The 426 Hemi did have some problems during development and post production. Let’s take a close look at them by answering the question, what problems did the 426 Hemi Have?
During development the engine blocks were cracking and the piston wrist pin assembly was failing. Post-production there was carburetor related issues like idling and difficulty starting. Earlier engines had some camshaft wearing. The blocks, combustion chambers and lifter bores were prone to cracking.
This article will explain each problem in more detail. In addition, I’ll provide the solutions taken by Chrysler to fix the problem, if any.
What Problems Did the 426 Hemi Have
Most of the topics written about the Hemi are all positive things. Even an article about its problems, like this one, it’s impossible not to compliment the elephant Mopar Hemi. After all, it is the greatest racing engine design in automotive history, so why not?
Although, like every other engine, it did have some faults. Yes, even the almighty Hemi design wasn’t immune to some design or engineering issues.
The cracking issues described below were more common in engines being raced or subject to a lot of abuse on the street.
Let’s take a look at those issues starting with the pre-production engine.
Pre-production/Race Engine Block Cracking
During the development of the 426 Hemi Engine, the engine blocks were cracking in the right side cylinder bores during durability testing.
This was discovered in the dyno room while running the blocks through a simulated 500 mile Nascar race. The blocks were cracking before the “race” was over.
Willem Weertman and Larry Adams came up with the solution which worked. To increase the amount of iron around the cylinder bores.
Read more about Weertman, Adams and everyone else who played roles in inventing and designing the engine in my article, Who Invented the 426 Hemi and Designed It.
Pre-production/Race Wrist Pin Failure
During the development of the engine, the heat that would build up in the big domed piston became a problem. The excessive heat was causing the piston wrist pin to fail.
The hot piston would expand around the pin bore until the assembly came apart.
The engineers redesigned the piston pin locks which solved the problem. The pin was moved up farther adding material to the crack-prone area.
Carburetor/Idling and Difficulty Starting for the Street
This may be the most common of all the 426 Hemi problems and affects the owners of the street Hemis.
The Hemi would experience some idling, difficulty starting and carburetor issues.
The carburetor issues were magnified with the Hemi due to its two four barrel carburetor intake system. This was especially true if the engine wasn’t started in a long time.
- The carburetors can easily become out of tune.
- The fuel in the bowls dries up quickly after the engine is turned off.
A weekly car show I used to attend always had a ’67 GTX Hemi car there. It was all original and I had to walk past this car and study it more than the wedge engines. It was always a disappointment when he started it up to leave because the carbs were having issues in need of adjustments or a rebuild. The car ran like crap and I used to wonder why he didn’t get the carbs rebuilt.
The street Hemi has a lot of valve spring force which wore out the lobes of the camshaft faster than other Chrysler big blocks. This was true even though the valve springs weren’t as stiff as the race Hemi.
Chrysler advised owners to perform regular rocker arm inspections and adjustments. In 1970, the Hemi changed to hydraulic cams and lifters.
The hydraulic set up didn’t require adjustments and the engine ran quieter.
If you’re interested in the cam specs of the street Hemi check out my article, Hemi Specifications.
Or my latest article about cams, 426 Hemi Camshaft Specs: Street and Race.
Block Cracking Problems
The main bearing saddle area of the block sometimes develops cracks. The cracks start between the main bearing oil hole and the main bearing bolt boss.
Sometimes the crack is directly at the main bearing oil hole and crosses the hole extending straight up to the camshaft.
The crack may eventually continue up towards the cylinder bore or camshaft.
The Hemi blocks produced in 1970 and 1971 have a thicker main saddle area.
Lifter Bore Cracking
The iron around the lifter bores was vulnerable to cracking. This was not considered a major issue and the production block design in that area didn’t change.
The after market Hemi blocks developed by Chrysler in the 1990s had boxed in lifter bores containing more iron around them.
Combustion Chamber Cracking
The combustion chamber would develop a crack or blow out a hole. This would occur on the opposite side of the chamber from the spark plug hole.
If this occurred in a regular street engine, it would be probably due to a lot of abuse or detonation.
In mostly racing applications, a crack may form between the spark plug hole and the valve.
Find out more about the cylinder heads which made the engine special in my article, What Hemi Means in an Engine.
For the past 40 years, I’ve been studying and researching Mopars and engines like the Hemi. I’ve read books, articles, magazines, watched videos, attended seminars and spoken to other Mopar experts.
Any questions or if you have more information you’d like to contribute, send us an email found specifically on our contact page.
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