Transmission swap

[jerry]

I have a 1968 Fiat Spider with a bad 5-speed tranny. I am considering buying a tranny out of a 1980 which is available locally. Will it fit if I use my old bell housing? I expect the gear ratios may be different but I think I can live with that. Are there any other issues that I should be aware of?

[spiderrey]

If I recall correctly, it should work. The bellhousing is the only difference fitment wise.

[baltobernie]

Swap your existing speedo drive to the new transmission.

[BEEK]

if you want to swap the speedo drive you will have to remove the tail housing and swap the drive gear on the end of the stack for the speedo. dont mix bearings as there are some differences in coller length and size.

[jerry]

I'm wondering why I would need to swap the speedo drive. Is it because of a fitment issue or an attempt to correct the speedometer reading because of the different gear ratios in the trannys? I don"t know the specs on the 80 tranny but wouldn't 4th gear have the same 1to1 ratio and 5th gear be close to the 1 to .912 of my old tranny? I'M thinking if I don't change the speedo drive the reading might be more accurate since I'm not changing the rear axle which I believe has a larger difference in the gear ratio.

If the reason to change the speedo drive is a fitment issue can't I just get the speedo cable from the parts car and use it?

[BEEK]

the speedo drive racio is determined by the rear end racio, where the 68 spider should have a 4/10 if original, using an enclosed driveshaft. if yours is not you could have a 4/30 if its a "chunk" style, whereas the 80 spider had a 3/90.
hope this helps

[jerry]

Thanks for the reply Beek. My 68 does have the original enclosed driveshaft. I'm not planning to change the rear end. I'm no math major so I'm not sure if your saying I need to change the speedo drive to get a more accurate speedometer reading or not. My thinking is if both trannys have a 1 to 1 ratio in 4th gear then, at least in 4-th gear, the speedometer reading should be spot on. My old 5th gear is a .912. I don't know what the number is for the 80 but wouldn't the differance between these two numbers be the only thing that would result in the inaccuracy in 5th gear? I'm really not to concerned with the accuracy in the first 3 gears since I probably won't be exceeding the speed limit anyway.

[BEEK]

the speedo drive calculates revolutions of the transmission so 1000 rpm in 4th gear = 1000 rpm of trans output, the pinion gear turns 10 times for every 4.1 times of the ring gear=.243 of trans output multiply that by tire circumfrence = distance travled= 165 / 80 /13 = 73.5" of distance travled / revolution of the axle = 16.91 mph / 1000 rpm.

now if you change the racio of the rear end to 10/39, which is the standard rear end for 2000 spider you get 17.81 mph.

so at 60mph your speedo will be reading 6% off.

hope this doesnt confuse.

[jerry]

Thanks for your patience Beek. I guess I'll be changing the speedo drive unless my new 185 70 13 tires would bring the reading closer to accurate or would it be even further off. Oh hell, I don't want to think about it anymore. Will I need any special tools for the job or should I take in to a repair shop to make the swap?

[BEEK]

easily done at home, i would order a transmission gasket set for the 80 5 speed and a front seal for the 68. a 13mm wrench and socket, a 10mm socket, for the pan, if memory serves correct a 30mm or a 32mm deep socket for the rear nut. a pair of circlip pliers, and a battery terminal puller. is all thats needed to remove the tail housing, drop the pan, shifter extension, and the bellhousing is a 17mm socket if i remember correctly. the speedo drive slides off, be careful not to loose the small ball bearing that locates the gear on the mainshaft. swap the external portion. and zoom zoom.

[baltobernie]

The speedo drive must match the rear axle. The internal gear ratios of the transmission are irrelevant. If there is any chance that the donor transmission is coming from a car that has a different rear axle ratio than your car, you must move the speedo drive from your car onto the new transmission, to preserve the speedometer accuracy. All bets are off if you are using non-standard wheel/tire combinations that are not 73" (1850mm) in circumference.

Also note that there are two drain plugs on the Fiat 5-speed. I would remove the rear one as well as the front pan, to completely drain the transmission. To refill, install the plugs, pan and gasket and place the tranny on a level surface. Remove the front fill plug and add about a quart of oil. Replace the plug, then tip the transmission upward from the front at about a 45° angle and hold for a minute. This will fill the tailshaft assembly. Return the tranny to level, wait a bit, then add remaining fluid (until it overflows). Replace the plug, and you are ready to re-install the transmission.