Turtle Gear Change

[drolston]

Because of an ill fitting repro floor board that takes five different kinds of fasteners to hold it to various stripped and rusted out screw holes, I will do almost anything to avoid taking the floorboard out of my '41 coupe, including attempting to change the turtle gear from underneath the car.

It takes some contortions and working by feel, but it can be done! A 1/4" drive ratchet can be used when the bolts are near all the way down, but further out than that, you have to use a 7/16 box end, one twelfth of a turn at a time. Luckily I could run the bolts with my fingers, once they were loose.

I could have avoided the problem altogether if I had changed from the stock 19 turtle to an 18 turtle when I changed from the 19 tooth gear to the smaller radius 17 tooth gear when the rear end was out to change from 3.78 ring and pinion to 3.25.

The 17 tooth fiber gear from Braton in the 19 housing just barely engages the worm gear on the drive shaft. The speedometer worked at first, then started skipping, then quit in about 100 miles. You can see the worn off teeth in the photo.

The turtle from Mac's is unmarked, but they swear it matches the 18. It is hard to measure from the curved surfaces, but best I could tell, the shaft centerline is about .030" closer to the mounting surface than the 19 housing. I left out the gasket to gain another .002. What's another little leak?

Long story for a little job, but I thought some future thread searcher might benefit from the experience.

[slowforty]

One of those cheesy ratcheting box wrenches may work.

[JM 35 Sedan]

I thought you would have followed up on this post.....

https://www.fordbarn.com/forum/showthread.php?t=229971

[ford38v8]

Originally Posted by slowforty

One of those cheesy ratcheting box wrenches may work.

I wouldn't call a tool cheesy if it performs the task it was designed for. Consider the need to remove and flip a box wrench multiple times in a tight spot such as described above. I think those ratchet box wrenches are a fabulous addition to my toolbox.

[slowforty]

Well I found the Quality varied a lot. The ratio in the swing of the wrench leaves a lot to be desired.And the ones I got worked but not well. But then again I got what I paid for.

[drolston]

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Originally Posted by JM 35 Sedan

I thought you would have followed up on this post.....

https://www.fordbarn.com/forum/showthread.php?t=229971

Good point, but that is a lengthy post more about rear end gears than the speedometer gear. Starting a separate thread about the turtle gear makes it easier to find for someone who is just dealing with that mechanism.

This forum has been a huge help for me in the numerous projects I have had on my '41 coupe and '34 pickup, but is not always easy to find the information needed. I have not found a way to narrow a search topic; if you search using two words, it does not find just posts that contain both words, it finds all posts containing either word.

I would be great if there were some way to flag posts with "how to" information, for search with key words.

[weemark]

thanks for the info. I have just bought A 17 tooth gear from brattons to try and correct my speedo however I think it will still be out even with a 17 tooth gear. Maybe I'm better just leaving the 18 tooth gear in it and marking the glass lightly with a waterproof marker to I know what speed I'm travelling at now?

[JM 35 Sedan]

Originally Posted by weemark

thanks for the info. I have just bought A 17 tooth gear from brattons to try and correct my speedo however I think it will still be out even with a 17 tooth gear. Maybe I'm better just leaving the 18 tooth gear in it and marking the glass lightly with a waterproof marker to I know what speed I'm travelling at now?

Hi Mark, Is this for your roadster or coupe? Which rear gear ratio, and rear tire size (diameter) are you running?
With 3.54 rear gears, #18 (39-11) turtle housing & gear, and 6.00 x 16 rear tires, the speedometer and odometer in my '35 fordor are reading right on the money based on GPS mph readings.

[weemark]

Originally Posted by JM 35 Sedan

Hi Mark, Is this for your roadster or coupe? Which rear gear ratio, and rear tire size (diameter) are you running?
With 3.54 rear gears, #18 (39-11) turtle housing & gear, and 6.00 x 16 rear tires, the speedometer and odometer in my '35 fordor are reading right on the money based on GPS mph readings.

hi john - this is for the roadster. I have an 18 tooth gear in the turtle, well at least I think its 18 tooth but ive never counted the teeth, I have 3.54 gears in the axle and running 6.50 x 16 on the rear. the speedometer reads about 24/25mph when im doing 30 - the needle bounces around a little so it could be one or the other. So even if I changed to a 17 tooth speedo gear the speedometer would still be out.

Someone put up a chart about turtle sizes for different gears/tyres but it said 20 tooth was fine for 6.50sx16 - the chart is on this thread https://www.fordbarn.com/forum/showthread.php?t=226134

[JM 35 Sedan]

[QU. E=weemark;1541332]hi john - this is for the roadster. I have an 18 tooth gear in the turtle, well at least I think its 18 tooth but ive never counted the teeth, I have 3.54 gears in the axle and running 6.50 x 16 on the rear. the speedometer reads about 24/25mph when im doing 30 - the needle bounces around a little so it could be one or the other. So even if I changed to a 17 tooth speedo gear the speedometer would still be out.

Someone put up a chart about turtle sizes for different gears/tyres but it said 20 tooth was fine for 6.50sx16 - the chart is on this thread https://www.fordbarn.com/forum/showthread.php?t=226134[/QUOTE]

Mark, I can't speak to the correctness of that chart, and if that chart was taken from the Green Book, or as some call it the Green bible, that would make me suspicious of its correctness like other info published in that book.

