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1982 Honda CX 500 TC Turbo
1983 Honda CX 650 TD Turbo


Transmission Undercutting

The CX tranny is a real clunker, and occasionally it will pop out of a gear, particularly second, when accelerating hard up through the gears. There is no such technique as "snicking" the tranny up through the gears! On the CXT, shifts require a deliberate, ponderous, full tug on the shift lever. Refining this technique greatly reduces the number of missed shifts. Yet, occasionally one will still sneak in every now and then when shifting from first to second under heavy acceleration. A missed shift is merciless, especially to a pushrod engine such as ours. Factored along with this, how long do you suppose it takes to bury the tach well into the red during a missed shift or false neutral with an engine which seems to deliver its power logarithmically? Less time than one can react and let off the throttle. Besides riding more conservatively, I found that having my transmission undercut eliminated missed shifts.

For about $150, R&D Motorsports in Safety Harbor, Florida, will undercut all your gears, including shimming when necessary. Their phone number is (813) 725-2446. If you ever should have your engine out and apart, I would strongly recommend you remove the tranny and send it in for this procedure. In my experience, the undercut gears do not diminish the "clunk" in the CXT transmission, as I had hoped it would. And it is very slightly harder now to get into neutral at a stop, yet not enough to be annoying. Those said, here is where the undercut gears shine: I have never, ever missed a shift under the most extreme of conditions after the transmission was undercut. That alone is worth the bucks.

Editors Note: The contact info for R&D Motorsports appears to have changed.
(727) 447-4552.
R&D Motorsports
1632 N. Hercules Ave.
Building G.
Clearwater Fla. 33765

When having your Honda Turbo transmission undercut, make certain you specify that you would also like to have them "NOS-cut the shift drum at fifth gear". It is absolutely imperative this be done at the time of undercutting. If the shift drum is not modified by this procedure, the lateral forces on the mainshaft created by the undercutting will eventually cause the mainshaft bearing to wear. Eventually this bearing will wear enough to allow the entire clutch assembly move forward just enough to unload the clutch in fifth gear, causing the clutch freeplay to disappear under acceleration only, and the clutch to slip in fifth gear only.This whole scenario played out in my transmission. Eight thousand miles after my tranny was undercut, the clutch began to slip. After going through three clutches in rapid succession, I then noticed that the freeplay was disappearing under acceleration only in fifth gear. Mark at R&D Motorsports, who originally undercut my tranny, examined the entire transmission and came to a conclusion as to what was causing the mainshaft to move forward upon acceleration in fifth gear, thus causing the clutch freeplay to disappear and the clutch to slip. Although the mainshaft M1 gear is butted right up against the transmission bearing, Mark describes the mainshaft as "semi-floating". That is, any axial forces present in the shaft will cause the shaft to move either rearward an amount equivalent to whatever end play is present, dictated by the rear shims, or forward an amount equivalent to whatever run out is present in the transmission bearing in the front. When the shift drum is rotated into fifth gear, the shift lever slides the M3 gear dogs into the mating slots in M5. M3 is a sliding gear, and M5 is held in place on the mainshaft with a circlip. The circlip prevents M5 from moving forward on the shaft, causing it and the shaft to be one axial unit. When M3 slides into M5, the dogs and slots engage only about 75% of the way. That is, the two gears are not butted right up against each other. This is normal operation. When dogs and slots are undercut then placed under load, an attractive force is created between the two mating gears. Mark determined that the whole problem was created by the fact that M3 does not normally fully engage with M5. When the gears are not undercut, this is not a problem, because the two gears are not attracted toward each other, hence no axial force is created. However, once the gears are undercut, while M3 is normally keeping a little distance from M5, the attractive force present due to the undercut dogs tries to pull M5 toward M3. Since M5 is held in place on the mainshaft with a circlip, that forward force pulls the entire mainshaft forward as far as any run out in the forward bearing will allow. This explains why the clutch freeplay disappears only upon acceleration, and only in fifth gear. Any axial forces present between in the other gears would cause the shaft to move rearward, not forward, hence the clutch plates would not be unloaded.

The solution? To make that M3 gear get all the way over into the M5 gear. This is accomplished by "NOS-cutting" the shift drum at fifth gear, which allows slightly more throw in the shift lever, thus allowing the M3 dogs to engage 100% with the slots in M5. In this way, with the two gears butted up tight against each other, the attractive forces between the two gears will cancel each other out, and thus null the axial force on the shaft. Although Mark has performed this shift drum lobotomy before, he says he is mildly surprised, because he normally would see these types of forces created and becoming a problem "only in high power sport bikes!". I guess he did not expect an aging 650cc bike to create such large forces within! "NOS-cutting" is his in-house term for the process, because this is a modification they must perform as a matter of course on undercut transmissions which are known to be going into Nitrous Oxide boosted engines! Anyway, NOS-cut it, and forget it.


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