Lister D Thread size?

Somebody must know! I am in the early stages of restoring a Lister 'D' (1948). What is the thread size of the water hopper drain tap ?
The drain tap is missing,although the boss is present,but the thread is a rusty mess!! Many Thanks Colin
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1/4 BSP. David Harris (see SEM) had, and probably still has, repro bosses for sale. hth Roland

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