Can someone tell me what the crank thread size is for the front of the engine?
ie. the bolt the crankshaft pulley is held on with? (Not the A/F, that's 1 5/16)

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

5/8-16 UNS, or 5/8-16 Whitworth form, depending who you believe. Same thread in both ends of crank.
Mini Mania and Minis Plus carry the 5/8 UNS tap.

If it's just full of wasps nest or crap, you can groove an old flywheel bolt and use that to clean it out like I did.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks Kev

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

It's a special non standard 5/8" 16 TPI whitworth form thread. Standard 5/8" whitworth thread is 11 TPI (turns per inch).
Whitworth threads use a 55 deg included angle form thread. BSF (British Standard Fine) threads are fine whitworth form threads. 5/8" BSF is 14 TPI.
Unified thread forms use a 60 deg included angle form thread.
The bolt and nut/hole rely on friction between the flanks of the mating parts. If the angles are different then there is insufficient friction force developed to stop things loosening as the flanks don't touch properly.
The thread forms, (55 deg vs 60 deg) do not allow a Whitworth threaded hole to mate properly with a unified threaded bolt irrespective of the TPIs being the same.
One of the first things you learnt when both Whitworth and Unified threads were used on british cars of the 50s, 60s and 70s is that you do not mix unified and whitworth thread forms if you want things to stay tightened.
The same problem arose when Rolls Royce aircraft engines were to be manufactured by Packard in the USA during WW2. The British had to supply Whitworth threading tooling so that the engines and parts were interchangeable as the USA was using unified threads everywhere.
In times past (19th century) there were no standard thread sizes. If you needed a threaded fastener you came up with your own design. Result chaos.
Most thread forms use less TPIs as the diameter increases but there are constant TPI thread series for particular applications. What applications? No idea.
There just happens to be a special series of unified threads, UNS series, one of which uses 16 TPI for all thread diameters which some have used in place of the correct whitworth form crankshaft bolts. There are also similar special whitworth form threads with the same TPI for all diameters but I don't have any info.
BMC provided a special tap to use to clean up damaged crankshaft threads - 18GA03.
If you are tempted to mate unified threads with whitworth threads use lots of high strength Loctite and check regularly otherwise you could end up with an expensive boat anchor. Not a good idea.

Hope this helps
RonR

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68 build Mk1 Cooper S
72 Clubman GT project. Total restoration required

That would have to be probably the only Whitworth thread left in the whole A series motor?
BMC, Rootes Group, Ford and most other UK makers got together in the late fifties and standardised on Unified threads.
So `officially' there was no more whitworth, BSF used (on motors) from the Minor 1000 motor onwards. Mind you some of the old stock MM suspension stuff (shocks) still had BSF threads (and their funny sized nuts).

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Excellent information from Ron and Kevin.

Can we make this a sticky please, questions about the crank thread seem to get asked every now and then, so would be great to have this excellent info readily available.

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Steve

I wonder if the strange thread was a machining convention for the machine tooling.
I would guess that the crank lathes came with a specific thread related to the manufacturing rather than the need to use a common thread. Do Morris Minor cranks use the same thread?

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Yes the Minor crank bolt is the same as a Mini, except for the crank handle appendage that it has.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Further to this topic, do the last of the A series engines used in JRA Rover Minis still use the special Whitworth crankshaft thread?

A bit more info.
One of the special constant pitch threads is the 26 TPI Brass thread which IIRC is of Whitworth form.
Also Lucas generators use BSF threads, IIRC on the pulley nut.

As far as I know the original A series engine was designed prior to WW2 but didn't go into production until the early 1950s when it first appeared in the Austin A30 in 803cc from. It is an Austin designed engine. It appeared later in the first Morris Minor Series 2 from 1953? The first of these used the A series engine and gearbox but retained the Morris rear axle. From around 1954 that changed to the A series rear axle. The first lowlite Morris Minors circa 1949 used the Morris side valve engines, gear boxes etc.

RonR

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68 build Mk1 Cooper S
72 Clubman GT project. Total restoration required