The first thing I would do is determine why the speedometer needle is bouncing. My guess would be the lubrication on the inner cable is either gummed up. or it's possibly lacking in the proper lubrication (i.e. graphite). Or there could be something wrong in the speedometer head itself.
My next recommendation would be to verify which turtle housing number you actually have, and how many teeth are on the gear that's in that turtle housing. If it has an 18 tooth gear, that may be as accurate as it's going to get with 3.54 gears and 6.50x16 tires. If you were running 7.00x16 tires I believe the 17 tooth gear would give a more accurate reading. JMO

[weemark]

John yes the chart is from the green bible - I have just realised that though.

I think the needle is bouncing a little due to a lack of graphite, I think its only lubed with oil and grease.

I have several turtle housings, the one im using though is a brand new one, possibly from macs, which is unmarked but I think ive also got an 18 tooth housing, ive got a 19 and a 21 too.

[G.M.]

Originally Posted by drolston

Because of an ill fitting repro floor board that takes five different kinds of fasteners to hold it to various stripped and rusted out screw holes, I will do almost anything to avoid taking the floorboard out of my '41 coupe, including attempting to change the turtle gear from underneath the car.

It takes some contortions and working by feel, but it can be done! A 1/4" drive ratchet can be used when the bolts are near all the way down, but further out than that, you have to use a 7/16 box end, one twelfth of a turn at a time. Luckily I could run the bolts with my fingers, once they were loose.

I could have avoided the problem altogether if I had changed from the stock 19 turtle to an 18 turtle when I changed from the 19 tooth gear to the smaller radius 17 tooth gear when the rear end was out to change from 3.78 ring and pinion to 3.25.

The 17 tooth fiber gear from Braton in the 19 housing just barely engages the worm gear on the drive shaft. The speedometer worked at first, then started skipping, then quit in about 100 miles. You can see the worn off teeth in the photo.

The turtle from Mac's is unmarked, but they swear it matches the 18. It is hard to measure from the curved surfaces, but best I could tell, the shaft centerline is about .030" closer to the mounting surface than the 19 housing. I left out the gasket to gain another .002. What's another little leak?

Long story for a little job, but I thought some future thread searcher might benefit from the experience.

You can't just put any turtle gear in any housing. The gears are
mostly different in diameter and the housings have a different depth depending on the OD of the gear. That's why your replacement gear
won't engage. G.M.

[drolston]

Originally Posted by weemark

hi john - this is for the roadster. I have an 18 tooth gear in the turtle, well at least I think its 18 tooth but ive never counted the teeth, I have 3.54 gears in the axle and running 6.50 x 16 on the rear. the speedometer reads about 24/25mph when im doing 30 - the needle bounces around a little so it could be one or the other. So even if I changed to a 17 tooth speedo gear the speedometer would still be out. [/URL]

Changing from 18 to 17 turtle gears would get your 25 indication only up to 26.5; maybe not worth the trouble. I went from 19 to 17 or 12% which was a pretty good correction for the 16% change in rear end gears from 3.78 to 3.25.

[drolston]

The 17 tooth turtle gear worked with no skipping or flutter of the needle, so the 18 turtle housing from Mac's did the trick. Finally got to do a GPS check on the speedo gear change. With 3.25 rear end gears and BFG 205/75R15 rear tires, the speedometer matches the GPS as close as I can read it in that unfortunately dim and low contrast instrument panel.

[JM 35 Sedan]

Originally Posted by drolston

The 17 tooth turtle gear worked with no skipping or flutter of the needle, so the 18 turtle housing from Mac's did the trick. Finally got to do a GPS check on the speedo gear change. With 3.25 rear end gears and BFG 205/75R15 rear tires, the speedometer matches the GPS as close as I can read it in that unfortunately dim and low contrast instrument panel.

Didn't know you have 15" tires until reading this last post.

[drolston]

Originally Posted by JM 35 Sedan

Didn't know you have 15" tires until reading this last post.

The rolling diameter of the tire is the driving factor, and the rim size affects that in combination with the mounted tire. Rolling diameter on my wheel/tire combination is very close to 28".

Just got back from the first long road trip with the new 3.25 gears. They took me over Afton Mountain on I-64 at 65 mph. Temp went up to 180, but otherwise, no strain. Cruised all day at 65 and averaged 16.7 mpg.

Visited cousins in Staunton, a very hilly town. Caught some stop lights on very steep hills, and got going easily without excessive clutch slipping.

Anyone else considering going to such tall gears needs to remember that my flathead is 284 cubic inches and has a ton of torque.

[G.M.]

Originally Posted by drolston

The rolling diameter of the tire is the driving factor, and the rim size affects that in combination with the mounted tire. Rolling diameter on my wheel/tire combination is very close to 28".

Just got back from the first long road trip with the new 3.25 gears. They took me over Afton Mountain on I-64 at 65 mph. Temp went up to 180, but otherwise, no strain. Cruised all day at 65 and averaged 16.7 mpg.

Visited cousins in Staunton, a very hilly town. Caught some stop lights on very steep hills, and got going easily without excessive clutch slipping.

Anyone else considering going to such tall gears needs to remember that my flathead is 284 cubic inches and has a ton of torque.

While driving you can time yourself over a measured mile at 60 MPH.
It should take EXACTLY 60 seconds to go 1 mile. This will tell you how
close your speedometer is. G.M.

[40cpe]

Originally Posted by drolston

remember that my flathead is 284 cubic inches and has a ton of torque.

You might have said in another post, but what cam are you running?

[sidevalve8ba]

Are you using a tachometer? I am curious how many r.p.m.'s you are turning at 65.

[Heavy Duty]

Great to see results of diff. and speedometer gear changes. Will order parts as you recommended. Thanks much