K-line valve guide liners

Depends what bronze, some will wear out very quick, other types will last a considerable amount of time. Colsibro is not ideal for road use but excellent for racing, it allows tight clearance to the stem but wears quickly. Buying ready honed to size colsibro guides is pointless on road bike.  

I believe there is a company in the UK that is the K-line agent, I would expect the cost to be much the same if not more expensive to fit liners than fitting new guides. Most automotive engineering firms would just copy the old guide themselves. 

One of our very  teckky guys Bob Matthews ?, has put a  lot of time and effort into researching the best cast iron/ valve material combination  and  getting some guides and valves  made for his very early model 18, Try to contact him. 

The relining of valve guides can be done by T&L Engineering in Bedford. I have had the valve guides on my 500 Nomad relined by them.

If Colsibro is no good for road bikes what should we be using?

Colsibro as others above have indicated gives a tight clearance to the stem, in some cases very tight with clearances that would frighten us Norton owners, ideal for racing but not for road.. Colsibro is not the best out there for road bikes as the tight clearance is sacrificed for wear rate, yes, it has a good wear rate but not the best these days. It will most probably survive the small mileages most do over ownership of the bike, but is not the ideal choice when better and cheaper is available for road use.  As you can see, if you fit a Colsibro guide which does not need reaming post fitting to the head as it has the same clearance as an original cast iron guide then you have an expensive sub standard valve guide, it will not last as long as a cast iron guide and wear quicker than silicon brass guide of the same dimension. 

Aluminium bronzes are far harder wearing and on paper look to be the best material, but harder to machine and some don't ream or hone. Silicon brass is now the go to choice. 

A granular cast iron was always fitted to Norton twins ,slow wearing ,and needing very little lubrication usually set up with more clearance than considered necessary these days. It coped with the poorish  level of oiling that the system provided . It was matched up with valves with silchrome stems. My 99 has been running on those I fitted about 30 years ago , It has been my go to bike since then and still goes fast enough for other members to ask --what have you done to that?.