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M20 Valve Seizures

Hi

Long story made shorter. Just had a rebore, new valves and guides, first trip out lost all compression after about 8 miles due to the exhaust valve sticking open. As I was booked to go on a rally the following weekend in Cape Bridgewater I popped her back to the workshop who did the work, he's well experienced in these engines, and diagnosed that the valve was duff. I double checked he'd allowed 3.5 thou clearance. I ran her without issue in hot (30+) weather about 10 miles or so back home. On the rally I suffered more of the same, but was able to run without issue if I kept to 40mph or below. If I strayed above 45 for any distance, based on 3 occurrences, the valve stuck open, on the last few casino needing to allow the engine to cool for 20 mins before shed close fully. This was over about 200 miles in total with the last 80 or so fault free but slow.

I need to wait until the weekend to look at the valve, but I do have the previous one, deemed faulty. Measuring that it appears to be about 0.1mm (vernier measurement) wider where the seize marks show, measuring across the non-seize parts. I tried heating in the oven, but couldn't tell if there was any real expansion, it just made the different metals stand out as the ferrous part showed discolouration.

I haven't altered the mixtures from before all this work, maybe I should have raised the needle? I did pop some 2-stroke oil in the tank for last two fills. Ignition timing is correct, and otherwise she pulled and ran well.

Both new valves are from the same supplier and most likely the same pattern manufacturer.

Has anyone had experience with poor valves, or sticking/seizing valves? Any recommendations for valve suppliers? Any suggestions?

Cheers

Richard

email (option): Dickie.bobbie@hotmail.co.uk

Re: M20 Valve Seizures

This happened to me after a recent rebuild and some long rides; exhaust valves on side valve engines live a harder life than we ever imagine! All other variables being correct (timing, fuel, lubrication,etc) As you have already found, what's required is to provide more exhaust valve stem-to-guide clearance.

You ask: Why now a problem and not before? I would venture to guess that the act of driving in a valve guide produces some stress and distortion in both the barrel and guide that relieves over time through repeated heating and cooling cycles affecting the guide shape and clearance. (I'm sure the group has some collective wisdom to offer on that opinion!)

I got pretty proficient at doing this procedure by the roadside; with an adjustable ream, clear the guide of rough spots from the siezures. Polish siezure marks off the valve stem. (I had access to an electric drill for that) The resulting extra clearance may seem excessive, but it's what's needed to prevent seizing. It may take several tries as you have already experienced; eventually a "sweet spot" is reached and the valve seizing cured.

email (option): teladelujo@msn.com

Re: M20 Valve Seizures

Nominal stem diameter of the inlet valve is .348" when new with a tolerance of +/- .0005"....

Valve guide bore size for the exhaust valve guide should be .3525"- .3535"...

Using the nominal stem size that works out to a valve stem to guide bore clearance of .0045"-.0055"...Even applying the maximimum possible stem size a .0035" clearance is therefore probably insufficient...

As you have a pattern valve I would measure the actual size of the stem and then apply a .0045"-.0055" clearance...

Note the guide should be reamed/honed to the finished size after fitting to the barrel...Ian

email (option): ian@wright52.plus.com

Re: M20 Valve Seizures

I would suggest you carefully check your ignition timing.
It needs about 11 mm advance, if i'm right.
Mine had only 7 mm advance wich caused exhaust valve sticking after about 6-7 miles.

Re: M20 Valve Seizures

Repeat valve stem damage on a newly rebuilt engine.

I'll get into trouble here for saying it and I have no intention of entering into a discussion but I can't avoid the suspicion that 'modern' fuels might contain less lubricating compounds and be less forgiving than the greasy low-octane stuff that was used for years.

If you're shopping around for valves and want the best available for our sort of motorcycle then I don't think that you'll do better than G&S Valves of Godalming.

http://www.gsvalves.co.uk/assets/vintage-motor-cycle-catalogue.pdf

Re: M20 Valve Seizures

Rik
Repeat valve stem damage on a newly rebuilt engine.

I'll get into trouble here for saying it and I have no intention of entering into a discussion but I can't avoid the suspicion that 'modern' fuels might contain less lubricating compounds and be less forgiving than the greasy low-octane stuff that was used for years.

If you're shopping around for valves and want the best available for our sort of motorcycle then I don't think that you'll do better than G&S Valves of Godalming.

http://www.gsvalves.co.uk/assets/vintage-motor-cycle-catalogue.pdf


The exhaust valve stem got little in the way of lube from old leaded fuels.
Lead was there principally to raise the octane equivalent

email (option): bsansw1@tpg.com.au

Re: M20 Valve Seizures

Hi

Thanks for the input everyone. I'll recheck the ignition while the head is off, but looks like my main work will be some light reaming and relief of the valve stem where the seize marks are. I'll pop the needle up one notch while I'm about it just as a precaution...and keep some additional upper cylinder lubricant in play for another few 100 miles, maybe just the 2-stroke oil as that's easier to hand

I'll update down the line

Cheers

Richard

email (option): dickie.bobbie@hotmail.co.uk

Re: M20 Valve Seizures

Check to see if you have oil going into the tappet cover as some pattern gaskets dont have the feed/drain hole cut into them.If you remove tappet cover then kick over engine you should be able to hear engine breath or fire up engine and see whether their is oil being blow around tappet area.Guess most of this oil mist is for tappet lubrication but some gets to valve stems... Dave

Re: M20 Valve Seizures

Appears to be all sorted now, at least on the basis of a reasonably long run and some higher speed (hahahaha) parts.

Yes Dave, plenty of oil and I double checked the oil hole in the gasket, I always seem to worry about blocking oil ways.

My trouble aren't over though, but that's another thread relating to the chaincase.

email (option): dickie.bobbie@hotmail.co.uk

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