KHK Oil Cooler?

Production K Models

KHK Oil Cooler?

Postby ClintsMotorcycles » Wed Mar 03, 2021 3:24 pm



I have read that the model K bikes had some overheat issues. I live in Florida and am more or less a fair weather biker. It gets HOT here sometimes. I do not like riding in the rain or if the temperatures are below about 65.

Would installing an oil cooler be something that I should consider? And, if so, plumbing it into the return oil line would be the way to go? If I were to install one, my preference would be to put it on the front safety bar fully exposed to the wind and not blocking the wind to the engine. But, since I do not have a front safety bar, any suggestions as to where I should mount it?

Thanks for any advice.

Posts: 28
Joined: Tue Sep 15, 2020 8:26 pm

Re: KHK Oil Cooler?

Postby Arvid Solli » Wed Mar 03, 2021 6:03 pm

An oil cooler is always a good idea if you live in a hot climate.
As for me, living up in the northern part of Sweden, the need for one is maybe five days a year, so I'll manage without....
I would suggest making a custom bracket low down on the front motor mount plates. You should be able to use the 3/8" bolts, and avoid drilling in the plates.
Start with strong scissors & cardboard to make prototypes (yes, you will need a few)
When your bracket prototypes start looking like something you can live with, take out a rattlecan of flat black and spray them to visualize the result.
If given thumbs up- go ahead and remake them in some mild steel.
Good luck!!
Arvid Solli
Posts: 6
Joined: Wed Feb 17, 2021 4:23 am

Re: KHK Oil Cooler?

Postby Simon » Thu Mar 04, 2021 1:49 am

Arvid, if you're anywhere near Örnsköldsvik which I visited once, you definitely don't need one lol! Lovely country, Sweden - I've enjoyed both times I've visited. Great bike culture too.
Posts: 158
Joined: Mon Oct 12, 2015 10:14 pm

Re: KHK Oil Cooler?

Postby hennesse » Thu Mar 04, 2021 11:33 am


An oil cooler is not a bad idea, but I've never heard of anyone putting one on a K-model. Oil filter, yes. And the K's came with one of those. I'd just be careful to not leave the bike running while you dash into the house, a store, or similar - it never takes as short a time as you'd expect. And if you get caught in a major traffic jam, don't sit there idling - just drive slowly down the shoulder of the road, between the lanes, or on the sidewalk. A ticket for driving on the sidewalk is probably cheaper than an engine rebuild. "But officer, I can explain..." And you can.

One problem with add-on oil coolers is that they, and the oil lines, are usually stuck in some place where they are susceptible to physical damage. A major oil leak at 60 mph can kill your engine before you even realize you have a leak. Even in a parking lot, a major leak will leave you dead in the water, until you can do some custom re-fabrication work.

Leave it stock. If it's too hot for you - it's too hot for your motorcycle.
User avatar
Site Admin
Posts: 553
Joined: Sat Mar 29, 2014 11:28 pm
Location: Warrenton, Virginia

Re: KHK Oil Cooler?

Postby Arvid Solli » Thu Mar 04, 2021 7:37 pm

Simon wrote:Arvid, if you're anywhere near Örnsköldsvik which I visited once, you definitely don't need one lol! Lovely country, Sweden - I've enjoyed both times I've visited. Great bike culture too.

Well, actually, I live 330 miles north of Örnsköldvik, in Luleå...
Arvid Solli
Posts: 6
Joined: Wed Feb 17, 2021 4:23 am

Re: KHK Oil Cooler?

Postby 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.
Posts: 58
Joined: Thu Jun 22, 2017 1:11 pm

Re: KHK Oil Cooler?

Postby panic » Fri Mar 05, 2021 6:06 pm

I still have a set of plans, drawings etc. (for an abandoned project) to put bypassed oil pressure using external lines through the upper valve cover to cool the guide, stem, spring and drain into the cam chest via a modified tappet.
[url=""]The Linkert Book[/url]
User avatar
Posts: 51
Joined: Mon Jul 29, 2019 5:33 pm

Return to K, KK, KH, KHK

Who is online

Users browsing this forum: No registered users and 1 guest