2006 Honda Civic Si Transmission Rebuild - ATS LSD & Final Drive Gear

2006 Honda Civic Si Transmission Rebuild Civic Rear

We've been singing the praises of Honda's latest version of the Civic Si for a few months now. But like any automobile made for the masses, there is room for improvement, especially for those wanting to race the platform seriously. In the interest of production, safety, or profit, manufacturers make all kinds of compromises in engineering a new vehicle-that's just the way the game works. The Si, while bestowed with much more than its predecessor, is no exception.

That's not to suggest a stock Si isn't already a good starting point, however, because it is. With K-series power and a limited-slip differential in its O.E. configuration, Honda's latest VTEC screamer has gobs of potential. Getting it from a street car to a competitive racer really doesn't take all that much, relatively speaking.

Not surprisingly the gearbox from the new Si uses a case that appears to be the same as those found in previous K20-powered offerings, namely the last gen. Si and all RSX's. Before the GearSpeed guys can crack it open, it has to be drained of fluid and the selector placed in neutral.
Not surprisingly the gearbox from the new Si uses a case that appears to be the same as th
The speed sensor and this 14mm reverse gear flange bolt are then pulled off
Next up for removal: these three bolts. Each conceals a spring and a shift fork detent ball
Next up for removal: these three bolts. Each conceals a spring and a shift fork detent bal

An area that could use fortifying, especially for the circuit, is the gearbox. We know the factory-included limited-slip differential has many Honda heads talking. Apart from that one bonus, the transmissions are basically the same ones found in the current RSX-S. With just a couple of mods, though, the tranny can be turned into a much stronger link in the power delivery system.

This was the impetus for our most recent endeavor with GearSpeed Hi-Performance Transmissions in Rancho Cucamonga, Calif. We got our mitts on a gearbox from an '06 Si that was headed for a racecar and had the crew at GS install a final drive ring and pinion from ATS that will take the ratio from 4.76 to a remarkable 5.06:1.

GS tech Jason Wishmeyer unbolts and removes an interlock bolt and the shift arm assembly next. Ideally the transmission shouldn't sit on a flat surface, as the input shaft sticks out from the casing and would bear the entire load of the transaxle. This is more important during assembly, so when it comes time the bell housing will be propped up to avoid placing weight on the shaft
GS tech Jason Wishmeyer unbolts and removes an interlock bolt and the shift arm assembly n
We didn't notice this at first, but the guys at GearSpeed pointed out that newer gearboxes seem to be gravitating toward speed sensors that employ a magnetic pickup, like this one for our trans. The pickup method is preferred because it eliminates any chance for a gear driven cable, the old method, to bind
We didn't notice this at first, but the guys at GearSpeed pointed out that newer gearboxes
The 17 fasteners holding the two halves of case together are zapped with an impact wrench. Since this is a brand new box the case will take some prying to get apart, but first a snap ring at the end of the countershaft will need to be opened up.
The 17 fasteners holding the two halves of case together are zapped with an impact wrench.

While the box is open they'll also plug an ATS LSD, primarily because we're not content the factory diff will hold up to the extremes of racing. The ATS diff can supposedly handle up to 1,000 hp. Finally, an Exedy clutch and flywheel will be installed once the tranny is reintroduced to the motor, which should ensure a smooth transfer of power to the gearbox.

Honda's 2.0-liter
K-series Transmission Gear Ratios

	K20A3	K20A2	K20A3	K20Z	K20Z3
	'02-up	'02-'04	'02-'05	'05-up	'06
	Base RSX	RSX-S	Civic Si	RSX-S	Civic Si
1st	3.27	3.27	3.06	3.27	3.27
2nd	1.88	2.13	1.77	2.13	2.13
3rd	1.21	1.52	1.21	1.52	1.52
4th	0.92	1.15	0.92	1.15	1.15
5th	0.74	0.92	0.74	0.92	0.92
6th	N/A	0.74	N/A	0.74	0.66
Final	4.39	4.39	4.76	4.76	4.76

A sealing bolt at the end of the case is removed and underneath is a snap ring that holds the countershaft in place. To remove the snap ring and separate the two halves of the case, Wishmeyer expands the ring using a set of snap ring pliers while pulling/prying the casing up so that the snap ring is no longer holding the bearing
A sealing bolt at the end of the case is removed and underneath is a snap ring that holds
Unlike older transmissions, K-series gearboxes (and likely most of Honda's newest engines) all use plastic oil guide plates like this one. We're guessing it's a manufacturing cost-saving measure
Unlike older transmissions, K-series gearboxes (and likely most of Honda's newest engines)

