WRENCH TECH: HONDA CRF450 TRANSMISSION UPDATE

One of the nifty features on the 2002 Honda CRF450 four-stroke is the separated oil cavities. Unlike a normal engine, the Honda has distinctly different oil compartments for top-end oil and transmission oil. This is a great idea, although by no means new. By separating engine oil from tranny oil, a CRF450 owner can use specialized fluids for each task. There is no cross contamination from sheared-off clutch particles, gear flashing and valve seats.
What’s the downside? The available pool of oil is greatly reduced. Instead of having 1500cc of oil running through the engine, filters, frame and pumps, the CRF450 has 650cc of oil in the engine and 670cc of oil in the transmission. Both quantities are more than adequate for the job at hand, but lower volumes demand increased vigilance by the owner. Normal blow-by on a YZ426 will not make much of a dent in the overall volume of oil, but careless maintenance will allow the smaller volume of CRF oil to become critically contaminated.

THE CRITICAL TRANNY INTERFACE

In addition to having separate oil cavities, the 2002 Honda CRF450 also uses a new method of oil distribution for the clutch and transmission gears. In order to reduce overall gearbox drag, Honda engineers incorporated a lower oil cavity for the clutch than for the gearbox. The idea, from a layman’s point of view, is to bathe the clutch in oil while only dipping the gearbox’s feet in it. Thus, the transmission gears are raised up onto an elevated platform in relationship to the clutch. There is no doubt that this is a performance-enhancing idea, because it keeps the spinning gears from having to slog through a giant pool of oil-and less drag equals more power.
There are three small catch basins on the clutch side of the CRF450 engine. These catch basins transfer oil from the clutch to the primary gears and then into the transmission cavity. The source of transfer is the paddle wheel effect of the clutch. Since the clutch is submerged in oil, when it spins, the oil is slung into the other cavities where it lubricates the gears and then spills back to the clutch basin through a transfer hole. As the engine runs, the oil in the clutch cavity is slung up to the transmission basin and the process repeats itself.

SO WHAT’S THE PROBLEM?

It is possible for the transmission cavity to come up short on its oil allotment. How so? The CRF450’s total oil capacity is 670cc at assembly and 590cc at draining (some oil stays in the gearbox). Since oil is added to the transmission through the filler cap on the clutch cover, the CRF engine will only accept 400cc of oil before the oil runs out of the CRF’s check hole. If you stop filling the CRF when the oil runs out of the check hole, it will only have 400cc of oil in it instead of 590cc. Once the engine is started, the oil will be slung up to the tranny, but it will not be enough oil. What will happen? In a worst-case scenario, a gearbox failure. In a best-case scenario, failure of the kickstart idler gears.

So, what should you do? Here is a possible solution. When you are changing the CRF’s transmission oil, fill the clutch up to the check hole and then start the engine and let it run for three minutes (on the stand). Turn the engine off and wait three more minutes. Now, refill the clutch cavity with the additional oil.
A better fix would be to measure out at least 600cc of oil into a graduated beaker and then not ride your CRF450 until you get all 600cc into the tranny. If you have split the cases or cleaned the cases out with solvent, use 670cc.