Cracked water jacket or blown head gasket...

Well there's oil in my anti-freeze and anti-freeze in my oil So it's either got a blown head gasket, or a guy was telling me his '72 with the 258 cracked the combustion chamber between the intake and exhaust valves. Apparently the early 258's have thin-walled water-jackets in the heads, and if you run unleaded gas (which burns hotter than leaded) the added heat will crack the heads... anyone else heard of such a phenomenon? At any rate I'm swapping in a later model 4.0L, I'm just wondering if anyone else has experienced this, or even heard of it.

Wil.

 

Sorry to hear that, Wil. Always gotta be something, huh?

 

I doubt it. I've never heard of such a thing. Even by '72, California was requiring unleaded gas, and there's no way that AMC would have supplied a different cylinder head for the 49 states than for California. Plus, the compression ratio of a stock 258 is only something like 8:1, which should be compatible with all grades of fuel.

IMO it's more likely that your engine was overheated sometime in the past and now the chickens are coming home to roost, so to speak. The 258 is very tough and durable - no major faults that I know of other than a tendency to manifold leaks.

Do you already have the 4.0L? IMO there's not much advantage to the 242 over the 258 unless you are planning to use the MPI as well. If you're staying with the Weber, I'd suggest you go with a 4.0L HO head on your 258 block, which will keep the long stroke of the 258 (torque!) as well as the heavier early 258 (pre-80) crank and block.

 

I've (knock on wood) never had a crack, but I have replaced a couple head gaskets(not on same Jeep). If you replace anything you maybe want to check the mating surfaces with a straight edge, mine were both OK. On one engine, I still had a slight leak after I replaced the head gasket with one that you weren't supposed to have to retorque. Retorquing solved the problem and it hasn't leaked since.

 

My thoughts were, that my engine has a gazillion miles on it, and putting a 4.0L head on it would be a waste of a head... unless I rebuilt the 258, but then having it bored and the crank ground, and buying new pistons and getting a new cam and lifters, would cost me WAY more than just buying a 4.0L

 

My point was there's no real advantage to going with a used 242 (4.0L) as opposed to a used 258, if you're not going to use the fuel injection that goes with the 242. The HO version of the 242 has a better cylinder head design, and has a shorter stroke and bigger bore than a 258, but otherwise its the same engine. A longer stroke and smaller bore, like the 258, generally means the engine has more torque at a lower RPM but less power than an engine of equivalent displacement with a short stroke and larger bore, like the 242.

 

You could allways use your 258 crank and rods in the 4.0, making it a 4.5L.R)

Kevo

 

My main concern right now is getting the most reliable engine for the least amount of money, not necessarily power. Eventually I'd like to get a big ol' V8 for it, but I can't afford that right now (nor the gas as the Jeep is my only vehicle). And I can get a 4.0L that has fewer mile on it than my 258, for less money than it would cost me to make the 258 as reliable as the 242.