At this point the main and countershafts, along with the shift fork assemblies, can be removed from the backside of the bell housing together, as can the pumpkin. Everything slides straight up and out of the transmission case with minimal effort. Wishmeyer warns, however, that some may have to hold the housing down and wiggle the assembly slightly as the gears are coming up.
At this point the main and countershafts, along with the shift fork assemblies, can be rem
When you order an ATS ring and pinion set for the DC5, this is what you get: an appropriately toothed and shot peened countershaft and ring gear for a final drive ratio of 5.062:1 and a bearing for boxes with a stock ratio of 4.389:1 which we won't need. Remember: a higher number means a lower, or shorter, gear, and short gearing gives quicker acceleration and moves you quicker through the gears. But because the engine must turn faster, gas mileage and top speed are lower. Tall gears, on the other hand, give longer acceleration and a higher top speed at the expense of quickness
When you order an ATS ring and pinion set for the DC5, this is what you get: an appropriat
In conjunction with the final drive kit, we also sourced an ATS limited-slip differential to replace the OE Honda diff. ATS says its diffs are race proven, which is mainly why the factory locker is being kicked to the curb
In conjunction with the final drive kit, we also sourced an ATS limited-slip differential

The reverse shift fork and lock cam are both unbolted and removed, as is the idler gear and shaft.
The reverse shift fork and lock cam are both unbolted and removed, as is the idler gear an
If we were just replacing the differential, we'd have to first get a bearing puller to remove the bearings from the top and bottom of the pumpkin, like Wishmeyer is doing here. If the gearbox is older, new bearings are probably in order when it goes back together.
If we were just replacing the differential, we'd have to first get a bearing puller to rem
The countershaft gears are then transferred to the replacement ATS shaft. The main shaft does not need to be taken apart for this procedure. Additionally, at a couple of points during the reassembly the synchro hubs need to be locked into position with a press and a collar, which is what Wishmeyer is attempting here.
The countershaft gears are then transferred to the replacement ATS shaft. The main shaft d

The other step to replacing only the diff is unbolting the ring gear from it. We're replacing both, though, so these last two steps are just for illustration
The other step to replacing only the diff is unbolting the ring gear from it. We're replac
Wishmeyer removes the bolt at the end of the counter shaft using a 12mm hex socket and impact gun. The fastener is a left-hand reverse thread that keeps the counter shaft gears secured.
Wishmeyer removes the bolt at the end of the counter shaft using a 12mm hex socket and imp
Everything begins to come back together now in the reverse order it was removed, as Wishmeyer oils up the bearings and lubes up the gears and pumpkin (not pictured). Again, if this were an older gearbox all seals and bearings would probably have to be refreshed.
Everything begins to come back together now in the reverse order it was removed, as Wishme

The pumpkin is placed in the back of the bell housing, and then Wishmeyer does his best to assemble the shafts and forks together to mount them as single unit. Like most Honda trannies, that's pretty much the only way to install the shafts. Following this the reverse shift fork, reverse gear shaft, and reverse idler gear are returned to their former locations.
The pumpkin is placed in the back of the bell housing, and then Wishmeyer does his best to
With the box's guts installed, a bead of Hondabond seals up the two halves of the case, as well as the mounting surface for the shift arm assembly. Like any good trans builder, Wishmeyer keeps track of where everything went and makes sure all the bolts and fixtures are properly replaced on the case.
With the box's guts installed, a bead of Hondabond seals up the two halves of the case, as
The final piece to getting power to the ground in this project is the Exedy clutch kit, consisting of a twin-plate pressure plate, six-puck cerametallic clutch disc, and lightweight flywheel. The lightened flywheel will reduce load on the rotating assembly, while the pressure plate and disc should ensure a solid, uninterrupted transfer of power from engine to drivetrain. Now let's go racing!
The final piece to getting power to the ground in this project is the Exedy clutch kit, co

CONNECT
Exedy Clutch 8601 Haggerty Rd South Belleville MI 48111 800-346-6091 www.exedyusa.com	ATS USA 527 Van Ness Ave Torrance CA 90501 310-320-5751 www.ppi-ats.com
Gear Speed Inc. 8727 Rochester Avenue Rancho Cucamonga CA 91730 909-476-7252 www.gearspeedinc.com