850 Spider gearbox ring and pinion swap

[Matt Zerega]

Greetings Gents. I've found surprisingly little information out there about working on the 850 gearbox...so I figured I'd share a bit of experience. Overall, I'm pleased with the elegantly simple assembly; it was simple to take apart with basic tools. So far, there's only one exception: removing the bolts that tie the ring gear to the carrier - there isn't enough room between the carrier bearings and the bolt heads to fit a socket, so I had to use a crow-foot wrench on the end of an extension attached to an impact wrench. It was a bit awkward, but it worked. Aside of that, the most difficult, unpleasant part was scraping and cleaning 53 years of crust, dry grease and dirt off every surface and out of every crevice on the case. It took a few hours and a whole lot of "solvent" (gasoline) to get the case clean.

Why I'm doing this: swapping out the 8/39 gearing with a 9/35 set I bought from Middle Barton Garage in the UK, will yield 70 MPH at 3,600 RPM (with the 195/50 16 tires I'm running on the rear of my Spider). With the stock gearing and tires, the motor needs to spin at 4,850 RPM for the car to reach 70 MPH. Fine for a stock motor at freeway speeds in 1969, perhaps. I'm building a 954cc turbo motor that I expect will almost double the factory 52 HP, so the taller gearing will be a worthwhile change, especially considering the speeds most people drive on California freeways.

Once the case was reasonably clean, here's how I took it apart:

• Pull out the little pin that’s visible once the side cover is removed. The pin holds the reverse idler gear shaft in place. Pull the pin and then tap out the shaft from inside the bell housing.

• Engage both of the gears by pushing them towards the ends of the case until they snap into place against the adjacent gears. This locks the whole assembly into place and prevents it from turning when you spin the pinion nut off (in a later step).

• Remove the spring clip that holds the big nut fastened to end of the pinion shaft, and remove the big nut. Note that the retainer fits through a hole in the side of the nut, and into one of the pinion-shaft splines. You'll need to remember this when you reassemble the gearbox.

• Measure the pinion depth from the machined carrier-cap face (Thanks, JohnT . When I measured, I got 1.936” with my depth micrometer. You'll need this reference when install the new pinion. A variety of shims - of varying thicknesses - are available (not sure where, yet) to adjust the pinion height. The shim(s) are installed between the pinion and the bearing that seats in the bell housing.

• Once the retainer is removed from the nut, remove the big nut off the end of the pinion shaft using an impact driver.

• Remove the three 13mm bolts along with their tabbed washers (visible in the photo showing measurement of pinion depth), that hold the pinion bearing in its bore.
• Now, gently tap the pinion shaft out through the bell housing. It’ll come out along with with the bearing that’s installed up against the pinion’s gear teeth, and the race that's installed just behind it. The race slides right off.

• All of the gears installed onto the pinion shaft are now loose; remove them from the case and clean ‘em up.

And just for entertainment sake, here's some interesting factory markings on the original pinion:

I'll share more as I get farther along.

 

[johnt]

Matt,
did I ever share how to sharpen 2nd gear synchro teeth? Yours look better than most. I always look for new 2nd gears on Ebay and keep them for inventory. Pick them up when you find them. I have a couple for future rebuilds. Sharpening the teeth is 80-90% as good as new.

 

[ramona300]

Thanks for this. I've got the same job ahead of me. I have the ring and pinion gear and a selection of gearboxes but have put off the job till I put in the new engine I'm building with the Suzuki head. My present set up with the supercharged 903 only gets to use top gear and third on the circuit because of the stock final drive of the coupe.

 

[Matt Zerega]

Matt,
did I ever share how to sharpen 2nd gear synchro teeth? Yours look better than most. I always look for new 2nd gears on Ebay and keep them for inventory. Pick them up when you find them. I have a couple for future rebuilds. Sharpening the teeth is 80-90% as good as new.

Do tell, John. Is there a URL to an existing post, or is this "teeth sharpening" procedure something you can describe in this thread?

