This item is for the disctinctive round head 'captive notch' type engine bolts and 5/16 x 25tpi nut combination, as fitted to early douglas models - i.e. 2 3/4hp (they may also fit 4hp models - but please email for dimensions). These bolts are CNC'd in stainless steel with correct threads and have a captive silver steel pin fitted in each bolt.
the Item listing is for one bolt (with silver steel pin fitted), one nut and one washer - all in stainless steel.
Original Douglas 2 3/4hp models secured the engine to the frame with two half clamps (part number 662d, shown in the first photograph) which matched up to the bottom of the crankcase which had similar half clamp lugs machined into them. And then (like so many other Douglas fasteners of this period!), Douglas used their own special engine bolts with a round curved head and 25 tpi thread to fasten the clamp/engine. In the first photograph in this listing - you can seen an original Douglas bolt (i.e. 100 years old), which although re-nickel plated, you can see an amount of underlying pittng and slightly worn thread.
however, if you look closely at the original bolt - you will see that it had been cast/forged with a 'notch' under the round head - which matched up with a corresponding notch cast into the frame clamp (662d), which meant the head could be round (not hex) and there was no need for a spanner on the head of the bolt when tightening the engine bolts.
Not suprisingly - these bolts are now very rare, and I would imagine most are now either corroded badly, or the notch/thread are very worn. I would also think it is reasonably common for later owners to fit (non-Douglas) later BSC 26tpi nuts in error. Although I have a few of these original bolts - as per the re-nickelled one in the photo, I decided to make a batch - as a couple of the ones I needed for my own project bike were missing, or threads not great.
The bolts shown here have been CNC manufactured in stainless steel by ourselves and replicate the gentle curved head and diameter of the originals - so they look spot on. Each bolt head is polished, so they look pretty too. However, we then use a milling attachment on our CNC to side-bore the shaft of each bolt directly under the head - to accept a 2.5mm Silversteel pin. The silversteel pin is a size fit and inserted with Loctite Retaining compound (i.e. industrial steel glue), which holds it captive. The hole is only bored for approx half of the shaft diameter, and being only 2.5mm should not affect the strength of the bolt. As can be seen in the accompanying photographs of our new bolts inserted into an original Douglas engine clamp, the pins match up with the notch in the clamps exactly and work very well.
Similarly, the nuts are CNC machined in stainless steel, and are of a slightly greater depth than commercial 5/16^ nuts (i.e. giving greater strength), and are also of the correct 5/16" x 25tpi Douglas thread form.
This item listing is for one engine bolt/one nut/one washer - all in stainless steel.
Note: Earlier models of Douglas 2 3/4hp models (1913 - 1919 I believe) also had an additional clamp/lug (part number 615d) to hold the brake pedal - which also employed these bolts. The last photograph shows a modified version of this 615d assembly, which I have fitted to my own 1919/20 project bike.
Sorry these are not cheap - but as well as requiring custom made taps for the threads, the extra effort of milling and fitting silversteel pins make them very labour intensive to produce
There is no reason why - if you are not fussed, that you could replace this type of Douglas bolt with standard modern 5/16" hex head bolts and nuts (and instead use a spanner on both sides when tightening) - but if like me, you would prefer to replicate the original Douglas design and originality - then these do look very nice when fittted