Strange thread

[Graham_Wright]

Five screws securing drive plate to flywheel, BMC 1.5 diesel.
26 tpi X 0.23".
I assumed it was 3/16" UNF and ordered more.
Too small, ordered 1/4" UNF too big.
Then I measured. I can't find a thread with these dimensions.
BSF is 1/4" X26 tpi is the nearest but doesn't fit.
Very unlikely to be metric but 26 tpi equals 0.97mm which could be 1mm. Diameter converts to 5.84 which could be 6mm. That doesn't fit either.
Weird.

 

[Boater Sam]

I am very confused by this. Could the flywheel have been changed or redrilled?
From memory there are 6 machine screws, 1/2" AF spanner head, 5/16" UNC
But it has been a while.
Phone Calcutt Boats, they will give you a definitive answer as they rebuild these all the time.

 

[Boathook]

Seems very close to 0 B.A thread with core 0.2362 and 25.38 tpi according to my zeus booklet.

 

[colind3782]

IIRC BSC is 26 tpi. I have a few on my old Royal Enfield.

 

[VicS]

1/4 " BSF ?

IIRC BSC is 26 tpi. I have a few on my old Royal Enfield.

BSF ?? My first thoughts

But same diameter as UNF which OP says is to big.

 

[colind3782]

1/4 " BSF ?

BSF ?? My first thoughts

But same diameter as UNF which OP says is to big.

British Standard Cycle BSC

 

[rotrax]

Seems very close to 0 B.A thread with core 0.2362 and 25.38 tpi according to my zeus booklet.

Exactly my thoughts.

0 BA was often used at the time.

A popular mechanical/elecrical engineering thread.

 

[rotrax]

British Standard Cycle BSC

BSC is the same as BSF in the 1/4 inch diamerter IIRC.

 

[B27]

We used to call this 'spanish pushbike thread'.
Anything that's not an obvious choice can still happen.

Coincidentally, I was tidying today and found a lot of BA spanners.
See also 'model engineer threads', basically a pitch that's easy for the lathe regardless of diameter?

 

[Graham_Wright]

I am forced to the conclusion it is metric. I found it unlikely it was metric but the gearbox is German (made in Italy!) so it is possible.
Unfortunately my extensive stock of screws went with the4 closure of my factory although the one I found was a good fit.

 

[Graham_Wright]

I am very confused by this. Could the flywheel have been changed or redrilled?
From memory there are 6 machine screws, 1/2" AF spanner head, 5/16" UNC
But it has been a while.
Phone Calcutt Boats, they will give you a definitive answer as they rebuild these all the time.

Those are the ones securing the flywheel to the crankshaft and they are UNF.