 

[Matt Zerega]

Anybody know how to safely remove the speedometer drive gear from the end of the pinion shaft...without damaging anything?

 

[Matt Zerega]

Everything's clean and I've installed a new synchro ring under the 1st/2nd slip sleeve. The front bearing has been pressed off the old 8-tooth pinion along with the depth shim.

I'm left with some important questions for you fellow Fiat enthusiasts (see below, pretty please

Noting my prior 1.936" (49.2 mm) pinion depth measurement...if I ignore the The factory service manual ("FSM") for a minute...and try to get the pinion depth roughly the same as it was...I note the following:

The overall thickness of the old pinion and its shim is 30.8mm. The new 9-tooth pinion's thickness without any shim is 30.1 mm...which appears to suggest I need a 0.70mm pinion shim in order to get the new pinion to sit as the same depth as the old one. The FSM says that depth shims are available from 0.75 to 1.20 mm in 0.05 mm increments, which would be bitchin'....if the damn things were still available.

Now...if I stop ignoring the FSM, I note that it describes a pinion-depth-setting procedure that relies on a "...number stamped on the pinion stem..." and a formula that doesn't apply in this case because 1.) there isn't anything stamped on the new pinon's stem, and 2.) the engravings on the old one are illegible. Instead of relying on markings I measured pinion depth from the carrier bearing-cap mating surface i.e. NOT as prescribed in the FSM, using the special factory measuring fixture.

The Fiat-factory prescribed formula is S = b + a which basically says (in this case, maybe?) add 0.01 to measured depth.... but "0.01" is stamped on the pinion face, not the stem. Anyway, since I don't have a depth measurement using the secret factory fixture...it doesn't make sense to add 0.01 to my measurement anyway.
S = shim thickness
d = measured depth (using factory fixture)
a = number stamped on stem (there isn't anything stamped on the new stem)

With all that said, it would be great if any of you super-knowledgeable Fiat guys could answer any or all of the following:

• You-all agree that I need a 0.75mm pinion depth shim? (ATTN: JohnT)
• Know where I can get a 0.75mm depth shim?
• How in the heck does one remove the speedo gear without damaging anything?

 

[Jeff Stich]

I have some of the shims, but I'll have to check if I have that size on hand. To get the speedo drive gear out, have you tried freezing the shaft?

 

[Matt Zerega]

I have some of the shims, but I'll have to check if I have that size on hand. To get the speedo drive gear out, have you tried freezing the shaft?

Thanks, Jeff. Anything less than 1.0mm will be better than the shim I've got. Let me know what you find. As for freezing and/or heat, I think heating the pinion shaft with a torch and then touching an ice cube to the speedo gear might help make it easier to get the damn thing extracted. I may try that, or I'll just rely on GPS satellites for speed - this car will have a Speedunio/Arduino 2560 and RealDash running on an Android phone...so I really don't need to speedo gear. It just annoys me to put something back together while eliminating usable features, and I don't like the idea of rendering the factory speedo non-functional.

 

[Matt Zerega]

Update: I'm continuing to learn and appreciate how challenging it is to get good information and parts for these old Fiats. This morning I received some new pinion shims in the mail from Treuchtlingen... Oops. I'll attribute this to my failure to communicate clearly across the language barrier. Fortunately, McMaster has ring shims that'll work.

I need 0.028" of thickness. The ring shims from McMaster come in a variety of thicknesses in packs of ten. Since I'm not sure exactly where the pinion will need to sit in order to get a good ring/pinion contact pattern, I ordered a pack of 0.007" shims. This will give me a little wiggle room.

I'll report back after I get a good contact pattern

 

[Matt Zerega]

Update: the standard ring shims from McMaster fit the pinion shaft absolutely perfectly. And thanks to a popular auction site operating in Italy, I've got a factory speedo gear and the washer that sits underneath. Now I just need to press this into the end of the new pinion.

