Headstock dimensions please

[RupertB]

Can anyone with a CB750 K2 or thereabouts do me a huge favour and measure the distance from top to bottom of the headstock please? That is to say the distance between the top and bottom yokes effectively - my Foale frame has a distance of 190mm according to my cheap tape measure - is this standard?
Reason I ask is the bottom yoke on mine is joined to the top yoke by a length of steel rod, and the top yoke has a sawn off bit of threaded metal from the original rod bolted into it.
Equally, anyone got a bottom yoke for sale?
Thanks in advance.

[mick]

Hi Can you post some photos of your top & bottom yokes and the threaded bar cheers, it might help us to help you  , cheers Mick.

[RupertB]

I've taken a few pics which should follow this post, but you will see the basic problem - I haven't got a steering stem! There's a top yoke, a bottom yoke, and a horrible metal rod which joins the top and bottom yokes with a couple of nuts and some washers, threaded through what looks like a cut off section of stem which has been stuck into the top yoke.
It has prompted a theory that someone bought a write-off K4 from America to put into the Foale frame, and a heavy frontal impact might bend a headstock and destroy a frame quite happily. That's only an idea though and doesn;t really matter.
My problem now is that I need to acquire a new bottom yoke with stem because there is no way I want to shove the old setup back in!
So I need to be sure that a new yoke and stem are right for the Foale frame. I guess they will be, but could do with some reassurance.

[RupertB]

The top yoke -

[RupertB]

.....and the frame headstock area.

[K2-K6]

Haven't got one readily available to give you accurate dimensions but, the one you have got maybe the original design solution.

If each yoke is located well to the bearings (seems to be so from the pics) then the only requirement is to pull the bearings into correct pre-load which the rod will do if of sufficient design spec (rods like this are used to hold buildings together and every bolt on the bike is essentially the same application). The fork legs will then keep the yokes in alignment, the tube is essentially redundant.

I'd guess that the headstock you have could be longer than standard so couldn't be adapted from a standard stem hence the solution that's used (odd but possible).

In front end damage the distortion usually happens on the legs just under the bottom yoke, the headstock of the frame itself and to the tube as it leaves the bottom yoke just inside the bearing race.

Can you get any info from another owner of this type of frame?

[RupertB]

Good idea - I might try emailing Tony Foale himself. I'm good at missing the obvious!

[Seamus]

These may help

http://cgi.ebay.co.uk/Honda-CB-750-CB750-SOHC-73-Bottom-Yoke-/270655844039?pt=UK_Motorcycle_Parts&hash=item3f04586ec7


http://cgi.ebay.co.uk/Honda-CB-750-SOHC-bottom-yoke-/370416018679?pt=UK_Motorcycle_Parts&hash=item563e83e4f7

[RupertB]

Everyone massively helpful as usual - I emailed Tony Foale last night and got a reply this morning! Apparently the set up I have got is actually what he made - a tensioned rod between the bearings. Looks Heath Robinson but as suggested earlier in the thread there is every reason for it to work, and work it must have done judging by the success of the frames.

Tomorrow sees the head and barrels off to North West Enamellers for blasting to get presentable for fin repair...

[K2-K6]

Aha! that makes for less work you need to do.

I've heard about Foale frames before but never actually seen one, it looks like he's pushed quite hard towards a light / strong solution and I guess that entails some more unconventional thinking to take a step outside what is standard.

It looks interesting, be good to see it when finished.

As a further extension to the original design logic it may have a specific torque requirement during assembly, in that taper roller bearings can and often do take a compressive load that maybe desirable as a form of damping to the rotation of the forks (i.e. a mild form of steering damper) if so, this would have to be set correctly prior to tightening the clamps around the fork legs. Be interesting to get an opinion from the designer for your benefit in re-assembly.

[RupertB]

Good point - Tony Foale is currently moving back to Spain from the USA so after mid-November he will be available more readily, from his willingness to help I am sure he will give me the answers I need. I doubt the setup it came with is original - several nuts and a big washer - but you never know!
The frame is surprisingly light (but then the last one I restored was on my Jota which felt like it was filled with lead) and with the front tubes is so incredibly easy to get the engine out. It will also make it very easy to put the engine back in without endangering the frame paint. I used a Guzzi engine support and couple of spanners and there it was. Talk about well-engineered.
I think this will be quite a fun rebuild, but it just depends how the money goes as to how soon and how well it gets done.