258 Main and Rod Bearing Size Identification

[Rescue Diver]

I'm doing a simple re-ring on a AMC 258 i6 / 4.2l I just picked up. I have the head at the machine shop for a valve job and I just tore down the short block. The rod and main bearings don't have STD, 010, 020, or 030 stamped on them anywhere??? They have the letters AM which I believe means they're factory American Motors bearings, but I'm not 100% sure. Besides that they have a part number that I haven't been able to find on the internet, and lastly, some have C-2 while others have C-20 stamped in them. I was thinking that C-2 or C-20 might mean they're for a crank that's been turned undersize, but guys at two different machine shops don't think this is the case. The shop that's doing the valve job looked at two of the bearings I brought in without measuring them and said they think they're STD because they have no size makings.


The main bearing has part number 3186677 and the letters M C L,
The rod has X-3230785 on the opposite side from the C-2.

Since this is a Jeep forum, I thought it would be a good idea to ask on here. Has anyone else that's done an overhaul of a AMC 258 i6 / 4.2l seen these bearings that can tell me what they are?

Can someone please tell me what the factory size is for the rod and main journals so I can try and dig up a micrometer?

Since the old bearings still look nearly new, and the crank is smooth, I was just planning to buy the same sized bearings and check the clearance with plastigauge without paying a shop to polish or turn the crank, but this has thrown a wrench in my plans.

 

[BrockGrimes]

Download this PDF and see pg 286:

Mopar Performance Parts Jeep Engines

Mopar Performance Parts Jeep Engines

jeep-cj.com

Also, Some info I found that may help you narrow your search:

"There are two generations of AMC 258 i6 / 4.2l engines. The first generation was introduced in 1971. This earlier engine has cast iron intake and exhaust manifolds with the two manifolds bolted together by 4 bolts and a gasket between. In 1981, AMC revised the AMC 258 i6 / 4.2l for lighter weight. A new block casting and fewer crankshaft counterweights were used. A new smoother exhaust manifold and aluminum intake manifold were introduced. V-belts were replaced with a serpentine belt, and a new lightweight plastic valve cover was fitted. Although these two generations have the same bellhousing and motormount attachments, and are mostly the same internally, the differences must be kept in mind when choosing used replacement parts.

The AMC 258 i6 / 4.2l can be identified by the 4th character of the engine build date code ("A", "B", "C" or "M"), stamped into the engine block below and between the number 2 and 3 spark plugs. An engine stamped with "B" is an early AMC 258 i6 / 4.2l with a 7.6:1 compression ratio. "A" and "B" are engines with 1-barrel carburetors, while "C" and "M" are 2-barrel. "M" originated in Mexico. Other codes ("E", "F", "K" and "L") are AMC 232 i6 engines."

 

[BrockGrimes]

From pg 286 of above link:​

CRANKSHAFT
The AMC 258 i6 / 4.2l crankshaft has twelve counterweights; the
crankshaft is counterweighted and balanced independently.
It has seven main bearings and six connecting rod journals.
The crankshaft has a main bearing journal diameter of​

63.489-63.502 mm (2.4996-2.500 1 'I), and main bearing
journal widths of (No. I ) 27.58-27.89 mm (1.086-1.098"),

(No. ​

​

2) 32.28-32.33 mm (1.271-1.273"), and (Nos. 3, 4, 5,​

6 and 7) 30.02-30.18 mm (1.182- 1.188").

The crankshaft has a connecting rod journal diameter of

53.17-53.23 mm (2.0934-2.0955") and a connecting rod

journal width of 27.18-27.33 mm ​

​

( I ,070- 1.076").​

The component parts of the crankshaft assembly are

individually balanced; then the complete assembly is

balanced as a unit.
​

​

Note:
On automatic Transmission equipped engines, the​

torque converter and converter flexplate must be marked

prior to removal and installed in this position upon

assembly.
Service replacement dampers, crankshafts, flywheels,
torque converters and clutch components are balanced
individually and, therefore, may be replaced as required
without rebalancing the complete assembly.
If the crankshaft is damaged to the extent that
reconditioning is not feasible, it must be replaced. The
engine must be removed from the vehicle for crankshaft
replacement.
For more crankshaft specifications, refer to ​

AMC 258 i6 / 4.2l Engine

Specificutions, 'Engine Assembly' section of this chapter.​

CRANKSHAFT MAIN BEARINGS

The main bearings are steel backed, sintered copper, lead

alloy precision type.
Each bearing is selectively fit to its respective journal to
obtain the desired operating clearance. In production, the
select fit is obtained by using various-sized, color-coded
bearing inserts.
When required, different sized upper and lower bearing
inserts may be used as a pair; therefore, a standard size
upper insert is sometimes used in combination with a .001"
undersize lower insert.
Service replacement bearing inserts are available as pairs in
the following undersizes: standard, .001", .002", .010", and​

,012". The size is stamped on the back of the inserts.

It may be necessary, in some instances, to use different

sized upper and lower inserts to reduce clearance by ​

.0005".

The bearing insert nearest to standard should always be​

installed in the upper location.

​

​

Caution:

Never use bearing inserts with greater than .001" difference in size in pairs.​

Upper - Std.​

Lower - .OO 1 'I undersize

 

[Rescue Diver]

Download this PDF and see pg 286:

Mopar Performance Parts Jeep Engines

Mopar Performance Parts Jeep Engines

jeep-cj.com

Also, Some info I found that may help you narrow your search:

"There are two generations of AMC 258 i6 / 4.2l engines. The first generation was introduced in 1971. This earlier engine has cast iron intake and exhaust manifolds with the two manifolds bolted together by 4 bolts and a gasket between. In 1981, AMC revised the AMC 258 i6 / 4.2l for lighter weight. A new block casting and fewer crankshaft counterweights were used. A new smoother exhaust manifold and aluminum intake manifold were introduced. V-belts were replaced with a serpentine belt, and a new lightweight plastic valve cover was fitted. Although these two generations have the same bellhousing and motormount attachments, and are mostly the same internally, the differences must be kept in mind when choosing used replacement parts.

The AMC 258 i6 / 4.2l can be identified by the 4th character of the engine build date code ("A", "B", "C" or "M"), stamped into the engine block below and between the number 2 and 3 spark plugs. An engine stamped with "B" is an early AMC 258 i6 / 4.2l with a 7.6:1 compression ratio. "A" and "B" are engines with 1-barrel carburetors, while "C" and "M" are 2-barrel. "M" originated in Mexico. Other codes ("E", "F", "K" and "L") are AMC 232 i6 engines."

Thanks! That manual is awsome!

 

[BrockGrimes]

I like it, LOL.

 

[IOPort51]

Archive Master.