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Discussion Starter #1
Hi all,

I'm in the process of changing out my 3.55 gears for 4.11s and am seeing what needs to be done to get my speedometer back on track.

Normally, it's pretty simple. I have a 19 tooth gear and my speedometer is spot on. To go to 4.11s, it shouldbe:

(19 / 3.55) * 4.11 = 22

They don't make a 22 tooth, so I'll need to change the internal ring gear in my transfer case.


However, something is amiss. When I looked inside the transfer case with a scope, there is a yellow gear inside, which is a 7 tooth.

The formula for speedometer gear is:

(Tire rotations per mile * gearing * drive gear teeth) / 1000
In my case, that's:

(630 * 3.55 * 7) / 1000 = 15.66

So, it should have been using a 16 tooth, not a 19 tooth. That's a huge difference.

I looked in my old transfer case, and it also has a yellow 7 tooth gear. However, I distinctly remember when I switched transfer cases, I had to change the speedometer gear to fix my speedometer. Even though they have the exact same yellow 7 tooth drive gear. It should have been a 1:1 swap.



TL/DR: Two transfer cases with the exact same yellow 7 tooth drive gear required different speedometer gears to get an accurate speedometer, and I don't know why. It doesn't make sense as that's simply not how it works. Also, for what it's worth, the speedometer works perfectly, so it's not like it's slipping.

Any ideas what would cause such an odd discrepancy? Normally, I'd just shrug my shoulders and move on, but I need to figure it out so I can get my speedometer working again with the 4.11s.
 

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Addicted to Junk
85 Bronco, 309ci I6 w/4bbl, np435, 4" lift, 37" Irok NDs, 4.56 w/ Detroit Locker and tru trac
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Did you examine both yellow gears side by side? Meaning are they EXACTLY the same? Both np208?

Tach vs non-tach cluster? That shouldn't make a difference though.
 

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Discussion Starter #3
I haven't removed them and held them side-by-side, no.
One is from an BW-1345 and the other is a BW-1356. However, there are only 3 Ford drive gears, and they're:

6 tooth - black
7 tooth - yellow
8 tooth - green

This goes for automatics, manuals, transfer cases, trucks, cars, etc. etc. So, having two yellow drive gears (which when I look at them with a bore scope both have 7 teeth) giving different speedometer readings when combined with the same speedometer gear is quite strange, indeed.
 

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Man of endless projects
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what size tires do you have on now? 32" in ur sig? jsut go by that and the 4.10 your putting in ow
32 would be 650 revs/mile. so 650 x 4.1 x 7 / 1000 = 18.65 = 19 tooth gear

if u look on the vin sticker it will say the original tire size it come with which is usually like ~29" which is about 720 revs/mile
720 x 7 x 3.55 / 1000 = 17.8 = 18 tooth gear.

but if it came with say a 28" which is about 747 revs/mile. then it comes out to 18.56 = 19 tooth
 

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Discussion Starter #5
That's the issue.
With 3.55s and my 32" tires, it already has a 19 tooth, and the speedometer is dead on. But, according to the above formula, it shouldn't be.

I also had a different transfer case in before (with the same gears and tires) and it took a different speedometer gear. I think a 17 tooth. However, both transfer cases have the same 7 tooth yellow drive gear inside, so they should require the same speedometer gear.

Why one transfer case needs a different speedometer gear than the other, when both transfer cases have the same 7 tooth drive gear is where my mystery is.
 

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Discussion Starter #7
Again, that's the issue.
The "correct" one (based on my current setup) is a 22 tooth, which doesn't exist. So, I have to change the internal ring gear. But the ring gear that would correct it is the yellow 7 tooth, which is already in there! Lol

Something is just not right.

Oh well, I'll have to pull it apart anyway, so I guess I'll see what's going on when I pull the tail shaft off.
 

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I'm pretty sure that green is 7 tooth and yellow is 8 tooth... I have both under my desk at work and can confirm in a couple days when I go in next (my work is maintaining low in-office presence). It's the number of tooth starts that matter which is more certainly confirmed by pulling the gear out and looking at it end-on.

EDIT:
See page 7 of my build thread, the green and yellow gears are pictured there. Unless they molded each in both colours, yellow is 8 tooth.

EDIT AGAIN:
This applies with certainty to the BW1345 t-case.
 

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Man of endless projects
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i know the BW1356 yellow is 7 tooth. taken enough apart to know and JBG sells 7 and 8 tooth for 1356 with 8 being brown

i really cant find crap for info with NP208 but in Ford world, 7 seems to usually be yellow when you look up manual trans and some others. but even trying to find a new 7 tooth NP208 gear is almsot non-existant, and have not seen yellow. an blue 8 tooth comes up easy. and im starting to wonder if it was originally 8 tooth.

saw a thread with this that kinda indicates it might have used 8 tooth. since your regearing you should know, did your truck originally have 3.50 gear ratio or was it 3.54 ratio? im assuming 3.50. it shows that 3.50 with New Process T/C has 8 while 3.54 had 7



when i looked up that 7 tooth part number D3tz-17285-a, i found a NOS that looks like this. maybe you thought it was yellow?


when you look up the 8 tooth d3tz-17285-a, it comes up as blue and with an NOS one looking like this

 

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Reading back through this, I think we have the answer to this mystery:
One is from an BW-1345 and the other is a BW-1356:
Per my familiarity with the BW1345 speedo gears, and @Kingfish999's familiarity with the BW1356, these cases do not use the same gears or color coding.
BW1345 yellow = 8 tooth
BW1356 yellow = 7 tooth

Also note that the BW1345 has the gear pressed onto a knurled steel sleeve (sleeve diameter is different for each size speedo gear), which the rear output yoke gets compressed against when the nut is tightened. Correct me if I'm wrong, but it looks like the BW1356 uses a key of some sort to drive the gear. The only BW1356 I've had was a slip yoke type in a pickup, with the speed sensor in the diff, so my knowledge is lacking on speedo gears for those.
 

