Engine First jeep rebuild plan
Hi all. First time Jeep guy but not first time working on stuff. Got a question about an engine I'm rebuilding.
I'm working on a AMC 258 i6 / 4.2l that was in a CJ7 I recently acquired.
When I bought it compression tests all indicated that I had a cylinder head leak between cylinders three and four.
The engine ran, but incredibly poorly as you can imagine. Compression was 120psi (cold) and 130psi (wet)for everything but 3 and 4 (as expected).
Pulled the head off and indeed, there is a hunk of gasket missing between three and four.
030 was stamped on the pistons and indeed the bore was verified to be at 3.78" at the bottom of the cylinder . Further investigation showed that this was an 85 block in my 83 CJ. So someone put in a reman at some point.
Further investigation showed what looked to be aluminum film around the top part of the cylinder of three and four. No other cylinders showed this, and I figured it was likely because of an overheating situation that those pistons started to melt. And then raise hell with the cylinder head gasket.
Or someone put in rings with wrong clearance. Not sure.
Can barely scratch your fingernail on it, but it's pretty visual. There is no scoring on any of the bores. Just this film.
Started to pull the bottom end apart and the clearances with plastigauge for 3 and 4 rod bearings are at .002. The bearings look scored. The journals don't look too bad, but you can catch a finger now just barely.
I've only pulled the center main crank bearing just for a look see.... its clearance is also at .002. The journal is about in the same condition as the connecting rod bearings. The main bearing shows some where but nothing like the connecting rod bearings.
Bore taper looks over FSM spec. I'm getting an average of .003 of taper. Looks like I'm heading to the next over bore. Hope I can get it cleaned at 0.04 over.
Let's see, other stuff... the timing chain was loose enough to start eating the wear tabs in the timing chain cover. The cogs showed a healthy amount of wear. Camshaft didn't look too bad, but some of its lobes were just rough.
Also some hamfisted PO managed to torque the rocker arm bridges to what I think was double their torque value. I had a hell of a time trying to loosen them gently,without bending the bridges.
Anyhow, with all the information, just looking to see what everyone thought of how this engine got in this condition. I'm imagining an overheating event, a lean condition or maybe bad ring gaps got me here.
PO had a Weber 32/36. I found some of the intake bolts were very loose. Probably an air leak.
My intention is to get it bored over .04 (hope to avoid going to .06)
I'm going to get the head reconditioned. Camshaft bearings replaced. Looking for a new camshaft probably a Crower. Maybe the Baja Beast (category one)
Hoping a crankshaft polish is all I'll need and all new bearings, but will get it ground to next undersize if machinist recommends it.
I'll put in a new oil pump, new roller chain, and cogs.
In the future I'm going to put a 38 Weber on it or a Motorcraft. I don't know. I'm debating a new intake manifold, but I'll see. Probably going to go the HEI ignition route.
Anyhow, looking for some insight of what you guys think of the mess I've got myself into and if you think I've got a good plan to fix it. I still need to tear apart more and I will but pretty sure this is what I've gotta do.
My intention with this Jeep is just to have a good Montana forest service road/ with the occasional Jeep trail outfit. It won't live on the highway a ton it'll be mostly on gravel roads in the mountains or getting to them. Not crawling rocks.
Thanks for any tips or advice!
I'm working on a AMC 258 i6 / 4.2l that was in a CJ7 I recently acquired.
When I bought it compression tests all indicated that I had a cylinder head leak between cylinders three and four.
The engine ran, but incredibly poorly as you can imagine. Compression was 120psi (cold) and 130psi (wet)for everything but 3 and 4 (as expected).
Pulled the head off and indeed, there is a hunk of gasket missing between three and four.
030 was stamped on the pistons and indeed the bore was verified to be at 3.78" at the bottom of the cylinder . Further investigation showed that this was an 85 block in my 83 CJ. So someone put in a reman at some point.
Further investigation showed what looked to be aluminum film around the top part of the cylinder of three and four. No other cylinders showed this, and I figured it was likely because of an overheating situation that those pistons started to melt. And then raise hell with the cylinder head gasket.
Or someone put in rings with wrong clearance. Not sure.
Can barely scratch your fingernail on it, but it's pretty visual. There is no scoring on any of the bores. Just this film.
Started to pull the bottom end apart and the clearances with plastigauge for 3 and 4 rod bearings are at .002. The bearings look scored. The journals don't look too bad, but you can catch a finger now just barely.
I've only pulled the center main crank bearing just for a look see.... its clearance is also at .002. The journal is about in the same condition as the connecting rod bearings. The main bearing shows some where but nothing like the connecting rod bearings.
Bore taper looks over FSM spec. I'm getting an average of .003 of taper. Looks like I'm heading to the next over bore. Hope I can get it cleaned at 0.04 over.
Let's see, other stuff... the timing chain was loose enough to start eating the wear tabs in the timing chain cover. The cogs showed a healthy amount of wear. Camshaft didn't look too bad, but some of its lobes were just rough.
Also some hamfisted PO managed to torque the rocker arm bridges to what I think was double their torque value. I had a hell of a time trying to loosen them gently,without bending the bridges.
Anyhow, with all the information, just looking to see what everyone thought of how this engine got in this condition. I'm imagining an overheating event, a lean condition or maybe bad ring gaps got me here.
PO had a Weber 32/36. I found some of the intake bolts were very loose. Probably an air leak.
My intention is to get it bored over .04 (hope to avoid going to .06)
I'm going to get the head reconditioned. Camshaft bearings replaced. Looking for a new camshaft probably a Crower. Maybe the Baja Beast (category one)
Hoping a crankshaft polish is all I'll need and all new bearings, but will get it ground to next undersize if machinist recommends it.
I'll put in a new oil pump, new roller chain, and cogs.
In the future I'm going to put a 38 Weber on it or a Motorcraft. I don't know. I'm debating a new intake manifold, but I'll see. Probably going to go the HEI ignition route.
Anyhow, looking for some insight of what you guys think of the mess I've got myself into and if you think I've got a good plan to fix it. I still need to tear apart more and I will but pretty sure this is what I've gotta do.
My intention with this Jeep is just to have a good Montana forest service road/ with the occasional Jeep trail outfit. It won't live on the highway a ton it'll be mostly on gravel roads in the mountains or getting to them. Not crawling rocks.
Thanks for any tips or advice!
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