by chuckthebeatertruck » Fri Mar 05, 2021 11:27 am
Let’s start by defining overheating – in my world; on flat heads – it involves two things:
1) Piston scuffing or seizure in bore
2) Uncontrolled detonation from high head/chamber temperatures
Many, many people want to say: “what about the bearings” and forget we are talking about an all roller motor with plain bushes on the cams when talking about Ks. They actually dislike large quantities of oil and don’t care too much about the temperature of the oil either. Roller bearings prefer a light mist of oil – plain bushes can just flat out take abuse that will amaze.
So, set the bearings aside.
Now, let’s get to controversy – despite the wive’s tale, flatheads only lose about 1/3rd their heat to the oil. No matter how hard you try . . . there is no way to positively cool the head with oil. On OHVs we can circulate oil around the heads and boost this a bit; but your antique air cooled motor is not oil cooled. It’s air cooled with a bit of help from the oil.
About now, the light bulbs pop on about how total loss systems worked. If the oil was responsible for cooling and you burnt the oil in the crankcase . . . not to mention they rolled with an average of 4 oz of oil in the cases . . . not exactly a lot of oil.
Oil coolers are often installed on plain bearing motors to allow light oils to be used and still maintain minimum viscosity. This keeps the bearings alive. We don’t have that issue here. We can basically have no oil and not seize the bearings. So, we don’t need to cool the oil for the bearings to have long life. Notice, for example, the main bearings are not positively lubricated – nor are the cams. It’s all splash, mist and hope. Not exactly good for temperature control.
The second use of oil coolers is to decrease chamber temperatures as well as extend valve train life by reducing spring temperatures. The first allows advanced timing without detonation; the second is service life.
On a flathead; there is no positive lube to the valve springs. In fact, the guides run pretty darn dry to boot. We also can’t control the chamber temperature by oil temperature UNLESS you add piston squirters.
So, it’s not always clear what the oil cooler does on a flatty. OHV yes; plain bearing, yes; roller bearing flathead – not so much. The oil won’t complain about being cooler – but it also won’t necessarily care until you’ve reached a very serious point and your pistons are already in a plastic state.
The other inconvenient fact is that oil coolers need air flowing through them to do much of anything. So, in traffic or at slow speeds below 25 or so – they don’t really do much but add an extra pint of oil to the system and create more spots for leaks. The extra pint can help – but not too much. Coolers do, however, make a lot of guys feel better and that can lead to a more satisfying ride.
Again this is specific to flatties (not just K models) – OHVs are a little different.
The final thing is that many guys new to flatheads, whether Ks or Indians or Big Twin Harleys, are freaked out by how much hotter they run. An average flathead will run 100 degrees hotter on the heads and 50 degrees hotter on the cases than a similar OHV bike.
The other big difference is that you have a long stroke and can run WFO pretty easily. It’s the burst of WFO that stick pistons. You’re now pushing more heat from the bigger displacement through the same basic parts designed for less heat. The pistons and top ring take a lot of abuse on flatheads and so when you really crank on it . . they struggle a bit. So, if you feel the motor slowing – DON”T shut it down – instead back off the handle a bit and let the intake charge cool the piston. If you’re caught in traffic – roll the left grip towards retarded. The motor tone will change quite a bit . . .
Otherwise, enjoy the bike. Keep it in tune, keep it filled with fresh oil, and you are unlikely to have an issue unless you idle away for minutes on end.
BTW – I run a 90” flathead big twin stroker with some wicked hot cams . . . I’m real familiar with trying to keep a big flathead’s temperatures in line.