View Full Version : TR4/4A Differential gearing

11-02-2013, 06:46 PM
I have a 67 TR4A with solid axle and 4.1:1 diff. It's fine around town but way too busy at highway speeds. I am considering converting to 3.7 gearing. I also see that Moss offers 3.45 gearing as an option. Does anyone have experience with the 3.45 ref performance etc? Also how much labor is involved in installation for the replacement gears? Any advice would be appreciated.

11-03-2013, 10:00 AM
The loss of acceleration is less than you might think. Really, the final drive ratio only matters up to 25 mph or so (when you shift out of 1st gear) and again near top speed. But you will, I think, definitely notice the difference when taking off from a stop. With the 3.45 gears, you'll have 16% less torque (and hence acceleration) at the rear wheels.

I'm actually somewhat in the middle of the same odyssey. When my current TR3 came to me, it had 4.1 gears and no OD. I first installed an overdrive, then later went to 3.7 gears when the 4.10 axle broke. Now one of my "back burner" projects is replacing the 4.1 gears with 3.45 (the 3.7 gears are in a different axle that I had on hand).

Unfortunately, changing the gears can be a lot of work. Without the special tool for setting the pinion depth, it becomes a trial and error process to find the right depth. It is adjusted with shims, so each time you check the pattern, then remove the pinion & bearings to add or subtract shims, put it back together and try again. There is a trick that helps (grind down the inside and outside diameters of the old races, so they will slip easily in and out of position) but there is still a lot of "rowing" in and out. Then once you find the pinion depth, repeat the same process with the shim pack that sets the pinion preload. Hopefully you'll get lucky and can keep the same shim packs for the carrier bearings (since you'll be using the same carrier), but you'll have to double-check and adjust those if necessary. The final proof is when you can see that the contact patch is centered on the gear teeth.
https://i258.photobucket.com/albums/hh260/TR3driver/Stag%20diff/th_DSCF0065.jpg (https://s258.photobucket.com/user/TR3driver/media/Stag%20diff/DSCF0065.jpg.html)

There are a few more photos of my Stag diff (not identical, but the internals are very similar to the TR diff) at https://s258.photobucket.com/user/TR3driver/library/Stag%20diff Along the way, you can see some of the special tools I made up, mostly just lengths of cheap hot rolled steel drilled and shaped to suit. The gear puller is a HF special, modified for extra reach with some coupling nuts & bolts from Home Depot. I got the micrometer and dial indicator from Enco https://www.use-enco.com/CGI/INSRHM as I don't really trust the precision of tools from HF. (Honestly, Enco may not be any better, but I think they are.) They frequently have a 1" dial indicator & magnetic base set on sale for under $25, but I don't see it at the moment.

11-03-2013, 02:54 PM
Thanks Randall for the information. This sounds like something I may have to have a good shop do. I also have a 66 4A with
overdrive and 3.7 gears, and have been thinking of going that route with the 67. How was the drive with O/D keeping 4.1 gears? I would like to keep costs down, but driving this car on long trips sounds like a pain. Thanks again.

11-03-2013, 08:23 PM
How was the drive with O/D keeping 4.1 gears?
Well, kind of hard to quantify. I also run tires that are shorter than stock (so the 3.45 will actually be about the same effective ratio as the 3.7 with stock tires). The 4.10 with OD wasn't bad, but still left the engine turning faster than I would prefer. I drove about 500 miles round-trip to VTR 2009 and certainly had no trouble keeping up with traffic at 70-75 mph. (My chase car swore I was doing 80 but I don't believe it :) )
My original plan was to keep the 4.10 until I could build my "hot rod" engine with extra torque to go with the taller gears, but as so often happens, things didn't work out quite the way I planned.

https://i258.photobucket.com/albums/hh260/TR3driver/TS13571L/th_DSCF0038.jpg (https://s258.photobucket.com/user/TR3driver/media/TS13571L/DSCF0038.jpg.html)

11-04-2013, 03:01 AM
I have two TR4's set up with 3.45 gears and also run a Toyota tranny on each car. I really like the 3.45's but will say that converting from a 3.7 to the 3.45 is not much difference and would not be worth the cost. Going from a car with 4.1 to 3.45 would definitely give you a loss in acceleration and hill climbing that you might be used to. A friend of mine and I drove on a long trip, he with OD and 4.1's and I had the 3.45's with the Toyota tranny. We paced each other fine with the exception of a couple of areas. First climbing very long hills, he moved way ahead of me. But then on the back or down side of the hill I was on him in about 30 seconds. The second area is on the downside of the hills, he would back off the accelerator in 4th and overdrive and I was in 5th gear. With him in the lead I would constantly have to brake to stay behind him. But I have to admit that driving down the freeway the 3.45 gears do help to keep the revs down. On my setup in 5th gear at about 3000 rpm I am doing about 74-75mph. So most often I am keeping the car under 3000 revs.
As to setting up the gears I found a shop to do the work.

11-04-2013, 02:35 PM
Thanks Charley,
I am leaning to just going to the 3.45 gears with no O/D. Moss gets $500 for the gears, and a local shop quoted me $375 to install. This will be a lot more economical than converting to O/D. If I can cruise at 65 at reasonable rpm that will be enough.I appreciate everyone's advice.