Just to post some info. Don't know if it is of any value. Removed a stock OEM oil pump from a 225 engine, it had the short rotor, but also did not have a key for the drive gear. Opened a box with a NOS TRW replacement, oil pump. It was made by Melling, Had the medium length rotors, keyed gear, but only 5 bolt mounting. It had the sixth mounting boss, but it was not drilled.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

That "checks-out" with what I have seen... 6 holes is the 5/8 (thin) rotor and 5 hole is the 7/8 rotor (medium thickness)

This is even called out in the Melling and other parts catologs, Kit # K66A, 5/8 thick rotor "6 Hole Pump" w/gear. K66CA, 7/8 thick rotor, "5 Hole Pump" w/gear.
Has anyone ever seen a 5 hole pump with a 5/8 (thin) rotor?

The word from Melling is that they are dropping the keyway / key from thier SL6 pumps. All the recent kits I got from them have no key. Dynagear and factory OEM pumps never had a key.
DD

The pump I have on Slantzilla is as Charlie described. Autozone, 32.95 when I put it on 3 years ago.

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Official Cookie and Mater Tormentor.