CB650 SC Oil Cooler for CB750's

Posted by sgtslag on Apr 14, 2012; 5:41am
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/CB650-SC-Oil-Cooler-for-CB750-s-tp3909678.html

A member of this forum privately asked me about my oil cooler system I installed on my '79 K model.  After doing some research on BikeBandit.com, comparing O-ring sizes, I discovered that the Oil Filter Housing on both the 1983 CB650 SC's engine, and the Oil Filter Housing on the CB750 K's, use the same O-ring; they also use the same oil filter.  This indicates that the Filter Housings, are identical, and interchangeable.

To boil it down, it is possible to take the Oil Cooling System off of a 1983 CB650 SC engine, and mount it onto a CB750K/C/S engine, as long as you get the long Filter Housing Bolt off of the CB650 SC.  The SC's bolt is longer, and it has a different P/N; this is the linchpin of the adapting process:  I assume the SC's Bolt will be a direct match, for threads, to the CB750 models' Oil Filter Boss, but I have not verified it.

The SC's Oil Cooling system consists of an adapter plate, with fittings for the oil lines which circulate the engine's oil through the radiator, then return it to the filter, and the engine.  The SC Oil Cooling Systems are frequently available on e-Bay for reasonable prices (usually less then $50).  The bolts, however, tend to be less commonly available.  Oil pressure throughout the Cooling System, will be the same as the oil pressure within the engine:  max. oil pressure, as per the FSM, is 72 PSI @ 7.000 RPM, for the 1979 CB750K.  I do not know the max. oil pressure on the SC engine, but it likely is very similar, which means there is no risk of excessive pressures.  My CB900C Radiator has performed flawlessly, on my CB750K, for over a year, and 5,000+ miles, without any leaks, bulges, or other issues.  I was advised that the CB900C systems were not designed for this much pressure, but that has proven to be untrue.

To install the SC's cooling system, remove the 750's Filter Housing, install the SC's adapter plate, and radiator (fashion a mounting assembly out of aluminum bar stock, two U-bolts to secure it to the frame down tubes, behind the front wheel, and some nylon locking nuts, to secure the mounting plate to the fork tubes).  Use the Filter Housing Bolt from the SC, to secure the whole to the engine.

This will circulate all of the engine's oil through the radiator, for cooling, as long as the engine is running.  Without an Oil Cooler, the CB750's will run their oils at around 250 F, at highway speeds.  With an Oil Cooling System, the engine oil will run at an average of 210 F, with an ambient air temperature of 80 F.  For faster warm-up's, and better thermal control, an oil thermostat can be inserted into the oil lines, between the radiator, and the adapter.  Installing an Oil Cooling system, will extend the life of the engine's seals, gaskets, and other components, extending the life of the engine, overall.

When performing an oil/filter change, add the same amount of oil, as per the FSM.  The Oil Cooling System's volume is small enough that it will not require additional oil be added to the engine.  Filters should be replaced with each oil change.  The two, large, O-rings (89 x 4.5), should be changed each time, as usual (note that only one, large O-ring, is typically included with the new Filter, so extra O-rings should be ordered, in advance, from a supplier such as BikeBandit.com).

If I had known that the SC's Oil Cooling System could be adapted to the CB750's this easily, it would have saved me a very great deal of time, as well as one-half of the cost of my system, adapted from a CB900C's system...  Cheers!