258 Question

[matt1971]

Hi. Matt here. As we know, the AMC I-6 engine goes way back to the Rambler days. When the 258 was redesigned for the 1981 model year, it featured a new plastic valve cover, and alot of aluminum components were used.
My question is, besides the plastic valve cover that can be swapped for a metal one, are the 1981 and up 258's better? Or are the earlier types better? Please elaborate.

[mudkicker715]

The 1980 and older 258's have a better crank.

[Amc1320]

It's one of the things I love about working on Amc cars, lots of things swap!

I currently have a 99 4 liter I put in a 74 gremlin to replace the 232 , I used the 232 starter and an intake from an 83 258 and it all works great

As far as better or not the later ones had a better intake and I'm not a fan of the old rocker shafts of the earlier 70's, but they work ok as long as they don't stop oiling

[doneagle]

It's one of the things I love about working on Amc cars, lots of things swap!

I currently have a 99 4 liter I put in a 74 gremlin to replace the 232 , I used the 232 starter and an intake from an 83 258 and it all works great

As far as better or not the later ones had a better intake and I'm not a fan of the old rocker shafts of the earlier 70's, but they work ok as long as they don't stop oiling

HI    I thank you had to use the 4.0 header with that combination ....right ..........don

[IowaEagle]

Heavier crank on the pre '81 models.  A bit smoother operation.

[BenM]

For most people, the older crank with more counterweights is good. Of course, it doesn't spin up as fast. The older blocks are thicker on top as well and are less likely to leak with the 4.0 head swap. The lightened crank isn't any weaker, so far as I've heard.

The newer blocks and cranks are lighter, about 60 lbs in total for a full engine. They're a bit more deeply scalloped around the head. They lightened the 4.0 a bit later on and there were a few problems with noise, but I've not heard about it in the 258. You can use the 4.0 main cap girdle if you're want.

The aluminum intake/exhaust set breathes better than the old cast intake ones and provides much more consistent fuel economy because water and the electric heater replaced the exhaust heated intake.

Also, in '82 the compression in the 258 was upped from 8:1 to 9.2:1. There was a different cam as well, but AMC never changed the rated power. It was the first cam redesign for the 2v engine since around '75.

[BenM]

Some camshaft and head info from TSMs:

Year	Vehicle	Disp	Carb	Adv. Intake	Adv. Exhaust	Overlap	I Center	E Center	LSA	I Lift	E Lift	Chamber	Comp Ratio
1973	All	All 6	1v	259	259	48	117.0	104	110.50	0.254	0.254		8.00
1973	CA	All 6	1v	259	289	68	117.0	89	103.00	0.254	0.254		8.00
1975	Pacer	232	1v	256.92	256.92	35.92	116.34	104.66	110.50	0.232	0.232	62.5-65.5	8.00
1975	Pacer	258	1v	256.92	256.92	35.92	116.34	104.66	110.50	0.232	0.232	62.5-65.5	8.00
1976	Pacer	232	1v	256.92	256.92	35.92	116.34	104.66	110.50	0.232	0.232	62.5-65.5	8.00
1976	Pacer	258	2v	263.37	263.37	42.36	117.11	103.91	110.51	0.248	0.248	62.5-65.5	8.00
1977	All	232	1v	256.92	256.92	35.92	116.34	104.66	110.50	0.232	0.232	67.84-70.84	8.00
1977	All	258	1v	256.92	256.92	35.92	116.34	104.66	110.50	0.232	0.232	67.84-70.84	8.00
1977	All	258	2v	263.37	263.37	42.36	117.11	103.91	110.51	0.248	0.248	67.84-70.84	8.00
1978	All	232	1v *	256.92	256.92	35.92	116.34	104.66	110.50	0.232	0.232	67.84-70.84	8.00
1978	All	258	2v	263.37	263.37	42.36	117.11	103.91	110.51	0.248	0.248	67.84-70.84	8.00
1980	All	258	2v	263.37	263.37	42.36	117.11	103.91	110.51	0.248	0.248
1982	All	258	2v	262	262	34	122	106	114	0.253	0.253		9.20
1983	All	150	2v	270	270	46	123	101	112	0.424	0.424	49.9-52.9	9.20
1987	Jeep	4.0L	Renix	270	270	46	98.5	98.5	112	0.424	0.424
1994	Jeep	4.0L	HO	270	270	46	120	98.5	112	0.424	0.424
1996	Jeep	4.0L	HO	253	259	43	107	96	114	0.408	0.414
2003	Jeep	4.0L	FI	253.3	259	41.6	107.3	121.3	114.3	0.408	0.414

[Amc1320]

It's one of the things I love about working on Amc cars, lots of things swap!

I currently have a 99 4 liter I put in a 74 gremlin to replace the 232 , I used the 232 starter and an intake from an 83 258 and it all works great

As far as better or not the later ones had a better intake and I'm not a fan of the old rocker shafts of the earlier 70's, but they work ok as long as they don't stop oiling

HI    I thank you had to use the 4.0 header with that combination ....right ..........don

Yes, used the 4.0 exhaust manifold and the Aluminum intake from the 83. All you have to do is grind the "ears" a bit off the intake so it clears the dowles, seems they moved them a bit along the way

[IowaEagle]

Thanks for the great information.  I will have to get that chart into the Eaglepedia.