I can’t believe it..

[Dellortoman]

Std bolts for calipers are 17mm M10 hex heads (as in not an allen head). The two halves of the calipers are held together by allen head bolts.

Hmm ... OK. I wasn't convinced by your assertion so I did some research. It seems we're both right.

It looks as if the early twins and triples with Ceriani forks had their calipers mostly fitted with hex head bolts (although there are a couple in Falloon's book with Allen heads). The Ceriani forks have the calipers fixed to the inside face of the fork lugs, with the bolts passing through the fork lugs and into threaded holes in the caliper.

The Marzocchi forked bikes have the calipers on the outside of the fork lugs, with socket (Allen) heads bolt going through the caliper flange into threads in the fork lugs.

I've had a look online at a bunch of photos of 500 twins (which is what Andy's bike is) and it looks as if they follow the Marzocchi setup, with socket head bolts.

 

[breganzane]

The Marzocchi forked bikes have the calipers on the outside of the fork lugs, with socket (Allen) heads bolt going through the caliper flange into threads in the fork lugs.

In terms of securing the calipers to the forks / rear sliding caliper bracket...

The front caliper bolts on the triples were always hex head (as in 17mm external hex, not socket head cap screws), Ceriani and Marzocchi bikes alike. The rear caliper was always socket heads.
Anorak mode off.

But given the discussion actually relates to a 500, this indispensable gem of information is not particularly relevant.

 

[Dellortoman]

OK. I bow to your and Quentin's superior anorak brains. That must mean 90% of the bikes you can find pictures of online have been swapped to socket heads like my bike, which was like that when I bought it. It doesn't have it's original front end so who knows what else the previous owner did? It's a 1976 3CL that left the factory with Ceriani forks and thin web wheels but it now has Marzocchi and thick webs. The original forks and wheels came with the bike. The wheels went to Bob Dixon and the forks to Maurice.

 

[Andy J]

Thanks for all the comments, I’ll let you know what I discover in a week or so.
BTW, the other 5 minute job that turned into a failure was swapping a 3 spoke for a 5 spoke rear wheel in the Formula. What I hadn’t allowed for is the hub is wider in the 5 spoke wheel and thus the spacer between the wheel bearing and the calliper carrier becomes redundant. It took a bit of head scratching before I worked it out. It then became a 5 minute job again. Doh!

 

[Andy J]

Hurrah, a functional front brake in time for the MGP. Replaced the calipers and master cylinder with the set off my series 2 F500 and all appears good. Just need to fit a battery and take it for a spin to make sure everything is working okay.