push rods

[jamesweed]

Hello, wondering which push rods I should use on my 1985 AMC 258 i6 / 4.2l block to go with a 7120 4.0 head. AMC 258 i6 / 4.2l or 4.0 rods? some other length that have been used? Ive compared both the lengths are different.

Thanks

 

[rlitavecz]

I used the stock AMC 258 i6 / 4.2l pushrods on my engine.

 

[CJim7]

Use your AMC 258 i6 / 4.2l pushrods

 

[73CJ]

I put the old push rods in and measured the clearance. Did the math and bought accordingly. Push rods are available in various lengths. And yes I know the lifters are hydraulic. But why not get it right from the start?

No issues and no clatter.

 

[jamesweed]

Thanks for all your comments. I still have my AMC 258 i6 / 4.2l rods as well as a 4.0 rod i snuck out of the junkyard. I suppose Ill put the head on and see what kind of preload I get with each one and go from there.

 

[CJim7]

Thanks for all your comments. I still have my 4.2 rods as well as a 4.0 rod i snuck out of the junkyard. I suppose Ill put the head on and see what kind of preload I get with each one and go from there.

Dont even bother with the 4.0 rods. I can tell you right now they are wrong in length
The AMC 258 i6 / 4.2l rods will be closer to what you need. The 4.0 head is similiar to the AMC 258 i6 / 4.2l head in height from deck to valvetrain. The difference is cam to deck between the AMC 258 i6 / 4.2l and 4.0 blocks.

 

[jamesweed]

OK, now the next question: I noticed when I got the 4.0 head from the yard it had different rockers on the #3 and 4 cyinders, I assume theyre different somehow because they had an x stamped on the head and the others are different. I thought that both heads could use the same rockers. True or false?

 

[CJim7]

OK, now the next question: I noticed when I got the 4.0 head from the yard it had different rockers on the #3 and 4 cyinders, I assume theyre different somehow because they had an x stamped on the head and the others are different. I thought that both heads could use the same rockers. True or false?

Side by side, the 4.0 and AMC 258 i6 / 4.2l rockers look identical dimensionaly.

 

[MN CJ7]

I did some of this research a few years ago and it might help you. The 4.0 head or using non stock cam, or other valve train changes will affect the lenght of the push rod to make the engine work/breath right.

Push Rod length seems to vary all over the place when I purchased them for 1978 CJ7 with 258. . I found the year of AMC Engine and the vendor you get the push rod from can make a huge difference. Wrong length can cause engine failure or poor operation.

Every push rod length was different and they did not have much to offer. Examples: Red Rooster 9.622 & 9.594”, ORiley 9.622”, Checker none, Napa 9.65”, CarQuest 9.594”. These are the offering for my 1978 Engine and some too long or short

I was able to get a photo copies of a few pages that list the years and length and the part numbers. The pages were from an old Melling Parts Book. Pioneer Inc, 601-483-5211, Meridian, MS, was the company that had the correct information and the correct push rods. They were very helpful.

The copies I have are very poor so I cannot post them.

The Engine Remanufacture installed incorrect push rods and caused a total engine failure so this does happen. The Push Rods were too long by, .12”, a lot considering lifters can take up .030” of play with hydraulic oil pressure. The LONG incorrect push rods (9.62” correct vs 9.74” installed) and bent push rod, collapsed lifter. one lifter out of pocket, and oil path problem took out the #1 connect rod bearing. They sent me a set of push rods to replace the bent one and they were the correct length, 9.62. Glad I measured the push rods with a 12” digital caliper and took pictures. Engine Remanufacture tried to blame me for the engine failure claiming detonation. Money was refunded in full.

Glad I did the research it saved my but.

I am pretty sure many of us here at JeepForum have issue with push rods and may not be aware. Non Stock cams can / will affect your valve train geometry and parts used.





This information was from an old Melling Parts Listing for AMC Engines


PUSH RODS AMC 258 4.2 liter stock cam & set up


MPR-301 9.594” x 5/16 1971-1974 w/o Rocker Arm Shaft
MRP-332 9.658” x 5/16 1971-1974 w/ Rocker Arm Shaft (BB ends)
MPR-301 9.594” x 5/16 1975-1976
MPR-301 9.594” x 5/16 1977 w/ Temp Sending Unit in front of engine
MPR-333 9.622” x 5/16 1977 w/ Temp Sending Unit in rear of engine
MPR-333 9.622” x 5/16 1978-1980
MPR-353 9.700” x 5/16 1981-1988



Rocker Arms AMC 258 4.2 liter stock cam & set up


MR-812 1971-1974 use w/ engines with rocker arm shaft

MR-801 1974-1982 use w/ engines with rocker arm and individual pivots
MR-740 1983-1990

 

[MN CJ7]

Another thought...

Performance shop/supplies will have adj length push rods..... These are used for setting the lenght of push rod so the rocker roller/tip is centered on the valve stem top during its movememt. The push rod of the right lenght will have a rocker arm & valve stem that rubs in the middle of stem and has min deflection of the valve stem.... Too long or Too short can make the rocker arm deflect and push the valve stems causing excess wear. It much longer and out of range like mine was...... will not allow the valve to close completly and could also lead to a collision with the valve and piston top.

These have a thread portion to push rod so it can be lenghtened. Once you have the correct length look at list and get the best match....

The push rod "ends" vary and work with lifter and rocker arm hole..... This is your oil path too.... Worn parts and the push rod / rocker arm holes do not mate up with a complete piston cycle........ will not lubericate the top end correctly.....