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NC30/NC35 HRC Oil Cooler

(and Radiator/Exhaust Header Clearance Issues)

June 24, 2004

(click on image for a larger version)

HRC Oil Cooler Parts Diagram
The HRC oil cooler is pictured at right. The cooler goes between the engine and the oil filter, cooling the oil directly with the bike's cooling system. Click on the picture and a clearer image will open in a new window - then read on.

The HRC oil cooler includes a new cover for the water pump (2), hoses and clamps (3)(4)(7), a heat exchanger (1), a special bolt for the heat exchanger (5), and a seal (6). The parts are still available from Honda, but tend to be expensive. As an alternative, G-Force Performance Center used a Honda CBR600F4i oil cooler on my bike, and modified the water pump cover to allow plumbing the hoses rather than buying the Honda HRC cover. The F4i comes stock with an oil cooler installed, so the parts should be commonly available at junk yards, or from Honda anywhere in the world. Oil coolers from other model Honda motorcycles may work as well.

The following series of pictures show how the oil cooler fits, and explains some of the clearance issues involved.

F4i Oil Cooler, Hoses, and Radtec Radiator
Pictured at right is my 450cc engine with the oil cooler installed. Note, the radiator fan is removed. Also, I am using after market radiators from Radtec (bought from RLR Motorsports). Only the lower radiator is shown. It is a close fit.

With the oil cooler installed, there is no room for a stock radiator fan because the oil filter is pushed forward by about 4 inches. For a race bike, this doesn't matter. Also, the Radtec radiators do not allow mounting a stock fan anyway.

Water Pump Cover
The stock and HRC water pump cover are the same, except the HRC cover has fittings for external hoses for attachment to the oil cooler. To mimic the HRC water pump cover, two holes must be drilled, and appropriate fittings added for the hoses. The smaller hoses shown in the picture are the hoses and fittings that were added.

The first hole to be drilled out is the drain hole on the stock cover. The hole must be drilled to a larger diameter and tapped to allow a screw in hose fitting to be installed. This hose is the out going water hose. The second hole is drilled on the flat spot below the center intake pipe. The flat spot is there for exactly this purpose (the same casting is used for both the stock and HRC parts). The hole is tapped and a fitting is installed.

Oil Cooler Hose Routing
To install the oil cooler, remove the oil filter and the threaded steel pipe that the oil filter screws onto. Use vice grips, or other clamping type pliers. Don't worry about the threads on the pipe. This part is no longer needed. The oil cooler is installed where the filter goes, oriented as pictured, and the special bolt is used to hold the cooler in place. Don't forget to install the rubber seal (also an F4i part) between the cooler and the engine. The torque setting is 47 lb/ft (according to the Honda CBR600F4i manual). The oil filter is then installed onto the special bolt like normal (this bolt has threads for the oil filter to screw onto).

The F4i oil cooler hose fittings aren't exactly the same as the NC30/NC35 HRC oil cooler fittings (see the HRC picture above for comparison).The F4i fittings come out at a wider angle, which requires the hoses to be routed as shown in the picture. This is of little consequence as there is plenty of space for routing. The main issue is the edge of my lower cowling's vent opening interferes with the upper hose. It would bend and pinch the hose, preventing water flow. I modified the cowling to solve this issue. I also bent the upper hose fitting on the F4i oil cooler down a bit to get the hose fittings closer together.

Upper View of Oil Cooler
Title Clearance with Radiator
These pictures show close-ups of the oil cooler and a better view of the hose connections to the oil cooler.

The oil filter is moved forward about 3 inches, almost touching the lower radiator, and leaving no room for the stock radiator fan.

Also, note how close the Radtec radiator is to the header pipe. The Radtec radiator is wider and a little thicker than stock. Pulling the radiator back (at the bottom where it mounts to the engine) allows the radiator to fit inside the lower cowling, and to make sure it clears the front tire under hard braking.

View of Oil Filter Clearance to Header Pipe
It is impossible to use this type oil cooler with straight header pipes. My bike has the stock, "bowed" headers. The bow is to allow room for the radiator fan, but also it allows room for the oil filter when using an oil cooler. This may be an issue for some race pipes as several race pipes are made assuming there is no need to make space for the radiator fan. Hence, they are straight, and don't allow room for an oil cooler either.

But, the stock headers don't clear the filter completely either. Depending on the tolerance of the outer diameter of the filter, and the fit of the left header pipe, the header may touch the filter. I finally took a ball peen hammer and pounded a dent into the header pipe to clear the oil filter. It didn't take much, but now there is about 1 mm clearance between the header and the oil filter. Every time I mount a new oil filter, or re-mount the header pipe, I check this to make sure the gap is OK. The temperature of the pipe is so great that if it touches the filter, it will melt a hole in the oil filter with disastrous results. Picture a pin-hole sized stream of oil spraying onto a hot header pipe. Can you say "huge white cloud of smoke!!" I've seen something similar happen at a race start. It's amazing nobody crashed. But, with a 1mm gap, the heat isn't an issue.

This picture also shows how close the lower radiator hose is to the header pipe (due to my Radtec radiator), but this clearance isn't a big issue. A wrap of fiberglass tape protects the rubber hose from excessive heat.

View of Header Pipe to Lower Radiator Clearance
Here's a picture of the right side of the bike, showing how close the Radtec radiator is to my right header. Again, this is because the radiator is pulled back further than a stock radiator. As long as they don't touch, it is fine. I may eventually wrap the header pipes in fiberglass wrap to prevent the headers from heating up the radiator, but because of the bow of the header pipes, they are close to the outside (non-cooling part) of the radiator. The radiator fins are not near the header pipe, so there isn't much heat transfer from the header pipes to the radiator fins.

This is a bigger issue with race pipes with straight headers. The straight headers run more toward the center of the radiator, right next to the cooling fins. This is bad because of heat transfer from the header pipes to the cooling fins, and also because there is a possibility the header will melt the thin aluminum and cause a hole in the radiator. I have seen this happen to an NC35 racer I know.


Unfortunately, one of the problems with after market parts is they often require special effort to mount them and prevent damage or other issues. Because of the tight space in the front of the engine, using aftermarket radiators, exhaust systems, and an HRC style oil cooler can be very problematic. It is important to verify that the header pipes do not touch the oil filter or radiator. Similarly, the radiator must be placed so the front tire doesn't touch as well. Contact between any of these items may cause an unfortunate early end to your race, or worse, a crash.

The good news is, the Radtec radiators, and a Honda F4i oil cooler are much cheaper than the real Honda HRC parts, and more commonly available as well.

Last Updated: Mon, Jun 28, 2004

Copyright © 2003 Michael Lohmeyer, All Rights Reserved