Guy Croft Beta inlet manifold tapping

[mangocrazy]

On my GC Beta inlet manifold (the version that is angled to avoid the block-mounted distributor) there is a tapping on the inlet to cylinder #4. I've tried various bolts and studs I have in my stash and none fit. An M12 bolt of any pitch drops straight through, so I'm guessing it's either M14 (which I don't have) or an Imperial/US size. I've got the verniers on it and the i.d. of the thread as far as I can make out is 12.58mm / 0.495". Does anyone know the correct thread size/pitch? I'm assuming that this is the vacuum takeoff point for the brake servo?

[mangocrazy]

On further investigation it looks like it could be M14 x 1.5 - the i.d. of the thread 'sort of' matches and a 1.5 thread pitch guage seems to fit well. If anyone could confirm that would be good, otherwise I think it's off to eBay to buy an M14 x 1.5 allen bolt to try it out.

[WestonE]

Hi Graham

Take care it could be BSP or BSP Tapered which locks as you increase the engagement. It will be listed in GCs books. Mine are somewhere in a crate not unpacked post move!

Eric

[mangocrazy]

Hi Eric,

Yes that's just the kind of thing I'm afraid of. For now I just want to blank it off with a bolt so I can't drop anything down there. Apparently M14 x 1.5 is the size that certain Triumph motorcycles use as a sump oil drain plug, so I've ordered one of them. We shall see...

I've been through all the relevant thread pitch info in my little Zeus book, and as far as I can tell it's not BSF, BSW, BSP, UNF, or UNC, but I'm sure there are plenty of other exotic thread pitches out there it could still be.

Graham

[mangocrazy]

Eric,

Your tip about the GC book(s) was spot on - found this on page 114 of Guy's first book:

Graham

[WestonE]

Hi Graham

I am glad my idea worked.

Happy Christmas

Eric

[mangocrazy]

Hi Eric,

And the same to you and yours,

Graham

[betabuoy]

… I'm assuming that this is the vacuum takeoff point for the brake servo?

Yes that’s the one.

[mangocrazy]

… I'm assuming that this is the vacuum takeoff point for the brake servo?

Yes that’s the one.

Hi Chris, thanks for the confirmation, much appreciated.

Graham