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3.9 Diff

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John Moore

John Moore

Luke Skywalker
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I've read that guys with 1275 Bugeyeslike to swap the 4.22 rear end for the 3.9. Is it better for HWY cruising? I assume it would help keep the revs lower. Was the 3.9 used in all later spridgets or only certain years? Is it just plug in an play or other mods required for the bugeye axle?
 
3.9 is easier to find, 3.7 is difficult.

A nice running 1275 will pull both nicely.

Steve
 
So, if I were looking for a 3.7 what year, make & model donors should I be trying to find?
 
Look at the both of this page. It will show the RPM differences for the different differential. I've got a couple 3.9's if you need one. It is a plug and play change, pull the axles, unbolt the drive shaft, remove the brake line Tee and then the nuts that hold the diff in. I can swap the rear on the track car in 30 minutes without rushing.
 
Unless you are drag racing, the 3.9 matches fine with a street 1275.
 
Chris,

Eventually, I would like to get one. There are a few places I can look locally that have spridget parts, I just didn't know which year cars had them. They also seem to do pretty well on ebay. But seems like it would be pricey to ship.
 
3.7s were installed in some later RB Midgets, mostly the last couple of years, so look at 77s through 79s. New gears are available but you'll have to have the gear mesh set up. Best bet is to find a complete pumpkin so it's a plug and play job.
 
"oldgreynorm" on e-bay usually lists at least one per month. They seem to be coming down in price..this summer they were going for $160-$180, his last one just sold for south of $100 I think.
See this one..200297960226
Ends in 3hrs and is at $50 now..it will go higher I am sure. SHipping on it to HOuston is quoted at $30..as you're in NY..as he's in Western PA..should be same or less I would think.

If he says it looks good..it is. I have bought many many parts from him, they are all as described or better. My favorite vendor for Spridget parts.
 
Devin,

I'll check that out. I want to get one eventually, but have bigger fish to fry with my bugeye body restoration.
 
I've run a 3.7, 4.2, and a 4.5 with both a 4 speed and later a 5 speed in a MK II. As mentioned, it is a half hour job. Watch for used complete sets which are quite cheap. If your not satisfied you'll be able to sell it for what it cost.
 
richardjbondy said:
I've run a 3.7, 4.2, and a 4.5 with both a 4 speed and later a 5 speed in a MK II. As mentioned, it is a half hour job. Watch for used complete sets which are quite cheap. If your not satisfied you'll be able to sell it for what it cost.
Click to expand...

When you say "complete set" do you mean the entire axle or the diff you slide in?
 
64rocksprite said:
"oldgreynorm" on e-bay usually lists at least one per month. They seem to be coming down in price..this summer they were going for $160-$180, his last one just sold for south of $100 I think.
See this one..200297960226
Ends in 3hrs and is at $50 now..it will go higher I am sure. SHipping on it to HOuston is quoted at $30..as you're in NY..as he's in Western PA..should be same or less I would think.

If he says it looks good..it is. I have bought many many parts from him, they are all as described or better. My favorite vendor for Spridget parts.
Click to expand...

I sold one at peak gas prices on ebay for $280! It seams as gas prices rose, so did the cost of 3.9 and 3.7 diffs. And as it fell, as did the diffs. I also noticed there was a influx of sellers pushing 3.9 3.7 diffs lately. This helped to lower the price too. I suppose they saw the returns and everyone ran to the barn to rip there old axles apart. Simple economics. Supply and demand.... Now you can see 3-4 listed at once and score one under $100!
 
John:

You just want the diff, but check the marking and\or count the teeth.

At a highway speed the 3.9 lowers my 1275 revs by 200-300. The 3.7 is about another 100-150 rpms.

Pat
 
There is a 3.7 on eVilBay right now for a Buy It Now of only $1200!!

Item # 120361350619

https://tinyurl.com/9fuw8a

Are they out of their Vulcan minds? (Bones on Star Trek)

I've been looking for a 3.9, I did luck into a 3.7 and have a 4.22 in the car.

ChrisS what are you wanting for one of yours?

Paul
 
richardjbondy said:
I've run a 3.7, 4.2, and a 4.5 with both a 4 speed and later a 5 speed in a MK II. As mentioned, it is a half hour job. Watch for used complete sets which are quite cheap. If your not satisfied you'll be able to sell it for what it cost.
Click to expand...

OK not wanting to hijack this thread but just when were you going to tell us about your car? Or am I the only one who doesn't know. It makes me feel all warm inside.

that is stunning! but I really do want to know more

https://tinyurl.com/73c6dv

drool salivate :cheers:
 
JP, do you ever visit britishv8.org? There is all sorts of useful info there. It is all about engine swaps (not just V8).
 
Gear ratio is di stamped on the machined outer edge of the diff case where it bolts to the axle housing at about 11 O-clock as you look at it from the pinion end. At least that is what I seem to remember as I sit in a nice warm house not willing to go double check! You will have to scrape grease to find the stamp. I'm mentioning that because people told me it was stamped on the dff housing but I could'nt find it till I pulled the diff and did extensive cleaning. Someone might like the info!! Also nice wheels rjbondy!!!!
KA
 
JP,

There will be a lenghty article on the Zetec Sprite with lots of detailed how to photos at www.britishv8.org very, very soon. The editorial work is complete and I'm just waiting. I welcome anyone's questions but the publication might be the best way to start.

Thanks for the interest,

Rick
 
Just Vega 13x5 inch steel wheels, 7/16 inch studs, and Moon wheel covers. All very inexpensive, but most of all easy to clean. I have to drive on a dirt road for 3/4 of a mile before I hit the pavement.
 
how did you do the windshield?
 
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