Wages of sin no doubt...

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Onder said:
Head a full overhaul. Implanted on bike. Started second kick. Rode a few miles. Never started again for two days. Looked at ignition of course it wasn't that. Did a differential compression test. Bingo. Hung intake I thought. Pulled head. Tiny piece of metal on seat ate into seat and valve.
These things are sent to try us.
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Is your engine standard or tuned ? If you just want it to plod around on you could probably just keep cutting it back until it seals
Maybe have a look at the pushrod length/valve geometry there's nothing to loose

And if it doesn't work get a new valve seat fitted
 
Baz , I think has hit the nail, you only have 2 options as I can see, 1,keep cutting the seat till it seals, or 2, put in a new seat, more involve but its then done and fixed.
 
Appreciate all the input. The head was done with new guides and valves by Jim Comstock so I cannot believe it was just tossed out the door. If you look at the pix you can see a piece of metal sitting on smack on the valve seat surface. It held the valve off the seat. I prodded it with a screwdriver and it flew off. Pits are both in the valve and in the seat. The pit is rather deep in the seat.
Lapping the valve again seems to make no difference in the pits. No doubt nothing to lose by lapping away. With a new valve I am sure it would run fine at least for a while. What the worry is will other parts of the seat give up the ghost as well. Naturally I do not want those pieces in the cylinders.
Torn between lapping a lot and new valve and taking it to somebody and having it cut. It has been cut once unfortunately.
 

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Onder said:
Appreciate all the input. The head was done with new guides and valves by Jim Comstock so I cannot believe it was just tossed out the door. If you look at the pix you can see a piece of metal sitting on smack on the valve seat surface. It held the valve off the seat. I prodded it with a screwdriver and it flew off. Pits are both in the valve and in the seat. The pit is rather deep in the seat.
Lapping the valve again seems to make no difference in the pits. No doubt nothing to lose by lapping away. With a new valve I am sure it would run fine at least for a while. What the worry is will other parts of the seat give up the ghost as well. Naturally I do not want those pieces in the cylinders.
Torn between lapping a lot and new valve and taking it to somebody and having it cut. It has been cut once unfortunately.
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I wouldn't lap it back if I were you
It needs cutting with a valve seat cutter then lapping
Cheers
 
I would have a new seat put in and let them seat your new valve. There is a cost but minor one and the problem is solved.
The well known machine shops always have waiting lists but I had a HD machine shop do a T140v head with guides, seats and valves in no time.
 
Probably could lap for ages and not made any improvement. Yes cutting is the correct approach but finding someone to do it in reasonable time....that is the issue.
 
Still wondering where it came from. Since it ran and then didn't, it probably wasn't there when you put the head on. I can think of nothing in the carb that hard. I suppose it could have been lurking in the air cleaner. Is there a chance it's a bit of top compression ring? How about something broken off a spark plug? I suppose it's also possible that it was in the cylinder when you put the head on and it took a while to end up in that spot.
 
If one knows a "real good and experienced" tig-weldor i reckon it could be fixed as follows:
If then tig-braze it with Ampco 45 and not cusi3 and afterwards have the seat a hint retouched with a serdi.
Preheat the head to not have too much temperature gradient, and do not i repeat let the seat get in fusion you want the Ampco 45 flow into the nick.

Kind regards

Christian
 
@kommando

Unfortunately that's what neway publishes but the truth is that one has to check very carefully the concentricity afterwards as their cutters tend to tweak the mandrels.

Kind regards
 
Hudriwudri said:
@kommando

Unfortunately that's what neway publishes but the truth is that one has to check very carefully the concentricity afterwards as their cutters tend to tweak the mandrels.

Kind regards
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I use Neway cutters and when I first got them I did lap as well, but have stopped lapping as the neway finish I find does give a good seal, well at least on pre 65 trial winning Tiger Cub heads.
 
@kommando

I guess that the typical original Brit valve seat is a lot softer than even late 70's jap stuff.
I noticed that on my Yamaha singles and twins that neways have a tendency to run a lil out of concentrity which was the reason to ultimate reason to settle for the Hunger valve machinery/tool range.
So I hope you did not get my statement as a critic as i personally think that neways are for what they are great tools.

Kind regards and nice evening

Christian
 
Onder said:
Probably could lap for ages and not made any improvement. Yes cutting is the correct approach but finding someone to do it in reasonable time....that is the issue.
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You should have plenty of machine shops in the UK to my knowledge.
If the shipping wouldn't be quite prohibitive I'd say send it over but i can with best will not imagine that there ain't no good reasonable engine shop in the UK, c'mon guys.

Kind regards

Christian
 
I have discovered that fine grinding, filing, lapping and honing are skills that are prerequisite to looking after a Commando. It is a very fine motorbike , easy to work on, they are definitely a
piece of art, they look good, sound good , smell terrible, and the cats don't like them.
 
Yes, have appointment friday with a shop to face the seat. Ill have a go, if that won't work Ill have to man up and have the seat
replaced. It is particularly gauling (pun) because my Fullauto head is already out for stud repair and so now I have to repair another head! Grrrr.
 
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I would investigate getting that divot laser welded and compare the cost of that vs a new seat.
 
I once had a small triangular piece from the end of a cast iron valve guide pass through my Triumph motor. I thought the loss of power was due to a tuning error, until I pulled the head off the motor and saw the mark on the piston. It had dented the seat of the exhaust valve, Paul Dunstall also mentions this can happen, in his tuning notes.
 
Bronze guides. No hint of any separation or damage them at all.
 
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