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MG Midget and Sprite Technical - Rocker lock nut thread size

Hi all, found a loose & stripped lock nut on my rocker shaft, went straight for a 5/16 unf too loose!

Took out the screw and measured the diameter & pitch, 26 tpi and 0.275" dia.

Three thread sizes are similar:-
M7 (unlikely) 25.4 tpi and .275" dia,
9/32" BSCycle 26 tpi and 0.281" dia. 60 degree angle.
9/32 BSF 26 tpi and 0.281" dia. 55 deg thread angle.

Spoke to a couple of suppliers and they say all these nuts are 5/16" UNF.

Stumped.

Tony
Tony G

All the engine threads are UNF or UNC. It should be 5/16" unf. If the lock nut is stripped, it would indicate that the stud is also in a bad way. It's easy to check, just try one of the others.
Cheers John
HALL JOHN

Checked the adjustment screw and its good, checked the threads per inch as 26, no UNF or UNC at this pitch.
The rocker is a pressed type (2A964) showing a nut (6K654) so I believe must be a 9/32" BSF or BS cycle thread. M7 is very very close!
Tony
Tony G

It's not UNF or UNC. I didn't think they were used on 1275s, but Minispares can provide replacements. Might be worth investigating if it's original or not first, though.
Philip Dodd

Thanks Philip, I will try Minispares.
MGB Hive and another supplier list 6K654 for pressed rockers, however, their nuts are both 5/16 unf and won't fit.
Mini is the way to go.
Tony
Tony G

It should be 5/16"UNF, Have you tried a 5/16"UNF nut on one of the other good rockers to check? It sounds to me like the threads profile has been damaged through over tightening. Metric, BSF & BS are definitely not used anywhere on the A series engine.
Cheers John
HALL JOHN

Sorry to disagree, but I don't think it's UNF. He says it measured @ 26 tpi, which is neither UNF or UNC. I think the part numbers he reference are correct :

http://www.minispares.com/product/classic/12A1215.aspx

and the locknut :

http://www.minispares.com/product/classic/6K654.aspx

Philip Dodd

Hi John, 5/16" UNF is way too loose.
All 8 of the adjustment screws are the same and threads appear fine.
An engineer friend measured the thread at 26 tpi and dia. of 0.275", he went straight for a m7 which fitted but was a tad tight, he did not have any BSF or BSC nuts in stock to try.
Said the thread was not UNF (24 tpi) or Whitworth.
As the screw was hardened, it easily cut the M7 to size for a temp. fix, until I find the correct nuts.
Looking at Moss parts list, it appears this smaller dia. screw and lock nut was fitted on early 1275's and prior to the sintered and cast/forged rockers
Tony
Tony G

How early is early? I remember changing mine on a 74 midget. I used some stainless half nuts I had spare.
Cheers John
HALL JOHN

They are 9/32 BSF we had some made some time ago and can supply them at 2 each plus vat.

Peter
Peter Burgess Tuning

This thread was discussed between 03/08/2013 and 05/08/2013

MG Midget and Sprite Technical index

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