Fj40 3.7 diffs

[BushBen]

Ive just aquired two complete diffs drum to drum from a very early fj40. Im not sure what year but I know they are out of a three speed. Did all three speed cruisers have 3.7s ? I have taken the back of the diff housing off and I can count the teeth on the ring but not on the pinion. Does anybody know how many teeth there should be on a 3.7 ring?

[keencdk]

Put it in neutral, lift one wheel and mark the top of the wheel and the tail shaft at their current locations. Spin the wheel and watch both the tail shaft and wheel. Keep a count of how many times each of the marks comes to the top. When it gets to 37 on the tail shaft, take the number of turns of the wheel and you will know the ratio. If the number of times the wheel spun is 20 then it is a 3.7:1, if it is 18 times then it is 4.11:1

[loosecanon]

wow, were did you learn that?

[keencdk]

Had the same issue about a week ago, bought a complete diff housing with locker installed but the guy didn't know what ratio and is really hard to count teeth when it is all together. 4.11 is 37 teeth on crown wheel and 9 on pinion which makes the shaft spins 4.11 (37/9=4.11) times to make the wheels spin once.

If you only lift one wheel and leave the other one on the ground then the diff will actually make the wheel that is lifted spin twice as fast cos that's what diff's do so 37 spins on the tailshaft will mean 18 on the wheel lifted. If you lifted both wheels then both would spin 9 rotations either way giving a 4.11 ratio.

To get a 3.7 ratio the wheel would spin 20 times with one wheel lifted or 10 times with both lifted.

[91Mav]

I though 40's had 4.10's instead of 4.11's. My old 40 did. They have 41 teeth on the crownwheel and 10 on the pinion.

[BushBen]

Thanks for that. It was very helpful And a great tip too. Ive been hunting for 3.7s for years but Im starting to think they are just a myth. I counted my teeth and there are 37 so its 4.11. Ive always thought 3.7s would be a good easy and cheap alternative to a 5 speed swap but now that Ive done the swap Im so happy with it that I might give up on the 3.7 quest.

[SCANAS]

Plus a set of 3.7 would sell for more than a 5sp

[RED60]

Thanks for that. It was very helpful And a great tip too. Ive been hunting for 3.7s for years but Im starting to think they are just a myth. I counted my teeth and there are 37 so its 4.11. Ive always thought 3.7s would be a good easy and cheap alternative to a 5 speed swap but now that Ive done the swap Im so happy with it that I might give up on the 3.7 quest.

Using only the crown wheel tooth count is flawed... both 3.7 and 4.11 have the same crown wheel count (37).... its the pinion that has the different count...
if you look closely as you rotate the crown wheel, on its outer circumference (you may have to wipe the oil off as you go), it will have either 9x37 or 10x37.. (4.11 and 3.7 ratios respectively).... also 3.7s aren't a myth, uncommon yes, but they do exist..

[BushBen]

Nice one Red. I reckon that was the tip of the day. You were spot on and as I wiped the oil away there it was just as you said but unfortunately it said 9x37. So it looks like I lucked out this time but Im keen to continue on my quest to find the holy grail of cruiser parts. Just for interest I checked the ring of a 1984 bj42 diff Ive got and it had nothing stamped on it so it must be just the early diffs that have that feature.

[4X4 shorty]

Nice one Red. I reckon that was the tip of the day. You were spot on and as I wiped the oil away there it was just as you said but unfortunately it said 9x37. So it looks like I lucked out this time but Im keen to continue on my quest to find the holy grail of cruiser parts. Just for interest I checked the ring of a 1984 bj42 diff Ive got and it had nothing stamped on it so it must be just the early diffs that have that feature.

I pulled i diff out once for the lokka and it had 3.7 gears in it I don't know what model 40 it was out of though

[little rusty]

i think the yanks got 3.7s after a certain year... google knows.

i will double check now I have a maths lesson on gear ratios but pretty sure I have a set of 3.7s from my old V8 40rty....