 

[Matt Zerega]

Success! I must say, I didn't leave the pinion in the oven long enough. I set the oven to 170F but I left the pinion in there for only about ten minutes...and I didn't preheat the oven. I froze the speedo gear... but it was terrifying there for a minute, while I pounded on the little gear with a brass hammer; it stopped about a quarter inch before it was seated. The force with which I had to pound, to get it seated that last quarter inch, did NOT feel right. Fortunately, the part survived Anyway, if you do this, make sure the pinion is heated to its core...to at least 150'ish F. Make sure to freeze the little speedo gear...and move fast-but-carefully when you drive it into the pinion. The critical thing: you've got to make sure you start the speedo gear perfectly straight. If you don't, you run the risk of binding irreversibly. Anyway, I'm ready to get this gearbox assembled and the new 9/35 ring and pinion set properly. And with four of the 0.007" standard ring shims, the pinion depth with this new part will be nearly identical to the depth with the original pinion. Of course, none of this accounts for differences in any dimension between the old 39-tooth ring gear and the new 35-tooth gear.

 

[Matt Zerega]

I've got the gearbox preliminarily assembled with pinion ring-shims intended to get the new pinion sitting at the same height as the old one. Anybody's who's installed new ring and pinion before...and run it problem and noise-free for several years afterward...care to weigh in? Here's what we see after spinning the brand new ring gear around several revolutions in both directions:

Yokon Gear (a helpful resource) states that you want to see a pattern that's centered between Face and Flank, and that it's not necessary to see it centered between Toe and Heel. Here's Yukon's reference images:

If this was your gearbox, what would you do?

1. Button it up and run it!
2. Decrease pinion depth

 

[johnt]

Do tell, John. Is there a URL to an existing post, or is this "teeth sharpening" procedure something you can describe in this thread?

Matt,
Here's a link to some transmission rebuild information, but not much on tooth sharpening. I'll include that below with pictures.
https://forum.retro-rides.org/thread/153149/1967-fiat-850-coupe

Dremel tool Small slip-on sanding drums give a near perfect point (shown are 80 and 120 grit). Fine grinding stones work too with the correct diameter to get tooth points fresh again.
The 1-2 shift collar is also renewed the same way. 3rd and 4th synchros need nothing. Even 1st gear synchros last for ever; 2nd gets the most abuse!
EXTRA TIP! Move the 4th synchro band to 2nd and move 3rd to 1st.
Install them Reversed to function on the least worn side. Both sides of the band get worn, but reverse them to get the coursed surface toward the shift collars. The course surface on the band provides friction to slow-down or speed-up the gear based on up or down shift.
Objective is to use most worn synchro bands in the 3rd and 4th since 3-4 shifts are the least abusive on this style of syncro. New bands are nice-to-have but you can accomplish the same 2nd gear shifts with reversed 3-4th bands.

In my experience 2nd gear needs all the work when it comes to eliminating synchro tooth grinding. All the other gears don't grind given the way I shift. Your shifts might be different than mine; I always have grinding from 1st to 2nd and 3rd to 2nd.
The most worn-out transmission can shift smooth if you shift slowly at low Rpm, but who does that?

 

[ramona300]

Greetings Gents. I've found surprisingly little information out there about working on the 850 gearbox...so I figured I'd share a bit of experience. Overall, I'm pleased with the elegantly simple assembly; it was simple to take apart with basic tools. So far, there's only one exception: removing the bolts that tie the ring gear to the carrier - there isn't enough room between the carrier bearings and the bolt heads to fit a socket, so I had to use a crow-foot wrench on the end of an extension attached to an impact wrench. It was a bit awkward, but it worked. Aside of that, the most difficult, unpleasant part was scraping and cleaning 53 years of crust, dry grease and dirt off every surface and out of every crevice on the case. It took a few hours and a whole lot of "solvent" (gasoline) to get the case clean.

