Ste
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Registered: 5th Mar 03
Location: Taif, Saudi Arabia
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Thanks to Dave A for the pictures and guide.
Changing the final drive (FD) in the gearbox is a common mod in the motorsport world but no so much in the modded car scene, mainly due to people thinking that gearboxes are complicated. Well they are to be truthful but with the small block gearboxes (f10, f13 and f15) swapping the FD is a pretty simple job to do.
It is worthwhile noting that this is also a great modification to the lazy old C20XE engines, but far more difficult due to the pinion gear not being removable from the gear cluster shaft (read on and you will see what I mean)
so what are the benefits of swapping the final drive? Basically, the final drive is the last gear that the gearbox drive sees before power goes through the differential and to the drive shafts. the final drive is always in gear it is never disengaged, it is even in use during reversing. With this in mind, by adjusting the ratio (amount of turns it makes per revolution of the engine) it will affect the way the gearbox delivers the power. The higher the ratio of the FD the shorter it is through the gears, so it will lower the top speed of the car but it will also get there a lot faster!
Final drives compromise of 2 cogs, the bigger one is known as the 'crown wheel' and the smaller is the 'pinion'
to calculate the final drive, count the number of teeth on the crown and divide this number by the number of teeth on the pinion. this will give you the FD ratio. for example: a 4.18 FD has 71 teeth on the crown and 17 teeth on the pinion, so 71/17= 4.176 rounded up its 4.18.
to begin, find out what gearbox and ratio you have by looking at the end-plate and just below it to the left on the alloy part of the box there is a code, for example mine was: ********F13 C 3.74 meaning my box was an f13 close ratio with a 3.74 FD.
I got my 4.18 FD off ebay for about £60. you can get them from f10 and f13 wide ratio gearboxes as fitted to mk 3 astras, novas, some 1.4 16v corsas etc... but only the wide ratio models.
Firstly inspect the new FD and make sure there's no damage to the teeth. if all is ok, then the fun can begin! Its not a hard job to swap the FD over, just time consuming and a bit fiddly at times. Start by moving things out of the way such as the SAI pump, PAS bottle etc. then disconnect the pin that holds the gear linkage to the selector fork on the gearbox. it makes it easier later to have the car in neutral when you do this. when this is done, remove the selector assembly, its held in place with 4x 10mm bolts. when you take it out its a bit fiddly but it will come out with a bit of twisting and wobbling. as with any engine or gearbox maintenance, don't ever force anything. When looking down at the gearbox you should now have this:
now get the car up on axle stands and remove the road wheels.
Next job is to remove the diff cover. this is held in place with 11mm bolts. make sure you have a big box ready to catch the oil! it will go everywhere!
by using a clean box to catch it in you can reuse the oil (if its new) or otherwise have 1.8 litres of 75w/90 gear oil ready.
leave it to drain, it takes ages. while its draining remove the driveshafts.
once its all drained, move to the passenger side of the gearbox. on the end of the box there's a plate, its held on with 13 and 11mm bolts remove just the 13mm bolts at this point (shown in red below). set back from these bolts there is also 4 more 13mm bolts. (2 above the endplate, 2 below shown in yellow below) remove these and now the gear cassette is loose from the bellhousing, have the oil tub ready to catch the oil again, wiggle the end of the box a bit it may need a few taps with a rubber mallet or a block of wood but it will come free and empty oil everywhere. Pull the cassette out so there is about 5-10mm gap between it and the bellhousing. this will allow the oil to drain fully.
NOTE: in this pic I have removed the endplate by taking out the 11mm bolts, you do not need to do this at this point.
while that's draining the oil its time to get the diff and crown wheel out. if you look where the passenger side driveshaft goes into the box you will see a big castellated nut, its held in place with a 10mm bolt holding a toothed plate. mark the position of the teeth where they sit next to the locking plate so it can be replaced in the same position later, I marked mine with engine marking fluid (tippex)
remove the bolt and plate and using a bit of angle iron across the teeth remove the castellated nut, like so:
this is how the angle iron sits over the teeth and what the nut looks like once removed:
it seems to take forever to get off, but it does eventually. when you remove it from the gearbox, have a helper ready to catch the diff and crown wheel as it will now be loose. remove the diff and crown from the box and inspect for wear. then clamp it in a vice:
remove the crown wheel (15mm bolts) and put the diff to one side:
take the new crown wheel, ensure the mating faces are clean and dry and bolt it onto the diff. using a torque wrench tighten the bolts up to 100nm.
now that's the diff and crown wheel done
move back to the cassette and it should now be fully drained. The cassette will slide out from the gearbox without too much hassle but may need a wiggle from side to side. it may also be necessary to lower the gearbox by supporting it with a jack and taking out the 2 bolts from the gearbox to body mount and lowering it a bit.
when its removed it should look like this: (you can also see the gear selector we removed earlier)
the bit we need to remove is this top cog, the pinion:
remove it by taking off the circlip, then it may slide off easily or if you are less lucky you will need a small puller to do it:
(take note of which way up the pinion is before removing it, and replace the new one the same way up)
replace it with the new pinion, using a bit of wd40 to ease it on. if its stiff going back on use the old pinion as a drift and tap it onto the shaft.
Ensure that the magnet is still in the bottom left corner, and that the D shaped washer is still on this shaft:
clean up all of the mating faces with gasket scrapers and make sure they are free from grease and dirt/ old gasket. use blue silicone based gasket sealing compound (hermatite blue) on both of the mating faces and basically reassembly is the reversal of removal, here's an example of how to seal using the blue sealant:
refit the cassette and tighten up the 4 13mm bolts (shown in yellow above)remove the endplate, now clean that up and seal using the gasket sealant, replace all of the bolts, torque the 11m ones up to about 20nm and the 13mm ones to about 25nm
refit the diff and crown wheel, spin the castellated nut to hold it in position, tighten to the marks made earlier, refit the locking plate and 10mm bolt, make sure the diff spins freely, replace the diff cover using sealant and tightening the bolts to 18nm. refit the selector fork again using sealant, replace driveshafts, wheels etc.
Now fill up with oil and run the engine for 20 mins to check for leaks. if all is ok, go for a test drive 
A haynes manual is a good idea as it explains in detail about how to remove the driveshafts and filling the gearbox up with oil.
I would rather lose by a mile because i built my own car, than win by an inch because someone else built it for me.
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