Lotus Elan

Pinion flange dimensions

PostPost by: 2tmike » Tue Jun 01, 2021 7:53 am

I'm sure I've seen a sketch on here with dimensions and hole centres but after much searching, I can't find it again.....
Basically I bought a pinion flange at Castle Combe that is the correct OD for my 3.77 diff but I'm not sure about the hole drilling.
What I have is, measured by eye to hole centres :-

Flange OD = 92mm
Hole PCD = 78 mm
Hole spacing, rectangular
Long spacing = 58mm
Short spacing = 48mm
22 splines
Can anyone advise if this is correct for an S4 please?
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PostPost by: jono » Tue Jun 01, 2021 8:59 am

I may be missing the obvious here but can you not just check it against your prop flange?
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PostPost by: 2tmike » Tue Jun 01, 2021 9:39 am

jono wrote:I may be missing the obvious here but can you not just check it against your prop flange?

Not easy to get a good measurement with it on the car and a broken trolley jack......
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PostPost by: Craven » Tue Jun 01, 2021 11:05 am

Not sure if this helps!
diff.jpg
diff.jpg (40.82 KiB) Viewed 445 times
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PostPost by: 2tmike » Tue Jun 01, 2021 6:07 pm

Thanks for that, it does look like I've the right flange for a 3.77 diff within the limits of my measurement!!!!
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PostPost by: ceejay » Tue Jun 15, 2021 11:36 pm

Having three different ratio diffs with non matching PCD was cured by marking out and re-drilliing the flanges to match the rear pcd of the prop shaft yoke flange. No more problems with mis- matched bolt holes, and no need to swap parts around when changing the diff.
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PostPost by: 2cams70 » Thu Jun 17, 2021 10:58 am

Note the PCD diameter of the pinion flange is a function of the differential ratio because the driveline experiences greater shock loading the taller (numerically lower) you go in ratio.

Take for example the following extreme example. Car from a standing start in first gear and the driver side steps the clutch at full throttle. The rear wheels under those conditions present more of an "immovable object" to the driveline up until the point where they loose traction the taller you go in rear axle ratio. The same principle applies to a lesser extent in regular driving. The greater the PCD of the flange the less shear force in the bolts for any given torque input and bolt size.

I would not want to use the small PCD flange on ratios taller than 3.9:1 never mind aftermarket ratios like 3.1:1. Propshafts flailing around under the car aren't pretty! Aside from the flange and bolts the extra stress affects the propshaft itself too of course.
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PostPost by: ceejay » Thu Jun 17, 2021 11:04 am

Been swapping between 3.9 to 3.5 diff for nearly forty years... No issues what so ever. Its not like we are sending 200HP plus through the lile old elan driveline. Old Ford stuff is tough.
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PostPost by: 2cams70 » Thu Jun 17, 2021 11:11 am

ceejay wrote:Been swapping between 3.9 to 3.5 diff for nearly forty years... No issues what so ever. Its not like we are sending 200HP plus through the lile old elan driveline. Old Ford stuff is tough.


The Elans actually continued on with bolts and flanges that Ford had subsequently improved and upgraded in models post Mk2 Cortina. Yes the Elan is a lighter car. 200HP has no meaning in the context of driveline stress. It's torque that matters.
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