression check and I should have. Like an idiot, I thought the previous owner knew what they were talking about. The kid said that the speedo had been rolled over and that the engine had a lot of miles on it. I'm learning very quickly that this kid had no idea what he had or much about vehicles. Purchase price for this rig: $2,500.

Motor was running like crap and figured it wouldn't hurt to do a complete rebuild on it, but now I am questioning that idea. I am mechanically sound, so tearing into the motor isn't a problem for me. Was planning on having this apart for most of the winter. I don't know anything about carbs, so that is why I am putting in the TBI, that and I live in Colorado, so the motor needs to run at altitudes from 5,000 to 11,000 feet.

 

Yep, I'd go ahead and bring the cooling system up to snuff, and see what I had then. Is the body off the chassis now?

 

Yep, body is off, there were some frame and body issues, so I took it apart. Will be patching a lot of rust and repairing a few cracks in the frame. Makes for a good time to do engine, tranny, and xcase work.

 

You might want to do all your rust and frame repairs, and test the cooling system again. The front clip comes off easily, so you aren't making much extra effort by doing some more running tests before you pull the engine. Up to you.

But an overheating 401 is suspicious, for the reasons stated above.

 

I'm with Tim. Get it back together, fix what you can without tearing it apart and then see if it runs like it should. Compression test, Oil pressure, etc. will tell you what you need to do. But in some cases you can fix the ills without opening her up.

You got a great deal on the jeep. Way cool! Love the paint job.