46 replies to this topic

[dklawson]

OK, this is another would-be thread form post but I hope it doesn't run the way the crank tail tap thread went.

On another board I was reading a post where a guy made the argument that the Mini drain plug was NOT 5/8-11 (UNC) as I have been led to believe for years. The proposition he made was that the tapped hole is really a Whitworth thread. His argument for this was the radiused crests and roots of the tapped hole.

With it's narrower thread angle I guess a Whitworth threaded drain plug would go into a 5/8 UNC tapped hole and maybe that's why the HeliCoil drain repair kits have worked over the years. However, does it make sense that BMC would have tapped the drain plug hole Whitworth when the rest of the gearbox being threaded UNC and UNF?

Thanks for your observations and comments.

[newdawn1st]

Couldn't possibly comment myself but Martin from MRA Mini's on here may be of some help, as far as I'm aware he does a lot of machining for mini parts so would be a good start point

[dklawson]

Thanks. After the last Whitworth thread string that ran on this board I didn't think anyone would reply at all.

[icklemini]

Well the helicoils i use for stripped threads are unc and they work...

whitworth or unc? - um to be honest who cares!

Edited by icklemini, 12 June 2009 - 09:40 PM.

[MRA]

The sump plug was a newly designed part specially for the Mini, and therefore I use UNC the same as the rest of the box..... I have machining drawings for the gearcase, but all it has is the machined surfaces and holes no thread info at all

[Ethel]

Wasn't the crank unique to the Mini too?

Do Minispares say anything on the subject regarding their taper repair plug. I imagine they'd need to get the thread form right for it to work.

[MRA]

Yes in theory, however it was a modified design based on the inline moggy, A40 etc.... and as they where using the Whitworth form it carried over, bottom pulley bolts are the same......

[Cosford Cowboy]

To close the question, I have a full gearbox case drawing set and they do show the drain to be 5/8" UNC

[Tom7]

BMC didn't use any whitworth threads on the post war Morris Minor, I don't see why they would on a mini considering they didn't carry over BSF to them and that was used on most BMC bodywork and pre A-series mechanicals (bizarrely with Metric fine threads!..another story) I'm amazed they used a course thread at all! A lot of ally stuff got fine threads such as Minor/Sprite gearboxes and they really needed a course and probably larger thread in some places.

[icklemini]

old thread revival...

(pun intened)

[dklawson]

But... the A-series did maintain a "Whitworth form" thread for the crankshaft. Not a true Whitworth thread... but a Whitworth form having the crest/root radius and Whitworth thread angle. The reasoning is typically presented that the Whitworth form thread is stronger than the UNF/UNC series 60 degree threads.

Regardless there are a few different thread forms on the Mini other than UNF and UNC. The reason I started this thread years ago was in response to discussions elsewhere regarding new, aftermarket drain plugs that did not fit the Mini properly.

[[email protected]]

We actually have a drawing for this and it states 5/8"-11BSW

[MRA]

But... the A-series did maintain a "Whitworth form" thread for the crankshaft. Not a true Whitworth thread... but a Whitworth form having the crest/root radius and Whitworth thread angle. The reasoning is typically presented that the Whitworth form thread is stronger than the UNF/UNC series 60 degree threads.

Regardless there are a few different thread forms on the Mini other than UNF and UNC. The reason I started this thread years ago was in response to discussions elsewhere regarding new, aftermarket drain plugs that did not fit the Mini properly.

Whitworth was used on the bypass pipe in the head, I have official drawings of 1275 head castings with whitworth.

[MRA]

We actually have a drawing for this and it states 5/8"-11BSW

Simon is that an old drawing, because I have DAM6639 which is the later case and that uses 5/8 x 11 UNC Class 2B Whitworth is better though

[Cooperman]

Welcome back Martin. I hope all is well for you. We've missed your input and common sense engineering on here.