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ok, you play Phil and Errol can play Rita

Bloop, bloop, over my head....

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groundhog day?

Apologies for the delay in measuring the HR and FB diff widths. The rain didn't help, then I went through the "there's a hole in the bucket" saga with the floor jack (but the seals are too small, dear Liza, dear Liza...). With thanks to Craig's advice, the new Armstrong jack got it's christening.

I took the rear wheels off both vehicles. I put a stack of wheel spacers onto the rear drums to give me a flat surface to mount to (I later subtracted this width to give the measurements below). Used some straight lengths of aluminium, sash clamped to the drums to give me two points to measure off (and to make sure the drums were seated). Used Grace's new tape measure (don't tell Grace). The measurements below are drum-to-drum with brakes installed... the same distance that a pair of wheel mounting faces sees. Be wary that some other references quote axle flange-to-flange (i.e. with brakes removed).

The HR measured up as 1421.5mm.
The FB measured up as 1428.5mm.
i.e. the FB diff assembly is some 7mm (0.28") wider.

Allowing for some measurement error could get it closer to the 0.4" (10mm) track difference that the workshop manuals indicate:
FE-EH's have a 54.5" track across all models.
HR commercials have the same 54.5" track.
HR non-commercials have a 54.9" track.

So I reckon:
HR non-commercial wheels have +13mm offset (from Clay's measurements)
FB wheels have +19mm offset (from my measurements).
On the same diff, HR non-commercial rims sit (19-13)x2 = 12mm wider track.
Moving from an FB diff to a HR diff, the diff is 7mm narrower, but the wheels are 12mm wider, so 5mm wider track all up (allowing for error in all the field measurements above, 0.4" as per the workshop manuals).

I reckon the commercial wheels are about +16mm offset, to give the same HR/FB track when fitted to a HR diff.

Cheers,
Harv