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Hello,

I was looking thru some spare spark plugs I have and I found 2 diiferent lengths of threads. See image.

The model of the engine is a B&S 31P677-3373-G2

From Sears the spark plug is this: Part #491055S and here is the write up:
Lawn & garden equipment engine spark plug
Part #491055S
The manufacturer no longer makes this part, and there's no substitute part

I think the one with the shorter threads I am NOT using, and purchased by mistake.

I don't know if I found the one with shorter threads does not work, this goes back more than a year.

The write at B&S says this:

Briggs and Stratton Spark Plug​

491055S
$4.09
Genuine Briggs and Stratton replacement part for small gasoline engines. This OEM part ensures proper fit and performance to maintain the life of your Briggs and Stratton engine.

• Provides a path for electrical energy from the ignition coil to create a spark
• Maintains an optimal temperature to prevent fouling and pre-ignition
• Designed to withstand high temperatures and pressures
• Replaces Old Briggs # 491055, 594056, 805015, 72347GS , 72347
• Used on Most 3-Cylinder Liquid Cooled Briggs Engines
• Designed specifically for outdoor power equipment

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Then at AMa it displays this:

• OEM Champion Spark Plug for Briggs & Stratton 491055, 491055S, 491055T OHV Engine
• Specs - Refer to images 2 & 3 for more information
• Includes - (1) Spark Plug; comes as shown in the first image
• Please confirm your OEM part number, along with your unit's model, spec, type/code, and serial number to ensure this is the correct part for your unit. This part may replace multiple OEM numbers, so verifying every detail is important for a proper fit. If you're unsure or have questions about compatibility, feel free to contact us!

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One thing is the write up at B&S does not mention OHV engine. I have an OHV engine.

Either way B&S and Ama both point to the same long threads spark plug ( are they exactly the same? I don't know, they look like it )

But the question I have is, "will the one with the shorter threads work too?" Right now the engine is running very well with the longer threaded plug, I am not swapping plugs to see what happens.

But does anyone know why some plugs have shorter thread length? Does it depend entirely on the design of the cylinder head? I wondered for more than 20 yrs, but never asked.

Thanks,
Jim

Often times, equipment manufacturers will assign their own part numbers to spark plugs, but the plugs are produced by Champion, ND, NGK or any variety of plug manufacturers according to their design specifications.

Use the Champion RN12YC plugs in this engine gap at .030 inch. These are a long thread reach for these OHV engines.

The short thread reach plugs were use mostly on the older flat head engines.

To answer your question, the plug length is designed for the cylinder head thickness but also the heat range of the spark plug is important to make sure it can fire under harsh conditions. The Champion and NGK plug charts online explain a lot of this in more detail.

A plug that has shorter threads may fit but work poorly because it will not be far enough inside the combustion chamber.

Auto Doc's said:

Often times, equipment manufacturers will assign their own part numbers to spark plugs, but the plugs are produced by Champion, ND, NGK or any variety of plug manufacturers according to their design specifications.

Use the Champion RN12YC plugs in this engine gap at .030 inch. These are a long thread reach for these OHV engines.

The short thread reach plugs were use mostly on the older flat head engines.

To answer your question, the plug length is designed for the cylinder head thickness but also the heat range of the spark plug is important to make sure it can fire under harsh conditions. The Champion and NGK plug charts online explain a lot of this in more detail.

A plug that has shorter threads may fit but work poorly because it will not be far enough inside the combustion chamber.

Click to expand...

Thanks so much for the detailed answer and the I think you said a 12yc that comes to mind but whatever you said that is the one that I either have or have used in the past and for other Briggs& Stratton engines even I believe 14.5 horsepower OHV engine but that's all good to know thanks again.

Jim

Actually the shorter thread spark plugs that are depicted in that picture might be for for a machine I purchased that has a twin cylinder flat head. I probably bought it for that machine and completely forgot.