Answers to 420A Headgasket Questions

[brassc2007]

I was doing some searching and found some information on the 420A headgasket oil leak problem and their corresponding fixes. First of all, the reason that the 420A is notorious for leaking oil from the headgasket is not mainly the head gasket materials' fault. The reason for the 420a head gasket oil leak is that when Chrysler designed the 420A they overlooked the fact that at least one headbolt was too long, specifically the one near the 4th cylinder, causing the gasket to separate.

I found some information pertaining to shaving the bottom of bolt a couple mm or so to fix this problem. The only problem with this solution is that the bolt is "torque to yield" meaning that the length, strength, and material of the bolt was taken into consideration for the torque specs.

When you are getting ready to replace the headgasket replace the bolts with either OEM replacements or ARP studs. The OEM replacement bolts were modified to fix this problem. It is also a good idea to use a MLS (multi-layer steel) headgasket. Also, always make sure you have the head gasket the right side up.

Finally, since the head is aluminum, you may consider taking the head to a shop and having it checked for warpage; the smallest amount of warpage can make the biggest difference. Having the head checked should only cost you 35-50 dollars depending on the shop.

While replacing your headgasket with an MLS gasket, you may notice a tab on the back side of the motor, near the 4th cylinder. Your old gasket probably does not have this tab, but it's okay; this tab is one of Chrysler's improvements made to their headgaskets to fix this leak problem.

There is a new torque sequence that you should follow while replacing your headgasket with an MLS headgasket. Be sure to follow this sequence carefully!


2.OL DOHC TORQUE PROCEDURE WITH MLS GASKET INSTALLATION

NOTE :THE 4 SHORT BOLTS ARE PLACED IN THE CORNERS.

A. Torque all center bolts to 34 Nm (25 Ft Lbs), Torque the 4 corner bolts to 27 Nm (20 Ft Lbs).

B. Torque all center bolts to 68 Nm (50 Ft Lbs), Torque the 4 corner bolts to 47 Nm (35 Ft Lbs).

C. Re-torque all center bolts to 68 Nm (50 Ft Lbs), Re-torque the 4 corner bolts to 47 Nm (35 Ft Lbs).

D. Tighten all bolts in the specified sequence (Fig 1) an additional 90°(1/4 turn).

While replacing your headgasket, as long as the head is removed from the block, you may also consider replacing your timing components and water pump.


If you cannot find the source of your oil leak, then I recommend cleaning the exterior of the engine thoroughly with degreaser and a rag or Scotch Bright pad. Then, drive the car several miles and check the engine for leaks. The source of the leaks should be apparent if the engine was previously clean.

If there are no exterior leaks, then your engine may be burning oil, as indicated by a puff of blue smoke from the exhaust. The cause of these leaks may be worn valve seals or piston rings.