Ignition Contact Set Replacement Guide for the 1956-1959 GMC PM251
Keep your classic GMC running strong by understanding how to choose and replace your ignition contact set.
- The ignition contact set (points) is a normal wear item that requires replacement every 10,000-15,000 miles.
- Always replace the condenser at the same time as the points to ensure reliability.
- Standard Ignition is a reputable aftermarket brand for this part.
- Properly 'gapping' the new points with a feeler gauge is a critical step for correct engine performance.
Understanding the Ignition System in Your Classic GMC
The 1956-1959 GMC PM251 uses a traditional points-style ignition system. This is a simple and effective design, but it relies on parts that wear out and need regular service. The ignition contact set, or points, is the primary wear item in this system. The constant opening and closing causes the electrical contacts to burn and pit over time. Additionally, the small rubbing block that rides on the distributor cam wears down, changing the point gap and affecting ignition timing.
Symptoms of a Failing Ignition Contact Set
When the points in your PM251 begin to fail, the signs are usually clear. Ignoring them can lead to a breakdown, so it's important to address them promptly.
- Hard Starting or No-Start: This is the most common symptom. Worn, pitted, or incorrectly gapped points cannot provide the clean electrical signal the coil needs to produce a strong spark.
- Rough Idle: An uneven idle or engine shaking can be caused by inconsistent spark timing due to worn points.
- Engine Misfiring: You may feel the engine hesitate, stumble, or backfire during acceleration. This happens when the spark is weak or timed incorrectly.
- Loss of Power: A general feeling of sluggishness and poor performance can be traced back to an inefficient spark.
Choosing the Right Aftermarket Part
For a vehicle of this age, new original equipment (OEM) parts from General Motors are no longer available. Your only option is the aftermarket. Fortunately, brands like Standard Ignition have been making these parts for decades.
| Brand Tier | Brand(s) | Assessment |
|---|---|---|
| Recommended | Standard Ignition / Blue Streak | Standard Ignition, and its premium line Blue Streak, has a long-standing reputation for producing quality ignition components for American vehicles. While some classic car enthusiasts feel modern parts don't match the quality of old stock from decades ago, Standard is generally considered a reliable choice for replacement points and condensers today. |
Pro Tip: Always replace the ignition condenser when you replace the points. The condenser is a small capacitor that prevents the points from arcing and burning out prematurely. A weak or failed condenser will cause a new set of points to fail very quickly. It's an inexpensive part that is critical for the longevity of your tune-up.
Known Issues, Recalls, and Bulletins
There are no official recalls or Technical Service Bulletins (TSBs) from GMC for the 1956-1959 PM251's ignition system. The NHTSA recall database was established in 1966, long after these trucks were built. All known issues are related to the normal wear and tear of a points-based ignition system, which requires periodic maintenance as a tune-up item.
Frequently Asked Questions (FAQ)
How often should I replace the ignition points?
As a general maintenance item, ignition points should be inspected and likely replaced every 10,000 to 15,000 miles, or as part of an annual tune-up. Given their low cost, regular replacement is cheap insurance against being stranded.
What does 'gapping the points' mean?
'Gapping' refers to setting the correct maximum opening between the two contact points using a feeler gauge. This gap is critical for performance. If the gap is too small, the coil won't have enough time to build a full charge, resulting in a weak spark. If it's too large, the timing can be off, leading to poor running.
Can I replace the ignition contact set myself?
Yes, this is a very common DIY job for classic truck owners. It requires basic hand tools, a feeler gauge, and access to the distributor. The process involves removing the distributor cap and rotor, unscrewing the old points and condenser, installing the new parts, and setting the correct gap.
Should I convert to an electronic ignition?
Many owners choose to replace the original points and condenser with an electronic ignition conversion kit. These kits eliminate the need for regular point replacement and can provide a more consistent spark. However, a properly maintained points system is very reliable. The choice depends on whether you prefer originality and simplicity (points) or a more modern, maintenance-free setup (electronic).
Technical Specifications
This is a mechanical set of electrical contacts located inside the distributor. It functions as a switch for the ignition coil. The set includes the points and typically a new condenser.
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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.