Has anyone come up with a suitable plug (or bolt) to cover the threaded holes in the rear frame lugs (in front of the rear wheel spindle)?
I have never seen a photograph where the holes have actually been plugged but there must be someone out there that has done it.
On the vast majority of frames those sidecar fitting holes are threaded 18mm at 1.25mm pitch, just the same as 18mm spark plugs, so it would be very easy to make something vintage-looking out of a couple of old 18mm plugs.
Alas, no. I tried some 18 mm spark plugs and they don’t fit.
On mine, the thread is 10 tpi and the diameter is either 3/4″ or 23/32″ (or something else entirely).
An internet search suggests that 10 tpi bolts are either BSW or UNC.
One would think that sidecar fixing bolts would all be the same in the 1930’s.
All very interesting stuff.
On all but one of the Scotts that have been thru’ my hands, those lugs have been 18mm metric. The one exception is an early (1928) TT bike that has 3/4″ Whitworth threads @10tpi. Ironically, when the bike was new it was sold to Holland, and fitted with a sidecar for the Dutch TT as Assen. You would think that metric threads would have been far more appropriate !
[attachment=0:1egim7qv]IMG_2116.JPG[/attachment:1egim7qv]Well, as I couldn’t find anything metal to fit the holes, I tried Oldguy’s suggestion and bought some plastic plugs off Ebay (less than £2 for both) and, he was right, they look ok.
I will still be on the lookout for some metal plugs, but the plastic plugs are certainly a lot better than the threaded holes.
A bit of information to add to the pile. I’ve been making up fittings for these holes that will support the bobbins used for a paddock stand on a competition bike I’m building for a customer.
It’s a 1929 Flyer frame. The left hand hole is threaded for a 3/4” x 20 tpi (60deg) bolt and the hole on the right is threaded for an M18 x 1.5 bolt. I believe this is the 18mm spark plug thread pitch.