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@AbandonedBronco which t-case are you using/planning to use?
I've done a pretty extensive search for the various BW1345 speedo gears, and ended up having to find the best I could in JY's, then measured it machined my own. Speedo gears for the BW1356 are obviously still available, per KF's link, so that case may be the better path.
 

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Discussion Starter #12
Reading back through this, I think we have the answer to this mystery:


Per my familiarity with the BW1345 speedo gears, and @Kingfish999's familiarity with the BW1356, these cases do not use the same gears or color coding.
BW1345 yellow = 8 tooth
BW1356 yellow = 7 tooth
They don't?? Why would they change the color coding?
If that's the case then that would be it.

From what I've been seeing online, I am not seeing "transfer case specific" or "transmission specific" drive gears, but maybe I'm not looking at them correctly. I thought the drive gears were universal, like their speedometer gears.

But if so, that would be the case.
Yes, one is a BW-1345 with a yellow gear and the other is a 1356 with a yellow gear.
I also, for what it's worth, have an NP-208 with the blue/green gear that appears to have more teeth.


Not 100% sure on what transfer case I'm going with. The 1345 is in there now, it works great, and has the best low range ratio (2.72), but I'd heard the 1356 was stronger. I fully rebuilt the 1356, but the weld fix I did on the pump arm broke, and the pump spun in the case. I haven't pulled it back apart and dealt with it.
 

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Discussion Starter #13
Edit: (I read the search results wrong).



From what I'm finding it looks like both the yellow gears for both the 1345 and 1356 are still 7 tooth:


 

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Here are my pictures of drive gears pulled from BW1345 cases. I've coloured the end faces black for clarity. When viewed from the end:
Green = 7 starts
Yellow = 8 starts
159662


Note that they both appear to have approximately 7 teeth when viewed from the side, but that is not the correct way to count the teeth. This is required for the drive gears to mesh with any of the driven gears
159663
 

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Man, where was this guy 8 months ago? I called transmission shops and driveline part distributors all over North America trying to find a good BW1345 8-tooth gear. I ended up having to buy an entire used t-case ($200) from a wrecker just for a good 8 tooth gear, then replicated it since I didn't think I'd ever be able to find a new one.

Note in the ad that this guy is making these gears, and he is not matching the original colours
 

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boy im been failing at this. i was under the assumption you originally had a NP208, not a BW1345. also i assumed you were talking about your old 81. between here and the 6 other facebook groups, its hard to keep all the posts and vehicles straight
 

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Discussion Starter #17
Man, where was this guy 8 months ago? I called transmission shops and driveline part distributors all over North America trying to find a good BW1345 8-tooth gear. I ended up having to buy an entire used t-case ($200) from a wrecker just for a good 8 tooth gear, then replicated it since I didn't think I'd ever be able to find a new one.

Note in the ad that this guy is making these gears, and he is not matching the original colours
Ah. I totally missed that about those being reproductions / not color matched! That's cool he's making them.
Now I'm curious if there's documentation somewhere for the color coding.

I did find this one as well:


That looks to be more of an "original" part, and in that case it's an 8 tooth for the yellow.
If that's the case, that'd make more sense with what I have, as that'd calculate out to about an 18 tooth. However, that's with perfect 32" tires, but that's never the case. Installed height (with the weight of the truck, etc. compressing them) comes out to about a 30 to 31" rolling height, and that calculates to a 19 tooth. If so, that'd still require me to use a 22 tooth to fix for the 4.11. So, if I run the 1345, I'll have to get a 7 tooth drive gear to put it back.

boy im been failing at this. i was under the assumption you originally had a NP208, not a BW1345. also i assumed you were talking about your old 81. between here and the 6 other facebook groups, its hard to keep all the posts and vehicles straight
I hear you on that! I'm about the same with the number of groups and vehicles. Still! I'm impressed you remembered my other setup. :D
Yeah, the NP208 is in the garage, but in this case, it was switching between a 1345 and 1356.
 

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I did find this one as well:

Good find!
I have a hunch that this one is also a reproduction, despite being the correct color for the application. Notice the coring in the end face from the injection molding process, and a fair amount of flash at where the mold's parting lines would be. The coring reduces shrinkage when the mold is cooling, so that the finished part holds better tolerances. Looks like a good piece.
159693


The original gears are pretty amazing pieces of work actually... still injection molded, but the parting lines are very difficult to find, and the wall thickness is fairly large (considering how small and precise the gear teeth are) which typically results in lots of warping/shrinkage issues. The good used gear that I found in the JY appears to be original, and nearly new... possibly replaced shortly before the original truck was scrapped.

I might need a 7 tooth at some point as well, when I re-gear my axles. I plan to machine one, based on the chewed up green gear I have. If you can't find a good 7 tooth to use, PM me and we might be able to work something out. I can't "sell" them since I make stuff after hours on work's equipment, but if you cover material/shipping then its just a matter of me running the program twice. If I make way more than I need then that would raise questions as well.
Not sure yet if I'll do the 4.11's like you or go with 4.56 yet. I have a spare 3rd member already built with a Truetrac and 4.56 gears, so I'm planning to "test drive" it and base my decision off that, before I commit to a ratio in my solid D44 front. Also running 32's like you.
 
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