Browse Categories
Benchtop Basics 101>Episode 3>

Benchtop BasicsNo. 3

Sparky come home

We receive many tech calls and E-mails about spark plugs, and there seems to be quite a bit of confusion about the difference between long and short plugs, and which plug should go into which engine.  Polini has changed over to using only long plugs now in their engines, but many older bikes still remain in the field, and many have had their plugs replaced with ones incorrect for the application.  New owners of used bikes may not know what plug originally came in their motor when it comes time for replacement.

The tech tip below explains with pictures the differences between a short spark plug tower head and a long spark plug tower head (water-cooled engine examples used for illustration).

The Long and the Short of it...

long-short.jpg (156183 bytes)

On the left is a short-tower head (standard mount-style), and its correct standard plug, the L82C

On the right is a long-tower head (rubber mount-style), and its correct standard plug, the N2C

As you can see by the picture, the short plug head (left) has approximately 8 internal threads on it (measuring just over 11 mm), and the long plug head has approximately 12 internal threads (over 17.5 mm).

Good boy, Sparky!

long-short-good.jpg (201858 bytes)

When used with their respective proper sparkplugs, the protrusion depth of the electrode into each combustion chamber is correct.  The electrode is unshrouded and free to easily ignite the mixture, and yet does not protrude too far, endangering the piston.

Bad boy, Sparky!  NO!!!

long-short-bad.jpg (218659 bytes)

When the long plug is inserted into the short-tower head (left), the plug protrudes so far into the cylinder that it will easily collide with the piston, doing considerable damage.

When the short plug is inserted into the long-tower head, the electrode is shrouded, and would not be able to easily light off the fuel mixture, resulting in poor flame propagation, and significantly reduced power output.

sparkplug2.gif (1275 bytes)

For more information on spark plugs , see the TechFAQ!

This information is the intellectual property of Mid-South Minimoto and may not be reproduced, in whole or in part, unless authorized with the express written permission of Mid-South Minimoto.
Copyright 2000-2004 - all rights reserved.

This information is provided in good faith as a guide. We are not responsible for any typographical or informational errors or omissions.  Due to the many mechanical and technical variables, this information is to be used only as a guide, and is in no way guaranteed to be accurate or applicable to your engine, your bike, or your world in any way.

                 Follow MidSouthMini on Twitter