Good point north puller. Some stock appearing engines can pass as stock by casting numbers. I've seen some div1 tractors that don't even follow that logic. Our low speed limit equalizes it somewhat, even though there's no way to restrict M&W kits and the like. Here are our club's pure stock rules (aside from the General Rules):
Barnyard 2.5 MPH
1. Any regulations not mentioned for Barnyard can be obtained from Clintonia’s general tractor pulling rules.
2. Entry: A tractor may pull in any other class if it meets the criteria. Oliver 88, M-M U, Farmall M, Massey 44, Case DC, Deere G, and like tractors may not pull in classes below 5500#. Oliver 66, M-M R, Farmall H, Allis WC, Deere B, and like tractors may not pull in classes below 3500#.
3. Rims / Wheels / Tires: Tractors must have OEM diameter & width rims. This also applies to welded rims that have been fitted to replace steel wheels. If a tractor was not originally equipped with pressed steel centers, OEM cast iron or spoke centers must be used. Use original hubs only. Tire size may be one size over OEM width for that make/model of tractor. No sharpened tires. “Road worn” tires will be inspected and are subject to track officials’ discretion.
4. Engine: Engines need to have a stock block and be naturally aspirated unless originally equipped from the factory with a blower or turbo. 10% over factory high-idle RPM (see chart) is allowed using a working governor. Stock heads, manifolds, and carburetors are to be used. Engine parts not listed in a manufacturer’s parts book for that specific model cannot be used. No electronic ignition systems or coils that vary from original shape. The only exceptions include magneto/distributor and 6V/12V generator/alternator conversions.
5. Appearance: If a part is not listed in the manufacturer’s parts book, it cannot be used. No aluminum or chrome is allowed except for exhaust pipes. All covers and guards shall be in place and made using original specs (steel or iron). The tractor seat must use stock mountings. No fabricated engine shrouds allowed. Any altered items are subject to officials’ discretion.
6. Weights & Hitch: No weights shall extend beyond eleven feet from the center of the rear axle. Hitch point will extend beyond the radius of the rear tires and 19” or less from the ground. Wheelie bars are recommended and may become mandatory.