Re: My Nighthawk: 750SC

Posted by sgtslag on Jun 03, 2013; 1:17am
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/My-Nighthawk-750SC-tp4041475p4041679.html

Here is some information on the CB750/900 oil cooling systems (I added a CB900 oil radiator to my CB750K, with a thermostat)...

The CB900 oil pan, and pump, can be bolted onto the CB750's.  The CB900 has a dual-circuit oil pump:  one circuit pumps oil, from the pan, directly to the engine; the other circuit sucks oil from the pan, and pumps it directly to the radiator, no thermostat, always pumping oil through the radiator, regardless of oil temperature...  They have to be cold-blooded as hell -- that radiator sheds a lot of BTU's!

Your options are to install an OEM CB750 oil pan and pump, which will eliminate the radiator system, and return it to factory design.  Secondly, you could install an oil thermostat, like I did, but you would still need to cover/mask the radiator in cooler temperatures (see below).

My 1979 CB750K had a maximum oil pressure of 72 PSI, at 7,000 RPM -- worm clamps won't hold that kind of pressure!  Use T-bolt clamps:  worked perfectly, all speeds, all loads (heavy saddlebags, with a passenger, at highway speeds).  I took the CB900 radiator, cut the hoses, and I inserted an oil thermostat (Perm-Cool Oil Thermostat):  below 180 F, 90% of the oil bypasses the radiator, routed directly back to the engine; at 180 F, 95% of the oil goes to the radiator, with 5% bypassing it, returning directly to the engine.  Below 60 F, the oil temperature never got above 180 F, so the radiator needed to be covered to get the oil up to minimum operating temperatures (180 F).  The oil cooler kept the oil at 210-212 F, under all riding conditions and temperatures -- loved it!  Oil ran at 250 F, sans cooler, which is the thermal limit of conventional oils -- also hard on seals, gaskets, rubber, etc.

Running the oil at 180-210 F, is far better than the OEM design of 250 F.  Check your oil temperatures before you pull that cooling system:  cover the radiator, and run it on the highway for 20-minutes (check oil temperatures by inserting a thermometer in the dip stick hole, if possible), note the air temperature; then uncover the radiator, and re-test, compare results to make an informed decision.  The thermostat works well, except when the air temperature drops below 60 F.  Installing an oil temperature gauge can help you run your engine at its best (install the sensor in place of the oil pressure idiot light's sensor, beneath the carbs, on top of the transmission), covering the radiator when necessary, uncovering it when the oil gets warmer.  Just food for thought.  Dropping the oil temperature from 250 F, to around 210 F, can double the life of the engine...  Cheers!