The MGA With An Attitude
Crank Dog Nut THREAD SIZE -- CS-104B

Crank dog nut installed Crank dog thread dia. Crank dog thread pitch

Sorry folks, cancel all of this. Good try, but no cigar.

By popular demand, thread size for the crank dog nut is 5/8-16-UN.
This falls in between 5/8-11-UNC and 5/8-18-UNF.
Good luck finding either a bolt or a Heli-Coil repair kit with that thread.

But here is source:
1 x 5/8" x 16 TPI UNS (Unified National Special) Crankshaft Plug Tap to suit the Classic BMC/British Leyland/Austin Rover Mini range, both 'A' Series & A+, plus all other similar Engines (FWD & RWD). Precision Tool in Ground HSS, supplied by the Toolmex Corporation, Natick, Massachusetts, USA, part number 5-750-6082.

Truth is, this is not a Unified thread form; it is a Whitworth thread, and a non-standard one at that. It is 5/8-16-BSWS x 1-1/16" long full thread with 55-degree thread angle with rounded crests and roots. The last "S" in "BSWS" means "Special" because of the odd pitch. If you use the UN tap listed above to clean out the female thread, try not to go more then a couple of threads deep to remove damage or burrs on the leading threads.

The following information was supplied by Fletcher Millmore. The "secret" info comes from the 1100/1300 WSM, even has it's very own section number, A5:
"Flywheel Retaining Screw Thread The flywheel retaining screw thread in the end of the crankshaft is not Standard Whitworth but is Whitworth form: Diameter 5/8 inch 16 T.P.I. 1-1/16 full thread. If it is found necessary to clean up the thread, the operation must be confined to cleaning up. This thread is highly stressed and must always be up to full size".

On the transverse engine flywheel (front wheel drive cars),
Tightening torque on this is stated as 110/115 lb-ft, but this has proven in practice to be insufficient. I've taken lots apart to find that the flywheel has been moving on the crank in almost every case, causing fretting damage up to actually shearing the locating key and letting the flywheel spin. General opinion from Vizard etc is 150 lb-ft or more, especially on high performance applications. Very high output engines with special flywheels get 200+, and the threads are fine with that.

I examined a number of A & B series bolts on my own, and tried swapping the various bolts/holes, and the fit is identical no matter the combination. This is even confirmed by the quote from the seller of the "wrong" tap.

Of course, on the front crank bolt, there should never be a reason to torque as much as the flywheels need, though I can tell you that having that bolt loose and shearing a crank key can lead to some fun puzzlements about why the car runs so badly. Eventually you check the valve timing and find it is 90 deg late!. -- FRM

Thank you for your comments -- Send e-mail to <Barney Gaylord>
© 2010-2015 Barney Gaylord -- Copyright and reprint information