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Differential reduction ratio

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Differential reduction ratio #1
Pezzalonga Brian C Teramo, Abruzzo, Italy Sign in to contact 1959 MG MGA 1500 "721"	Topic Creator (OP) Apr 11, 2024 07:30 AM Joined 4 years ago 120 Posts

Hi...can someone please definitively identify the ratio on this diff? Supposedly from an MGB, which should be a 3.9 if standard, but looks like a 4.3 to me. Thanks. It has 10 splines.

Edited 1 time(s). Last edit at 2024-04-11 07:32 AM by Pezzalonga.

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Re: Differential reduction ratio #4
Pezzalonga Brian C Teramo, Abruzzo, Italy Sign in to contact 1959 MG MGA 1500 "721"	Topic Creator (OP) Apr 11, 2024 08:13 AM Joined 4 years ago 120 Posts

Thanks for the quick reply guys.

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Re: Differential reduction ratio #5
MandoG Greg M Melbourne, VIC, Australia Sign in to contact 1959 MG MGA 1500 1959 MG MGA Twin-Cam	Apr 11, 2024 08:27 AM Top Contributor Joined 2 years ago 382 Posts

What's stamped on the edge of the Crown Wheel?
Does it match the stamping on the pinion?
i.e. 10/39 (not 9/41 as shown in my photo)
If all good then for your 1500 need to use your original spider gears (10-spline) to fit with your axles (check still 10-spline).
Cheers
Greg

P.S. I love that photo I took of my diff but I didn't get too many 'likes' on Insta sad smiley

Edited 3 time(s). Last edit at 2024-04-11 09:09 AM by MandoG.

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Re: Differential reduction ratio #6
RJBrown Randy Brown Queen Creek, AZ, USA Sign in to contact 1957 MG MGA 1500 1958 MG MGA 1500 Coupe 1960 MG MGA 1600 "Later" 1962 MG MGA MkII	Apr 11, 2024 09:32 AM Joined 12 years ago 4,307 Posts

In the first picture the only splined piece is the front of the pinion gear. Those splines are all the same on all MGAs I believe. I’ve never counted nor worried about that number.
The side gears seen in the picture in post #5, have 3 spline count choices 10, 25 and 26. Axle has to match side gear. The only other place I’ve had to count splines is the clutch disc/transmission input shaft interface.

Edited 1 time(s). Last edit at 2024-04-11 10:00 AM by RJBrown.

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Re: Differential reduction ratio #7
Endoman John Halton Bolton, UK Sign in to contact 1957 MG MGA 1959 MG MGA Twin-Cam 1959 MG MGA Twin-Cam Coupe 1989 Porsche 944 S2 & more	Apr 11, 2024 06:17 PM Top Contributor Joined 3 years ago 1,146 Posts

In reply to # 4783608 by copernicus Divide the 43 by 11 = 3.9. 10 splines would lead me to an early MGA 1500 as MGB's typically had 25.

Wrong splines that's halfshafts. MGAs did not use 3.9 as standard Normally 4.3, 4.1 on MKIIs and the occasional 4.55 as a competition alternative. 4.875 and 5.1 were also around from saloons and commercials.

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Re: Differential reduction ratio #8
copernicus Nick Kopernik Western, CT, USA Sign in to contact 1962 MG MGA MkII	Apr 12, 2024 06:30 AM Top Contributor Joined 10 years ago 6,919 Posts

3.9's were available as an option for MGA, but I'd expect that there were very few that came out of the factory that way. More likely 3.9's found in MGA were converted over from early MGB's. There's a good article (attached) from the Chicagoland MG Club that delves into "diffs":

https://www.chicagolandmgclub.com/driveline00/0601/go_brit_baby.html

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Re: Differential reduction ratio #9
bills Bill Spohn W. Vancouver, , BC, Canada Sign in to contact 1956 MG MGA 1958 MG MGA Twin-Cam 1962 MG MGA MkII Coupe 1965 Jensen CV8 & more	Apr 13, 2024 04:18 PM Top Contributor Joined 19 years ago 17,991 Posts

I ran the 5.125 diff in my TVR race car just for use in a couple of hillclimbs. It was great - would leave 30 feet of rubber at the start and topped out around 90 mph at red line, but it got there very quickly! I have also used 4.88 (hill climbs) and 4.55 - short race course. It gets a bit tiresome to have to change them for single events though, especially on the TVR as it was an IRS suspension.

I still have one TVR diff case, if anyone wants to convert an MGA to IRS.....

Bill Spohn www.rhodo.citymax.com/carstuff.html
Current: 1958 MGA Twincam (race car (170 bhp)),1962 MGA Deluxe Coupe (98 bhp)
1957 Jamaican MGA (200 bhp)1965 1971 Jensen Interceptor (350 bhp)
2009 Pontiac Solstice GXP Coupe (375 bhp)
2007 BMW Z4M coupe (340 bhp)
Recent: 1969 MGC roadster (175 bhp),Jensen CV8 (375 bhp),
1969 Lamborghini Islero S (350 bhp), 1988 Fiero GT turbo (300 bhp)
North Vancouver BC

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Re: Differential reduction ratio #2
sh8kin denis nalepa red bank,new jersey, USA Sign in to contact 1958 Austin-Healey Bugeye Sprite 1959 MG MGA 1500 1960 MG MGA 1600 1960 MG MGA 1600	Apr 11, 2024 07:47 AM Joined 13 years ago 141 Posts

Re: Differential reduction ratio #3
copernicus Nick Kopernik Western, CT, USA Sign in to contact 1962 MG MGA MkII	Apr 11, 2024 07:49 AM Top Contributor Joined 10 years ago 6,919 Posts

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