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E: Engine Cylinder Head Crack

LoneStar

Well Known and Active Website User
VOC Member
Hello all,

The rear cylinder head on my Series C Black Shadow has developed a crack at the exhaust valve guide bore. The bore is also belled out, measuring .566 x .585" at the inner end. This was likely caused by the piston having broken apart, with the separated crown hitting and bending the valve.

What's the best way to deal with this unpleasantness?

Thanks,

Dave

20200520_150928.jpg
 

brian gains

Well Known and Active Website User
VOC Member
are head castings that porous not unusual , I don' recall mine having that appearance or is that just a consequence of the spontaneous porting job ?.

jobs like that I split between dedicated welders and dedicated machinest and if you're lucky they talk to each other.

Or, Alp Racing Design. No affiliation etc etc but the fellah seems to have the gift with skills.
 
Last edited:

Vincent Brake

Well Known and Active Website User
VOC Member
Or leave it be and put in a full valve guide all the way up. Over forks and closed up by the top valve guide.

May be only solder a bit on outside.

Like the heads of Ernst Hegeler.

Easy repair.

Welding is messing up a lot there....

Or godo when you want to trash down the roads (riding i mean)

Not to much interf fit.
Say 2-3 thou.

A good bronze.
Alu Fe bronze?
Ampco 45?

Much wisdom send to you
 

peter holmes

Well Known and Active Website User
VOC Member
I think Maughan’s would fix it, but they are UK based, so probably no good for you.
 

roy the mechanic

Well Known and Active Website User
VOC Member
Looking at the picture, it has been "mended" before. If it was mine, by the time it has been welded, machined back, one-off guide made and fitted, I would just buy a new head and get most of the cost back from evil bay.
 

Vincent Brake

Well Known and Active Website User
VOC Member
Looking at the picture, it has been "mended" before. If it was mine, by the time it has been welded, machined back, one-off guide made and fitted, I would just buy a new head and get most of the cost back from evil bay.
Haha Roy,
Was it yours that i bought.
It was advertised correctly though
Presse Papier.

Cheers

But i guess your right
 

LoneStar

Well Known and Active Website User
VOC Member
Thanks for the ideas, everyone. I consulted with Glenn Bewley - the head can be repaired, but the cost approaches that of a new one. So, I'm leaning towards replacing it.

I hadn't considered listing the current one on Ebay, but it does seem to be a mainstream idea; quite a few Vincent parts there are clearly replaced rejects. No harm if disclosed, I guess.

VB - I'm not understanding your idea of a full-length guide. How does the rocker fork reach the valve stem? Any photos of this setup?

Cheers,

Dave
 

davidd

Well Known and Active Website User
VOC Member
VB - I'm not understanding your idea of a full-length guide. How does the rocker fork reach the valve stem? Any photos of this setup?
This is the set-up:

fullsizeoutput_a0.jpeg

fullsizeoutput_9d.jpeg

I think think that this would definitely repair it, but it seems like some serious overkill. I would be inclined to machine the hole round and fit an oversize guide.

Alp has done this repair because one of the heads had a broken a valve and cracked the guide:

Head Repair  (9).jpg

Head Repair  (14).jpg

Head Repair  (13).jpg

Head Repair  (11).jpg

David
 

LoneStar

Well Known and Active Website User
VOC Member
I see. Actually I've made and installed one of these, based on your earlier posts, to replace the ET122 guide lock ring. This in response to the ring having come loose at one point, destroying the threads in the head.

But I don't know if I'd rely on it to hold the guide in place in a loose bore. I think the cracks would have to be welded up before machining a new bore for an oversize guide - unless the guide bore could be made large enough to eliminate them; not sure how deep they go.

All this skilled work is beyond my abilities and equipment, and will cost about what a new head would - so that looks like the better option at this point.

20200520_150914.jpg
 

Bill Thomas

Well Known and Active Website User
VOC Member
I think we can worry too much sometimes, I know I do !.
Welding can bring it's own problems,
I have had a small crack between the valve seats, On my L/ning rep',
For more years than I can think,
But it's up to you, Really it's a money thing, I don't have any ! :D .
 

timetraveller

Well Known and Active Website User
VOC Member
HI LoneStar' manufacture of these is beyond my capabilities as well. I had a batch CNC machined and they are not intended to hold a loose guide into place. Rather, if the guide has become loose and the hammering has worn both the hole and the thread beyond use then the idea is that an oversized guide should be made, with an interference fit, and held into place by the new gadget. I have designed the new batch so that they will work with both inlet and exhaust guides and allow the use of a seal as well. The single design can be used as is on an exhaust with a ruined thread, on a similar inlet by slightly reducing the diameter of the lower oil seal retaining recess or, if the thread is still useable then the lower part can be turned off so the bottom is flush and it can be used to prevent the guide ever coming loose again. I think there will be a full description with photographs in the next MPH.
 

ClassicBiker

Well Known and Active Website User
VOC Member
They aren't that difficult to make if you have a lathe. After I saw timetraveller's post about them in another thread when I discovered my retaining ring had backed out and allowed the guide to self destruct and destroy the threads. The hole for the guide was also a little wallowed out so I turned a reamer guide and selected an over sized reamer to clean it up and made an over sized guide. MartynG posted an article how Maughans does the job. I used that article to figure out how to make the reamer guide. If you use the search and look for valve guide or valve guide repair you should find them.
Steven
 

oexing

Well Known and Active Website User
VOC Member
I wonder why there are numerous problems with valve guides on Vincents when you have two guides per valve ?? Poor shrink fits or unsuitable fitting replacement guides by cold bashing them in ?? When the lock ring goes you had obviously loose guides and maybe some seizures on valve stems so high forces on the assembly ! So take care what material you decide to use for guides, chromed stems a good idea as well, but availability . . . .

Vic
 

ClassicBiker

Well Known and Active Website User
VOC Member
No I don't think my father installed them incorrectly 40 years ago. Neither he nor I had any issue in forty years, but obviously something went wrong. I do know that the owner previous to my father had some kind of issue as of the three surviving retainers two were UNF threads rather than the original thread. I have no idea what the destroyed thread was as there wasn't enough left to make a determination.
 

LoneStar

Well Known and Active Website User
VOC Member
There appear to be diverse causes for guide problems. In my own case, there were two rounds of difficulty:

- The ET122 lock ring had loosened, and wore away the threads in the head as it vibrated. The guide itself wasn't loose. I fixed this by making and installing the alloy spacer unit as described by TT.

- Later, the piston crown separated from the gudgeon pin, striking and bending the exhaust valve. This also forced the entire guide sideways, enlarging the bore and cracking the surrounding alloy. This is the current problem, and due to the cracks I don't think it can be fixed with a simple oversize guide.

The one-piece bronze guide with extended cylinder, as Lintman posts, is a nice solution. I think in all cases, though, one needs an interference fit between the guide and its bore to insure rigidity.

Cheers,

Dave

20200426_183658.jpg
 

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