If you have taken a close look at the new WKA Tech Book for the 2000 racing season, you have noticed that there are some important changes, including the way 2 Cycle engines will be checked for minimum cc's. Specifically, Section 504 of the General Tech Inspection Procedures, on page TM36, details the use of a " 2 Cycle cc Measuring Plug." But what is this "measuring plug?" Where does it come from and why do we need it?
Some time ago, WKA's Director of 2 Cycle Tech presented the WKA Board with 6 Yamaha cylinder heads, each exhibiting a different method of circumventing the cc rules. He asked the Board to authorize some type of tech procedure to address this problem. The Board agreed and eventually contracted LAD Specialties in Bridgeview, Illinois to "engineer/design" a tool for that purpose. The 2 Cycle cc Measuring Plug is currently being made to WKA specifications by LAD. These are the same folks who manufacture the 2 Cycle Tech Tool that just about everybody uses to check for legal port heights. Like the Tech Tool, it is a precision gauge, made to the same exacting tolerances. The idea, in both cases, is to make checking critical dimensions at tech as easy, as foolproof, and reliable as possible. After
all, tech is not supposed to be a contest between the competitor and the tech man. It is about making sure that everyone knows that you won that victory within the constraints of the rules.
Of course, if nobody ever bent the rules, things would be a whole lot simpler. For some time, tech officials have had concerns about efforts to get around the minimum cc's rule. As critical as compression ratio is to performance in these engines, It is inevitable that some engine builders and individual karters would look for ways to decrease the volume of the combustion chamber when the engine is running , while "fooling" the tech inspector into thinking that the volume is at least the 11 cc's specified for 100cc Yamaha's and Piston Ports, 9 cc's for 100cc Controlled's and Euro 5's.
The most blatant, and most obvious, way to cheat on the cc rule is to "side drill'; the threaded portion of the sparkplug hole. By creating a cavity part way up the plug hole, the extra cc fluid that fills that cavity can make the engine appear legal. With the sparkplug tightened in place, this illegal cavity is blocked off and the engine actually runs with fewer cc's. Consequently, more compression. Pretty sneaky, huh? A less obvious way to accomplish the same thing is to machine the dome part of the head so deeply that, when the sparkplug is tightened down, the plug protrudes significantly into combustion chamber. Now, I know that many engine builders like to
re-cut their cylinder heads with the widest possible squish band. Then, to get the required cc's, they cut the dome as deeply as possible. Sometimes this means that the sparkplug threads are significantly shorter than the sparkplug itself. That means the sparkplug extends well into the combustion chamber, effectively raising the compression ratio. Until now there has not been a good way to deal with these tech issues.
After careful study WKA has settled on the cc Measuring Plug as the best way available to keep everybody on a "level field", at least as far as cc's are concerned. The tech procedure calls for the Measuring Plug to be torqued in place in the sparkplug hole. The Measuring Plug extends exactly .625 down the sparkplug hole, allowing plenty of room to reshape the combustion chamber dome without having the plug extend into the chamber. Like the sparkplug, the Measuring Plug covers any
side-drilled holes, making them ineffective. As I said before, the Measuring Plug is a precision gauge, dimensioned exactly so that the amount of fluid it displaces itself is exactly compensated for in the ' height of the Measuring Plug. A legal head that measures 11.2 cc's without the Measuring Plug will measure the same 11.2 cc's, when measured to the top of its hole, with the Measuring Plug installed.
With the use of the Measuring Plug, suddenly the issue of side-drilled sparkplug holes and excessively domed combustion chambers is a thing of the past. Since the Measuring Plug covers any
side-drilled holes, just like the sparkplug, they have no effect on the cc reading. Since the measuring Plug reaches into the sparkplug hole just like the actual plug, if it extends into the chamber excessively it will reduce the cc's in the same way that the sparkplug does. By adopting this new way to check for legal cc's, WKA has not only dealt with the obvious cheating of
side-drilling the sparkplug hole, but they also put a stop to ever deepening cylinder head domes. They did this without having to directly address the tricky issue of combustion chamber shape.
Since the introduction of the Measuring Plug, there has been a rash of postings on the Internet regarding how it would impact 2 Cycle racing. Many of these messages begin with the gloom and doom warning that many, maybe even most, Yamaha cylinder heads out there in use today will no longer pass tech. Some go on to say that these heads are still legal, but the new tech procedure makes them appear illegal. The fact is, if it does not pass tech, however the rules define the procedure, it is not legal. It's as simple as that. Critics have pointed to the fact that, if a head is cut deeply enough that the Measuring Plug extends significantly into the combustion camber, even if the chamber has enough cc's, the air bubble trapped in the head by the Measuring Plug will make it appear too small. While it is not in strict accordance with the prescribed procedure this issue can be addressed by stopping the fluid 1 cc before the limit and backing the Measuring Plug out a couple of revolutions, adding the remaining fluid, then tightening it down. Any minor air bubble will be "burped" out of the chamber. It remains to be seen if the rules makers will allow this or not.
In conclusion, if you have any question whether your head will pass tech, it's probably a good idea to invest $25 or so and get your own Measuring Plug. According to L.A.D. the same plug should work for the new iKF Tech Rules. At press time we could not verify this with any 1KF tech officials. The Measuring Plug Gauge is available through L.A.D. Specialties or their distributors. That way you can check for yourself and be confident that you will be O.K. when you get to the tech shed. See you next