Why I'm doing this: swapping out the 8/39 gearing with a 9/35 set I bought from Middle Barton Garage in the UK, will yield 70 MPH at 3,600 RPM (with the 195/50 16 tires I'm running on the rear of my Spider). With the stock gearing and tires, the motor needs to spin at 4,850 RPM for the car to reach 70 MPH. Fine for a stock motor at freeway speeds in 1969, perhaps. I'm building a 954cc turbo motor that I expect will almost double the factory 52 HP, so the taller gearing will be a worthwhile change, especially considering the speeds most people drive on California freeways.

View attachment 61558

Once the case was reasonably clean, here's how I took it apart:

• Pull out the little pin that’s visible once the side cover is removed. The pin holds the reverse idler gear shaft in place. Pull the pin and then tap out the shaft from inside the bell housing.

View attachment 61542
View attachment 61543
View attachment 61551

• Engage both of the gears by pushing them towards the ends of the case until they snap into place against the adjacent gears. This locks the whole assembly into place and prevents it from turning when you spin the pinion nut off (in a later step).

View attachment 61552

• Remove the spring clip that holds the big nut fastened to end of the pinion shaft, and remove the big nut. Note that the retainer fits through a hole in the side of the nut, and into one of the pinion-shaft splines. You'll need to remember this when you reassemble the gearbox.

View attachment 61544

• Measure the pinion depth from the machined carrier-cap face (Thanks, JohnT . When I measured, I got 1.936” with my depth micrometer. You'll need this reference when install the new pinion. A variety of shims - of varying thicknesses - are available (not sure where, yet) to adjust the pinion height. The shim(s) are installed between the pinion and the bearing that seats in the bell housing.

View attachment 61550

• Once the retainer is removed from the nut, remove the big nut off the end of the pinion shaft using an impact driver.

View attachment 61553

• Remove the three 13mm bolts along with their tabbed washers (visible in the photo showing measurement of pinion depth), that hold the pinion bearing in its bore.
• Now, gently tap the pinion shaft out through the bell housing. It’ll come out along with with the bearing that’s installed up against the pinion’s gear teeth, and the race that's installed just behind it. The race slides right off.

View attachment 61561

• All of the gears installed onto the pinion shaft are now loose; remove them from the case and clean ‘em up.

And just for entertainment sake, here's some interesting factory markings on the original pinion:
View attachment 61556
View attachment 61557
I'll share more as I get farther along.

Sorry to drag up this old thread Matt but I'm in a similar spot at the moment except I'm not actually doing the work. I currently have a gearbox in a local shop getting a new pinion and ring gear installed. The box is one of my spares and is in excellent condition. I was talking to the bloke doing the job this afternoon and he has done a trial fit and the pinion is miles out. He does not have the dummy pinion the manual talks about and I intend to take this section of the thread into him and see if it helps. Hopefully, the way you made the measurement helps. How has your installation worked out after some use?

 

[Matt Zerega]

Hi, Graham. I'm still leisurely modifying and reassembling the car; the gearbox isn't in the car yet. That said, the resulting pattern shown on the ring-gear is exactly as desired i.e., centered between the face and flank. Hopefully, what your guy is seeing is a contact pattern closer to the gear face, in which case, the pinion is too far away from the ring gear...and all he'll need is shims to correct the pattern/fit. You can buy them in a variety of thicknesses at McMaster.

 

[ramona300]

Hi, Graham. I'm still leisurely modifying and reassembling the car; the gearbox isn't in the car yet. That said, the resulting pattern shown on the ring-gear is exactly as desired i.e., centered between the face and flank. Hopefully, what your guy is seeing is a contact pattern closer to the gear face, in which case, the pinion is too far away from the ring gear...and all he'll need is shims to correct the pattern/fit. You can buy them in a variety of thicknesses at McMaster.

No, it was the other way, needs a lot more shims. The teeth bare touched. I will pay him a visit again in a couple of days I don't want to rush him.