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BUGEYE differential numbers

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paulsherman

Jedi Trainee
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I was cleaning one of my differentials and I notice some numbers. First was 9/38 and this should be the ratio, but what does 9/38 indicate. What ratio?
And there appears to be a serial number. Should that number be close to the serial number of my car?

Thank you
 
paulsherman said:
I was cleaning one of my differentials and I notice some numbers. First was 9/38 and this should be the ratio, but what does 9/38 indicate. What ratio?
And there appears to be a serial number. Should that number be close to the serial number of my car?

Thank you
Click to expand...

Heh - I got this one...

38 divided by 9 equals 4.2222222222...
AKA a 4.22 rear end.

Oh, and it indicates 9 teeth on one gear and 38 on the other! :jester:

Serial number on many parts is independent of car numbers.
 
Yep, 9 teeth on the pinion and 38 on the ring gear. Usually the 9/38 (or other ratio) is stamped on the outside of the assembly at about the 10 o'clock position when looking at it from the pinion flange side. BMC/BLMC diffs that will fit the Spridget typically have 7/8/9/10/11 pinion teeth and mid-30's to mid-40's on the ring gear. The 7/X stuff is real odd and not readily found in the US.

3.90 (10/39) and 4.22 (9/38) are the two most encountered gear ratios in the US. Most common in the Spridgets. 4.22's in the earlier ones and 3.9's in the later ones. The MG1500 transitioned to a 3.73 in September 1977 and is a good gear for the A-series for highway cruising.

HTH, Mike
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]The MG1500 transitioned to a 3.73 in September 1977 and is a good gear for the A-series for highway cruising. [/QUOTE]

Especially nice with a 5th gear, for freeway driving. :laugh:
 
Hi...New guy here, With the 3.73 gears and Datsun 5-speed, what is your 1275 engine taching say, at 70 mph? Thanks
 
DRH said:
Hi...New guy here, With the 3.73 gears and Datsun 5-speed, what is your 1275 engine taching say, at 70 mph? Thanks
Click to expand...

If you are asking me, that is mated to a 1500 not that it really matters in this equation.

It's about 3300-3400 rpm.
 
Rick is running a Datsun 1500. I really think a 1275 would have a hard time pulling the 3.7 on top of a 5 speed. I went up to a 3.9 behind a 5 speed and a stock 1275 and think I will drop back to the 4.22s.


Kurt.
 
But Kurt... you still have 4th gear. There are plenty of stock 1275 midgets with a 3.9 diff.

I use a 3.9/5-speed/worn out 1275 in my midget. It holds 65-70 mph in fifth just fine unless it is a steep grade, but then 4th gear is there like before.

My Sprite has a 3.9/5-speed/supercharged1275, and it can accelerate nicely in 5th gear ... even on a grade.
 
nomad said:
Rick is running a Datsun 1500. I really think a 1275 would have a hard time pulling the 3.7 on top of a 5 speed. I went up to a 3.9 behind a 5 speed and a stock 1275 and think I will drop back to the 4.22s.

Kurt.
Click to expand...

True enough, I was only commenting on the rev numbers @ 70MPH which would be the same for any inline 4 cylinder engine given the gearing involved.
Power is another question. :laugh:

If we want to talk about pulling up a hill in 5th with a 3.7 rear end then I'm all over that...
Switching from the economy head to the big port head and swapping the small downdraft hitachi carb for the pair of HIF4's made a big difference!

:jester:
 
Trevor Jessie said:
But Kurt... you still have 4th gear. There are plenty of stock 1275 midgets with a 3.9 diff.

I use a 3.9/5-speed/worn out 1275 in my midget. It holds 65-70 mph in fifth just fine unless it is a steep grade, but then 4th gear is there like before.

My Sprite has a 3.9/5-speed/supercharged1275, and it can accelerate nicely in 5th gear ... even on a grade.
Click to expand...

Ah, Trevor, I've taken a look at your powerplant and it definetly would not have a problem with the higher gears!!
And yes if you don't mind shifting down they will work as well. The car that I am talking about labors with 3.9's trying to hold freeway speeds. Its a 66 midget so a little heavier than a BE.

Kurt.
 
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