Engine Timing Cover Guide for the 1970-1977 GMC Jimmy
Everything you need to know before buying a new timing cover for your classic Jimmy's V8 or Inline-6 engine.
- The most common failure is an oil leak from the front crank seal or a coolant leak from the water pump gaskets.
- For this vintage of truck, aftermarket parts are the only viable option, with brands like Cloyes (OE-style) and Edelbrock (performance) offering high-quality choices.
- Consider a two-piece timing cover if you plan to do a cam swap in the future, as it saves you from having to drop the oil pan.
- It is highly recommended to also replace the timing chain set, water pump, and thermostat during this job.
Symptoms of a Failing Timing Cover
A bad timing cover usually fails in one of two ways: leaking fluids or physical damage. Because it's connected to both the oiling and cooling systems on many Small Block Chevy (SBC) engines, it can leak either oil or coolant.
- Oil Leaks: The most common failure is an oil leak from a worn-out front crankshaft seal or a degraded cover gasket. You'll notice drips or a coating of oily grime on the front of the engine and on the ground underneath.
- Coolant Leaks: On V8 models where the water pump mounts to the timing cover, the gaskets between them can fail. This results in coolant leaking down the front of the engine. A corroded timing cover can also develop pinholes that leak coolant.
- Cracks or Damage: Stamped steel covers can rust through over time, while cast aluminum covers can crack from impacts or improper installation. Any physical damage compromises the seal and protection it offers.
- Timing Mark Issues: While less common, a damaged or shifted timing tab can make it impossible to set your ignition timing accurately, leading to poor engine performance.
Choosing the Right Aftermarket Timing Cover
Since new OEM timing covers for a classic Jimmy are virtually nonexistent, the aftermarket is your go-to source. The good news is that the engines used in these trucks, particularly the Small Block Chevy V8s, are so popular that there is a huge selection of high-quality aftermarket options. They generally fall into three categories:
1. Standard Replacement (OE-Style)
These are designed to be direct replacements for the original part. They are typically made from stamped steel or basic cast aluminum and are perfect for a stock restoration or daily driver.
- Brands: Pioneer, Dorman, ATP, Cloyes.
- Quality: These brands offer reliable, budget-friendly options. Dorman and ATP are well-known for providing OE-equivalent parts. Cloyes has a long-standing reputation for quality timing components and is often considered a top-tier choice for standard replacements.
- Materials: Most are stamped steel, similar to the original factory parts. Some may be offered in a natural cast aluminum finish.
2. Performance & Dress-Up
If you're looking to add some visual flair to your engine bay or need added durability for a modified engine, these are a great choice. They are almost always made from cast aluminum and come in various finishes.
- Brands: Edelbrock, Proform, Mr Gasket, Spectre, Trans-dapt.
- Quality: Edelbrock is a premium performance brand known for high-quality cast aluminum parts that fit well, though some minor modifications may occasionally be needed for a perfect fit. Proform, Mr Gasket, and Spectre offer a mix of chrome-plated steel and polished aluminum covers that look great and provide good function for street performance builds.
- Materials: Typically polished, black, or chrome-finished aluminum. Some are chrome-plated stamped steel.
Pro Tip: Consider a Two-Piece Cover
For enthusiasts who plan on swapping camshafts in the future, a two-piece timing cover is a massive time-saver. This design allows you to access the timing chain and cam by removing just the front section of the cover, without having to drop the oil pan. Brands like Edelbrock and Cloyes offer well-regarded two-piece designs. Be aware that cheaper versions can sometimes be prone to leaking at the seam if not installed carefully with a good sealant.
Cost of a New Aftermarket Timing Cover (1970-1977 GMC Jimmy)
| Part Type | Estimated Cost |
|---|---|
| Standard Stamped Steel Cover | $40 - $75 |
| Cast Aluminum Cover (Natural/Black) | $60 - $150 |
| Polished or Chrome Cover | $80 - $180 |
| Two-Piece Performance Cover | $150 - $215+ |
Costs are estimates and do not include gaskets, seals, or labor.
Frequently Asked Questions
Do I need to remove the oil pan to replace the timing cover?
Officially, yes. The timing cover and oil pan have an overlapping seal. However, many experienced mechanics have successfully replaced the cover without fully removing the pan by carefully cutting the old gaskets, cleaning the surfaces meticulously, and using a high-quality RTV silicone sealer to bridge the gap at the bottom. For a guaranteed leak-free seal, dropping the pan is the recommended method.
Should I get a stamped steel or cast aluminum cover?
For a stock engine, a quality stamped steel cover is perfectly adequate and mimics the original design. A cast aluminum cover offers better rigidity, which is beneficial for high-performance engines, especially those using a roller camshaft that requires a cam button. Aluminum covers also offer better corrosion resistance and a more premium look.
What else should I replace along with the timing cover?
Since you'll have everything apart, it is highly recommended to replace the timing chain and gears at the same time. It's a low-cost part that is critical to engine health. You should also replace the water pump, water pump gaskets, and thermostat, as they all have to be removed for the job anyway.
Are there any recalls for the 1970-1977 GMC Jimmy timing cover?
No. A search of the National Highway Traffic Safety Administration (NHTSA) database shows no recalls or technical service bulletins (TSBs) related to the engine timing cover for this vehicle generation.
Technical Specifications
Fitment is primarily for the Chevrolet Small Block V8 (Gen I) and Chevrolet Inline-6 engines used in this period. Covers are available in stamped steel or cast aluminum. Key variations include one-piece vs. two-piece designs and compatibility with different water pump lengths ('short' vs. 'long').
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.