Guide to valve clearance

Some more help needed guuys,

What should be the clearance between bronze valve guides and standard Commando valves (provided by Andover Norton) ?

The engine has been fully rebuit and run OK for its first 130 miles. When I tried and started her today, she ran only on one cylinder (the right one). Ignition checked and spark plugs and spark leads swapped : no change

Investigating a bit further, I found that the left exhaust valve was stuck in its guide so that it could not be fully closed.

After having removed the cylinder head. I found that the 3 other valves were OK. No damage done at first sight to that valve. It is not "burnt". Pushrods are straight and not damaged. The corresponding guide looks OK too.

I wonder if having added break-in additive to the engine oil may be the cause of that problem. It does not look like having been over-heated but rather like the valve being stuck in its guide by gas or oil residues.

Thanks a lot for your suggestions.

L.

Sticky valves due to combustion residue and 'gumming' was an eternal problem with engines back in the 'good old days' - 50s and earlier.
Ashless type oils, and additives, had yet to be invented.

Sounds like your 'break-in additive' may well be the culprit.

Bronze guides run much tighter clearances than iron guides, all other things being equal (and lubricated).

As I think it has been said, when these new guides were pressed in it is very common to gall the walls. This creates a tightness in the guide. I have, in the past, reamed these because of this type of occurrence.

The reamers are such that they are so small that I needed 2 adjustable reamers span this tolerance. Your other guides may be fine but you better check them all.

It is not really recommended that bronze guides be reamed ?
They are supposed to be honed - or the bronze is smeared around = not a good running surface.

Better than being too tight though, maybe.

Merci beaucoup Messieurs,

What would you recommend in terms of guide-to-valve clearance ?
I imagine that about 0,002 inch (approx. 5/100 of a millimeter) is appropriate.

Maybe less ?

.0015 inch is adequate on bronze exhaust guides.

I have seen exhaust valves stick if they have been assembled with grease type prelube. Only a high grade of motor oil is permissible with tight new guides. Jim

Many thanks Jim.

I brought the cylinder head back to my machine shop. They confirmed your clearance values and will adjust the guides accordingly.

You can't always go by the spec in the manual,but often enough the minimum is the safe minimum.When you go tighter than the minimum,you have to ask yourself "Do I feel lucky?"Manual spec is 0.002"-0.004" clearance,new.

I wouldn't try to run an exhaust valve with a 5/16" stem and a bronze guide any tighter than 0.0025" clearance.You'd get away with less if you went slow,but I wouldn't guarantee it if you went fast.

It depends on the valve temperature,which depends on other things like how good your valve job is.How much seat eccentricity is there and what's the seat width?

I remember doing valve guides on a BMW R100S,which should have been 0.002" minimum clearance.The valve stems are 8 mm,close enough to 5/16".Everybody's saying run it tighter,so we used 0.00175" clearance.It wouldn't get much past 70 mph.,without the valves tightening.2 weeks later if he tried to overtake a car,the engine would be cutting out when he reached 80 mph.Until the valve job,this bike did 120 mph most days of the week.The valve job was good,apart from guide clearance.Seat eccentricity was less than 0.001",and valve seat width was right.

We reamed the guides another 0.00025" to make the clearance 0.002" (as per spec).End of problem.
He was lucky the valves only tightened a little to stop them fully seating,but didn't actually sieze.Siezure can be messy,when a piston crashes into a valve.

Thkx for that further explanation.

The head is back from the machine shop. They told me that they added "some more" clearance to the exhaust guides (a bit over 0,04 millimeters).

They told me too that the valves (provided by Andover Norton) were not of top quality.

I fully agree with someone saying in another post on this forum that "made in England" should not be considered as a proof of quality but as a WARNING!

Next time and when money for is available, I'll use Black Diamonds or another reliable product.