Gesammelte Weisheiten …

Race-Prepping an Oppy

  • Do not deck the heads. Flatheads need a large transfer port cross section. Flatheads need flow for power, not compression. The decks have all the same heights on Oppys. But they use different length rods to take up stroke differences.
  • Max. compression ratio schould be not above 8:1.
  • The 3-screw carburetor will probably be fine for what you are doing, it has the bigger bore compared to the 4-screw.
  • Carb mods:
  • Try to find a used aluminum flywheel. Yes, we all know they are „expensive“ if you only look at price but expensive can be determined by how well the product works and if you get what you pay for. Let me be the first to tell you that they are not „expensive“, once you buy it you will not regret it! And a stock flywheel with no govenor is a safety concert! DO NOT TURN STOCK FLYWHEEL OVER 4000 RPM! Do NOT remove the govenor! Instead, modify the spring so it will let the RPMs go to MAX 4000.
  • You will never make any power if you don’t advance the timing: of ignition as well as of valves.
    • Pull pan, cylinder heads, valve springs. Line timing marks of cam and crank, lift cam and advance it for 1 tooth. Put everything back together and re-adjust valve lash, done. Result: 360° divided by 62 teeth on cam is 5.806 deg per tooth -> the crank moves twice as the cam per stroke, so it is 12° ‚advanced timing‘ for the valves.
    • Use Timing Keys for Flywheel. ‚Advance keys‘ available for B&S kart engines, all briggs flatties use the same keys.
  • See if you can find a junk 46 cubic inch engine (find model number that starts with „46“) and rob the heads, camshaft, crankshaft, piston rods. The heads will say ‚MOD 46‘ stamped on them, and the 46 ci cam will also have a code with ‚—46‘ stamped on it.
  • Oppies come in 2 different bores: 3.4xx and 3.5xx. The Big Bores from Briggs are kinda rare. The common bore, 3.4xx, are the normal and cubic inch is done with stroke. You have 40 ci the short stroke to the 46 ci the longest stroke with a 42ci in the middle. These all take the same piston, which is the same as the 28 ci single. The difference is made up in rod length. All the blocks are the same dimensions. You can put any tins (= Kühlhaube) on any size. Using the big bore block with a 46 ci crankshaft is as big as you can get.
  • Use the oiling mods. You need those definitley with stock rods!
  • Port the intake port – and port and polish the exhaust port. Do NOT increase intake port size if not needed, i.e. port size = carb diameter. Do also NOT increase exhaust port size if you are not completely sure the port is not already too large anyways (as this is often true even though Briggs built it like this). Remember: It will only flow as much as the inside diameter of the valve seat (minus diameter of valve stem).
  • „Shave the eyebrows“ of the heads, but do not overdo it.
  • VERGISS Schlagwörter wie „Fire Slot“ und „Singh Grooves“! Ersteres wird im Oppy auf Grund von flachem Kolbenboden nicht benötigt und Letzteres ist wissenschaftlich nicht belegt (… man bilde sich seine eigene Meinung).
  • Make an exhaust header that will scavenge well -> a 2 into 1 style.
    • Why?
      • Combustion chamber scavenging happens when the exhaust gasses are pushed out of the combustion chamber by the piston. As the piston reaches the top of the exhaust stroke, it has less „push“ on the now thinner exhaust gasses. The exhaust gasses that have already left the combustion chamber that are traveling down the exhaust pipe begin topull a slight vacuum on the combustion chamber. About now the valve train has started to enter overlap and the intake is opening, but the piston has not traveled far enough to start pulling the mixture in rapidly. The vacuum created by the flowing exhaust gives the intake charge a „head start“ so to speak on the piston, thus creating a better fill. This concept works per cylinder, regardless of how many you have.
      • Exhaust scavenging, on the other hand, happens only in multi-cylinder engines with a collective exhaust system.
        As the exhaust is expelled from one cylinder through the collector it draws a vacuum on the other head pipes. When the other cylinders exhaust opens up, the vacuum in the tube helps evacuate the cylinder and increase velocity. This increased velocity and quicker evacuation in turn increase the amount of combustion chamber scavenging vacuum you have.
    • 2 into 1 header will make a broader torque curve than 2 seperate pipes.
    • Longer, smaller primaries for low end torque. Larger, shorter tubes for top end power.
    • Tube size corresponds to port size and displacement, length corresponds to rpm power band. Oversize tubes will move the power band up the rpm scale and kill low end power.
    • >1″ OD pipe per exhaust might be a starting point.
      • Example 1: I use 14″ long primaries shaped like the old Chevy Ram’s horn exhaust into a 1 1/2″ collector (90° sweep conduit), lenght of tail pipe: cut off where the paint burns off. Engine is stock, ports just cleaned up a little, and timing advanced. Rocks!
      • Example 2: 12″ Header pipes and about 18″ tail pipe.

Oil Mods on an Oppy

„The first thing I do is the crank. I remove material above the crank pins. Later you will see, with the rod on there it creates a sort of „shelf“ . The slung oil comes down and gets into the rod here:

But don’t cut the weights like I did (last picture)!


You can see where the grinding on the crank helps, leaving a „shelf“ to catch oil on top of the rod. (Disclaimer, I did not pay any attention to how I installed this rod, nor is it torqued down.It was done purely for photo use. It may or may not be installed correctly.)

Now the rods.


First I drill out the oil hole larger, I use a9/32 drill. Then I put a large chamfer on it, creating a funnel of sorts to catch more oil.


Then I drill a 3/16″ hole in the center of the cap, no chamfer here. (The hole pictured above is way too big….I dunno what I was thinking when I drilled it!)

Finally the pan.
There are 2 oil valleys that allow the oil to get down to the lower main bearing. Using a die grinder/ Dremel and a 1/4 ball grinder, I open those up just a bit.“