Correct push rod length for 258 / 4.2 be carefull???

Correct push rod length for 258 / 4.2 be carefull???


Senior Jeeper
1978 CJ7
Silver paint with black hard top on 33" radials & eagle alumium rims. Good looking ride and have done lots of work to make it look and run great.

Stock equipment is 258 carb, power brakes, power steering, hard top & hard doors (per vehicle gross weight) tilt steering wheel, big brake package with large rotars & drums, T18 Trans with low 6.4 granny gear, Dana 20 transfer, Dana 30 front axle, Dana 44HD rear axle with 3.73 gears. True Trac ft & rear.

I was around jeeps in the 70's and drove a 1973 Jeep Truck. My family had 4 jeeps at the time. We also joined a jeep club back then too. So about 6 years ago looked for a Jeep CJ and found the vehicle above.
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. They wanted $1000 to put the engine back together & return shipping & my warranty now void of any claims.

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

yes, this is a Fred Original
Please make this a tech article... I could not find a box/switch to do it myself, thx
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A large number of variables are involved in determining the correct length pushrod for your application. Pushrod length is affected by any of the following:

  • Block deck height
  • Head deck height
  • Head stud boss height
  • Rocker arm brand/design
  • Cam base circle size
  • Lifter design/brand/pushrod seat height
  • Valve stem length
Or to increase preload:

Push Rod Lengths
Jeep Push Rod Lengths
Length Part Number Engine Year
9.639 PR-437 4.0L 1987-96
9.728 PR-399 4.2L 1989-90
9.700 PR-353 4.2L 1983-88
9.622 PR-333 4.2L 1977-80
9.594 PR-301 4.2L 1972-76 Stamped Rocker
9.656 PR-332 4.2L 1971-74 Cast Rocker
Custom Length Push Rods
9.500 PR-310 Used with the aluminum head on decked engines.
9.547 PR-327 Used with the aluminum head on stock engines.
9.560 PR-310A
9.668 PR-303
Most of these push rods can be found on the web site in the "Valve Train" section.
If you believe you need a length that is not listed call and we will be happy to try and
accommodate your needs. You may also need a set of rocker arm shims to achieve
the proper lifter pre-load even with a shorter push rod. The part number is
HESRASK and can also be located in the "Valve Train" section.

Best article is from Crane:
did not thinnk to look at the 4.0 push rods
Are they same 5/16 and the same ends?
The early 258 Had bb ends and cannot be use interchangble with other lenghts

Hisco is using the Melling Push Rod ... the numbers are the same just left of the M.... off the part number.

I think the stock lifters take up .015 in hydrolic lift might be .020" not a lot... so pretty tricky to set up... and stil have the valve train wear built into the lifter hydrolic lift.

I did not follow the tech link when they want a straigth edge across the head gasket surface and scribe the rockers...

I did look at roller rockers for the 258 when doing my work... less wear and are adjustable... BUT $450 - $400 range was 1/3 the price of my long block... the 258 parts can be very exp. Purchased long block and if I were using the 4.0 head and had cam.... I think the roller rockers would be a big help and help with the wear and set up.... and spent the money more than likely

I saw guys on the sat morning motor head shows up the adj rollers with machine die on valve stem ends and adj them so the wear/movement was centered on the valve stem to min valve deflection and wear. They also had a ADJ PUSH ROD so the lenght was close and got rods to match the adj... fine tune with the adj roller rocker..... looked great

Bumping for info
haha me too! :chug:
So if you're not sure if your engine is totally stock or not is there a way to measure for correct length? I noticed there was no entry for 1981 on the second post but my PO said his PO did an "engine rebuild with some modifications" that he couldn't remember.
Use a pushed length gauge tool. I had to use one when I put it a bigger cam and had the head decked at the same time. It's not the easiest thing to use, I ended up asking the machinest for help, and he let me borrow a machinist ruler and explained the correct amount of play in the valve train, etc. Part of that deal was that I'd buy the correct length pushrods from him... plus whatever it was to go over the head...
Just shim the rocker bridges.
The main issue is when a head is rebuilt and the valve seats are ground deeper into the head. This causes the valve to sit higher making the rocker arm hold the push rod lower leading to excessive lifter pre load. Shim the bridges until you can just bearly rotate the pushrod with your fingers with the valve fully closed and the other valve on that cylinder just beginning to open.
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