Dominator 99 Crankshaft Drive Side Thread

I've just done a quick check with the only two crankshafts easily accessible in my workshop, but they are both dynamo cranks, not alternator type - that's too deeply buried and one of many "projects" yet to see light of day!  I can't remember if the shaft is a different size.

Anyway the figures I have are; shaft diameter 13/16" (only measured with a ruler not gauge) and the TPI is either 18 or 19!  I can't find a chart that lists this, although  taps are available.  It sounds like you need a die after your "accident"!

Uni-Thread do thread restoring files for £19.95.

Drive side crankshaft thread is seven eighth's ( •875" ) diameteŕ × 20 tpi. Checked with a micrometer and thread gauge.

This is from a 99 dynamo crank I have on the bench, hope this helps.                         

Ian - Are you sure that the figures you quote are for a 1962 Dynamo 99 crankshaft ?  That would be a very rare crankshaft indeed.  They did of course stop making dynamo crankshafts in 1958 though there were dribs and drabs in certain export models for some months later than that.  So, Edward, do you have a dynamo crankshaft in which the nut fits just beyond the sprocket or an alternator crank which has the extension for the rotor between the sprocket and the nut ? Cheers, Howard

RGM only lists a dynamo crankshaft nut which they say  is 0.81" (20.5mm) minor diameter.  Andover Norton list an alternator one but no sizes are given.

Where does Ian say his is a 1962 99?

on the basis that a 20tpi thread is •031"deep the core diameter or inside diameter of the nut should be about •812" which would be correct for the size RGM quote  which would be correct for the seven eighth's  shaft size I previously mentioned.                                                      It is likely that the alternator crankshaft would have a smaller thread dependant on the size of the rotor bore.

Hello again - I am sure it specified 1962 99 crank.  It must have been editted out whilst I was writing........ Cheers, Howard

dynamo crank 7/8-20

Alternator crank 5/8-20

Is die size, but 55 or 60 degrees uncertain

may be 20 series of cycle thread

I use 60 degree UN and it works fine.

"If it's an alternator crank with a 7/8" x 20 TPI you can get a die on eBay now for £14.95"

If it is an alternator crank with a 7/8 x20 tpi then it is probably the only one on earth...

What is the point of a new NUT when it's the crank that is messed up?.

Trying to screw ON a die over bad threads is quite difficult to get right. You may just want a skilled machinist to use his own tools and die to fix the crank as the end is usually the problem. Opening the "adjustable" die (a bit) to get over the bad threads then close it down on the threads, then  with draw/unscrew the die to clean the threads. Done it many times on many cranks...

Lionel, it's the dynamo crank that is 7/8" , the alternator one is 5/8".

Yes, I've got it!  The cranks I measured at home were dynamo cranks which are 7/8".  Edward has measured his at 5/8" x 18 tpi. My point about needing a new nut is obvious.  If you are repairing the thread you would always get a new nut - AFTER you had used the old one to clean the thread a bit.

I wonder if he's damaged the thread much or if it's more a case of being heavy handed with a hammer on the end of the crankshaft?

The 5/8 inch X 20 TPI BSCycle thread used for the alternator nut is still a Whitwoth thread form and so retains the 55 deg angle at each peak and valley of the thread.   The 5/8 inch X 20 TPI UNF thread form has a symmetric V pattern that produces a 60 deg angle between the flanks.  It also uses a smaller spanner size.  I have used the UNF nut on damaged crankshaft threads but like to use a good condition UNF thread file before fitting the nut.   Good Luck Edward, Howard

 Cycle thread angle is 60 degrees . It is Whitworth form ie a vee  , not a square ,buttress or acne thread form. Unf or unc is also 60 degrees so a Un thread file is the correct one to use  to dress up bruised cycle threads in the absence of the right die. An imperial thread pitch gauge is the one to use on old British bikes, its easy to see if the gauge fits the thread form correctly to identify the pitch.Cross reference the pitch and diameter usually identifies the thread, but there are a few exceptions so be aware. Hope this helps.

It's a whole art form in itself Edward!  Not difficult to find the info with Mr. Google!  This site (one of very many!) shows all the main thread forms in drawings so illustrates them quite well. https://britishfasteners.com/threads/bsf.html 

You'll see from this (if you haven't died of boredom!) that Whitworth (BSW) and BSF threads have a 55 Deg angle whereas BSCy, UNC and UNF are all 60 deg.  It means that with any given nut or bolt size you can get and interference fit by using the "wrong" thread form.

This site also, very usefully, gives you the tapping drill sizes with the various drills by letter, number or diameter.  I bought all the sets many years ago - from Screwfix I think.  Tap and Die sets are relatively cheap when you are only likely to use them a few times, as opposed to professionally every day.