All 4.63 ring and pinions fit 850 gearbox?

Matt Zerega

It's about passion, not practicality.
Greetings fellow rear-engine Fiat nuts. I understand that the 850 Spider's gearbox comes from the factory with an 8/41 (5.1:1) gear set. And I've read that 8/37 gears will fit the gearbox...but one question remains, in my head at least: how do I determine definitively if a particular pinion with 8 teeth and ring-gear with 37 teeth will actually fit? In this case, I see a set for sale that's purported to be for an "850 N"...and in photos, it looks awful similar.

Is it safe to assume that, an 8/37 ring and pinion advertised as being correct for an "850 N", will fit my 1969 Fiat 850 Spider's gearbox?
837gears.jpg
 

johnt

Daily Driver
Yes it will fit! Use the same pinion spacers from the trans case your using. Even better is to determine the pinion depth by using the procedure in the factory manual or any ring/pinion setup guide. Attached pic will give you an idea of how it can be done.
 

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Jeff Stich

True Classic
I understand that the 850 Spider's gearbox comes from the factory with an 8/41 (5.1:1) gear set.

All 850 Spider gearboxes used an 8/39 (4.88:1) final drive gear set. The 8/41 is from the 850 Special (Sedan).

...how do I determine definitively if a particular pinion with 8 teeth and ring-gear with 37 teeth will actually fit? In this case, I see a set for sale that's purported to be for an "850 N"...and in photos, it looks awful similar. Is it safe to assume that, an 8/37 ring and pinion advertised as being correct for an "850 N", will fit my 1969 Fiat 850 Spider's gearbox?

The "850 N" is shorthand for "850 Normale", which is the standard version of the Fiat 850 Sedan. It featured an 8/37 (4.63:1) final drive. There's also the later "850 Super" & "850 Special" versions of the 850 Sedan; the Super shared the 8/37 r&p, the Special had the 8/41.

Any factory Fiat 850 final drive (ring & pinion) gear set will fit into any 850 gearbox (the cases are all the same). I did this same 8/37 swap into my old '69 850 Spider gearbox long ago - worked great! (Ironically, it was John T. who did the backlash setup for me on that one). :)
 
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Matt Zerega

It's about passion, not practicality.
Thanks, Gents. With your guidance and a little math, I'm nearly convinced the slower, quieter engine speeds would be worth installing the 8/37 gear set. With 8/37 gearing and a 185/65 15 rear tire vs. the stock 8/39 set and 155/80 13 tire, the motor will spin 572 RPM slower (at 4462 RPM instead of 5034) at 70MPH...which is minimum speed on CA freeways if you don't want to see middle fingers and/or bright headlights in the rear-view mirror. The 8/37 gears alone (without changing tire size) saves 259 engine RPM at 70 MPH.

Summary: installing 8/37 gears and 185/65 15 tires results in cruising at 4462 RPM instead of 5034 RPM

For those who actually drive their Spider on the freeway, should a reasonable person expect a significant reduction in vibration and interior sound whilst cruising at 4462 engine RPMs as opposed to 5034?
 
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NM850

True Classic
Hmmm . I had 14” wheels (Alfetta) on a Sedan........I’ll reserve my opinion until I see pictures but my first thought is, it’ll fit in on the local cruise night
 

Frank L. Di Gioia

True Classic
The current tires are 195x50x15. The wheels are 15"X7". If I recall their diameter is the same as "stock" diameter tires. That would be approx. 22.5" total diameter. By pure luck they clear stock body sheet metal and will turn lock to lock in the front without body contact. The rears clear by maybe 1/4" or LESS and never scrub. Complete luck as they came from an older Honda Accord with staggered offset. These were 110mmX4. They were originally a copy of a Gotti French 3 piece rim from the 1960's-70's. The rims were/are 100X98 and require "wobble bolts" needed for the 1mm difference of 98mm vs 100mm pattern. I also had to machine center ring spacer to properly center the rims. I suggest aluminum rather than plastic rings for the centering rings. To be honest at this time in life (75years and counting) I'm more into "Cars and Coffee" than racing so it's a lot about THE "look" when parked.











crffing.
 

myredracer

True Classic
I just sold a sedan transaxle with 8/37 R&P. What I don't know is if these 8/37 transaxles have a different speedo gear set ratio? Or maybe the speedos were calibrated differently? A new 8/37 R&P set is available out there but installing one and setting it up is NOT an easy job. Better to find a decent used transaxle I think.
 

ramona300

True Classic
I just sold a sedan transaxle with 8/37 R&P. What I don't know is if these 8/37 transaxles have a different speedo gear set ratio? Or maybe the speedos were calibrated differently? A new 8/37 R&P set is available out there but installing one and setting it up is NOT an easy job. Better to find a decent used transaxle I think.
Finding one is the hard part. Plenty of Coupe and S boxes about. Berlina boxes are getting very rare.
 
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