Britbike forum Forums Thread Like Summary - BSA A65/50 - crankshaft drive side thread.

I am trying to fix a bad thread on my two spare crankshafts. One is the older one with 2 plugs for the sludge trap and one is the newer with only one plug. Threads on both crankshafts are 5/8" 20 tpi but which type of thread, CEI, UNF(S) or..?
Thanks!
Rgrds,
Lars

In Unified there is 5/8-18 UNF and 5/8-24 UNEF. 20 UN would be a constant pitch series which can be used on any diameter.
However, because UN uses a truncated crest and valley and BCE uses a rounded crest and valley they might not interchange.

Thanks Kevin for your input. As Allan wrote it would make no sense that BSA used something else than a BSC/CEI thread. Nuts, many times, were badly damaged which effected the thread on the crank. Both your and mine experience indicates it to be a 5/8" 20 tpi BSC/CEI thread.
Rgrds,
Lars

Not bring funny here, but why does a tap clean the nut up nicely afterwards? I wouldn't say the cranks are funky but the nuts are bring made with someone with a fraction of the skills god have them.

Just a thought.

Appologies Lars, theres several threads on the BSA crank, though I guess theres only one at 5/8", even still though the same logic applies. I have yet to see one which is different. The part number (29-2023) remained the same part number from 1962 until the bitter end. There isnt a UN size thread anywhere on a pre 69 bike (unless a previous owner put it there).

There are however plenty of nuts on the market which have not been machined properly. Same with clutch mainshaft nuts. Before I owned a comprehensive set of taps and dies I would end up buying a bag of 6 of each to find only 1 which fitted.

Hi Lars,

Good to hear that you got sorted but just to add to this thread (pardon the pun) here is my own experience with this particular issue.

On my recent engine rebuild I bought a brand new 29.2063 rotor nut from SRM and found that it would not screw on to the crankshaft threads easily. It was in fact so tight that I did not want to force it for fear of damaging the threads on the crankshaft. The nut that was on the engine before I started the rebuild was badly damaged. It had been butchered with a hammer & chisel in the past and also had a hole drilled and tapped in it, presumably for a grub screw? Despite it's poor state it screwed all the way onto the crankshaft threads very easily by hand. I have no idea if this was the original nut fitted to this engine but it was what was on it when I bought it.

Anyway I bought a brand new HSS 5/8" 20TPI BSC die from Tracy Tools and ran it down the crankshaft thread. I was surprised at the amount of material it moved but, as the new nut was so tight and the thread that was being cut looked good, I was not unduly worried and believed I was on the right track.

Once I had run the die all the way down the crankshaft threads and removed it I tried the new nut on again and it fit perfectly. Screwed on easily by hand and no slop. A nice snug fit as you would expect when something is right.

I am glad you got it sorted out

Something still fishy about the thread forms used on these cranks

The fact that Kevin E states "I was surprised at the amount of material it moved but" tells me that the PD or form is unique as compared to what the books "says"

When measure the PD on my cranks, the PD is larger than the typical 5/8" BSC form

The original question seems (ok I have a lot of "senior moments" these days) to be What is the thread FORM of the 20TPI thread.

it will be Whitworth form ie 55 degree included angle . Unlike the UNC/UNF series which are 60 degrees included angle. (UNC/UNF, being what you have termed "American thread") .

You can get away with it on smaller threads eg 1/4 Whitworth using 1/4 UNC bolts instead but you cant get away with it on larger ones like your 5/8" 20 TPI series.

Yes BSA did change some threads to the UNC (American) system in latter years but they did not change the crank threads.

confused ? I am .

D Madigan said "unless the parts book is wrong" ... I think it MIGHT be wrong ...when i saw that I though beauty and got a few 5/8" UNF nuts ... you guessed it they did not screw on !