Valve guide replacement MK3 Commando

.. on several bikes (although not a Commando) with no problems. The main thing is to make the head nice and hot (200 C) both before removing and fitting, and to thoroughly clean the protruding part of the old guide that will go through the bore.

After fitting you will probably have to ream to the correct clearance and purists would say you need to recut the valve seats although I have to say I've not neded to do that.

You do need a close fitting double diameter drift for both removal and replacement.

Its easy, especially with an RH4 head, to scrap it by changing guides, the inlet guide holes in the head are very prone to cracks. I would instead leave the guides in place and get the guides fitted with K liners which will restore the valve stem fit but leave the original guides in place.

I have replaced guides myself over the years on Triumphs and BSA's and all have worked but a replacement Norton head is too expensive to risk an error so I will use K liners myself when needed on a Norton head.

It is easy to score a bore, crack the head, and then to re-cut the seat accurately needs a decent machine. So yes, the home mechanic can change them, but what it costs to have them swapped and seats cut professionally makes it pretty pointless to do it yourself.

Our local specialist is £20 a corner plus the guides you will need to supply. They are considered expensive, but even this price makes it worthwhile.

Unless the guide is worn or leaking between it and the head, why change it, most will last 100K miles. 

It is far too easy to ruin the valve guide bore in the head by hammering or pulling the old guide out even with the head being heated up (as per the procedure in the manual)

I always machine most of the guide away (using a counter boring tool made from silver steel) to just leave a thin shell of the old guide that then comes out quite easily. The counterboring removes most of the guide metal so allowing whats left to collapse inward slightly, hence easy removal. 

The danger by 'doing by the manual' is that the old guide picks up bits of alloy and the hole gets scored by this as the guide is pulled or hammered out. No amount of cleaning the old guide (even by abrasive blasting) will stop this happening.    

Done this way and by using a good pipe freeze spray on the new guides and by heating the head they should be a drop in fit. Using the pipe freeze spray, make up a small insulated pot and a couple of wooden dowels stepped to hold the new guides, as you cannot hold the guides as you will suffer cold burns. Make sure the pipe freeze spray boils down to a liquid  in the pot as the action of it cooling looks as if its boiling. when it stops 'boiling' the guides are at there coldest. Liquid nitrogen is much better for doing this kind of work, BUT it is far colder than the pipe freeze spray from the diy store, so needs to be treated with due respect. Not just the danger of cold burns but loss of flesh etc!      

No wonder so many cylinder heads need remedial guide work, people following the manual just creates it! 

Measure the new guides, valves etc to make sure that the fit is correct. Unless your going racing don't be tempted to fit bronze guides, they will conduct heat better but will wear out faster. A lot of bronze guides on the market are crap, unknown material spec etc, (Colisbro is the grade bronze guides should be made of)  Valves the same.   I know AN supplied me with good valves compared to another supplier, their valves looked ok, but under a magnifying glass the surface finish on the valve stem was awful.

Good gear costs more as well!

I must have been typing the same time as Ashley and i agree with all his comments!

Second edit of this post!

If as you mention the valve to guide clearance is ok, a grind with paste or if required a touch with a valve seat cutter is all thats needed.

Seems this should have been about seats, definetly best done professionally. 

No 

I used to tell people 'i'm a mechanical fitter' but after a conversation with a colleague he told me i was a craftsman, explaining that a fitter is someone who just replaces parts ('fits them') A craftsman is someone who can make the parts as well! 

If you do decide to use an Engine reconditioner / Machine shop ask to see examples of their work or know someone who has had work done there. Don't get talked into have 'Unleaded seats fitted' as the valve seats in commando heads are perfectly suitable for unleaded fuel.   

... was always used to describe someone who could make parts fit ie not just someone who fitted new bits. It was always regarded as a skilled job in my day. I wouldn't actually expect an engineer to have hands-on skills.....

With regard to your original question, I would take the advice from people who've done it (or not) on a Commando head as my experience has been with others - particularly Velo and Enfield singles.

Polish the orange peel out, I no longer grind in seats but use Neway tungsten carbide seat cutters. The finish they give does not need any further work so the valves are not worn away by grinding away on the seat.

https://www.newaymfg.com/cutters

Does not make sense to buy a set for one head, I do a lot of different heads so it makes sense for me. That's if they are ever in my workshop as they are borrowed a lot, but never had to buy new carbide inserts. 

What is the history behind this cylinder head/? My impression is that it has had some corrosion occur and that has given the combustion chamber/cylinderhead gasket face the 'orange peel' appearance.

By all means try to lap the valves in. You will soon see if you are wasting time. A pass with a valve seat cutter will speeds things up.

Clean the inlet/exhaust ports up, combustion chamber as well and lap the head gasket face flat but  don't bother to polish the 'orange peel' out of the head, by the time it has got a layer of carbon on it, it will look the same as any other used head.  Plus polishing material away will lower the compression ratio!

Worth trying to recover the seats fitted, might take a little time and elbow grease but worth a go.

I use Owslebury cranks at Winchester for my engine work, not cheap but never had an issue either. 

To determine how far the guide is worn is quite hard to do for most owners. Comparison with a worn guide and new valve stem or vice versa gives an idea. On the exhaust side expanding gas regulates the rate of oil passing between the stem and guide. This works well, but is dependant on several things, and the wear or clearance between the guide and stem is just one.

15k miles I would expect guides to be as new, it is no real mileage. If they are bronze then they should be OK even at that mileage, as it depends on the bronze used. Colsibro is very good for racing guides, it allows very close clearances to be used, but it wears, that is the tradeoff, but should still be good for 40k + miles. 

If you have new valves then you can use them in the existing guides. On the old valves you might be able to measure the stem as a comparison to the new.

Worn exhaust guides or stems results in increase oil consumption, you'll notice the increase in topping up, or sticky and gummy black deposit in the silencers that you can feel with your finger. It does not affect starting or oiling the plugs.