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Classic, Vintage & Veteran For Coventry and Meriden Models. Anything pre-Hinckley goes.

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Old 07-17-2012, 12:35 AM   #1 (permalink)
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stripped exhaust port threads

I'm just getting started on a new project. It's a 66 bonneville 650 and I'm told it has a 1970 head. Started a little bit of disassembly tonight and found the one of the exhaust ports in my head has had its threads pulled out, and will no longer accept its screw in manifold. Is there a quick fix for this? Am I looking for a new head now? I got this bike cheap from a friend because she knew I would actually put it back together, and I am trying to make some progress quickly. Any help is greatly appreciated
Chris
Sorry about "strupped" in the title. I don't know how to change.it yet.
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Old 07-17-2012, 01:51 AM   #2 (permalink)
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I would talk to a local machinist before replacing the head.

If my exhaust port was stripped, I would turn out a new insert on a lathe, with self tapping threads of a slightly larger size. Any machinist with decent CNC skills could do it
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Old 07-17-2012, 04:08 AM   #3 (permalink)
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Quote:
Originally Posted by hippiechris View Post
Is there a quick fix for this? .
Mr Pete may read of your plight and confirm that you could get a a new spigot turned for an interference fit. This should not cost you an arm or leg. RR
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Old 07-17-2012, 04:24 AM   #4 (permalink)
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Check on Ebay, there is a company selling oversize spigots. 10 20 or 30 thou over for reasonable money. Used them to repair my head, very nicely made. Heat the head in the oven and freeze the spigot.

Rod
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Old 07-17-2012, 10:47 PM   #5 (permalink)
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I've got a 66 & the same thing happened to me. The exhaust system on the 66 model has a support stay connecting the two header pipes together at the front of the engine. Good, but not a perfect solution. This stay can be missing which then puts stress & vibration back into the flange spigots & threads.

After repair in addition to the original stay, I fitted later model ones which fit onto the crankcase..............no more problems. And apologies to all you sticklers for originality & trailer queen jockey's !
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Old 07-17-2012, 11:00 PM   #6 (permalink)
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So what was your fix?
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Old 07-18-2012, 12:03 AM   #7 (permalink)
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If you've still got some trace of a thread left in the head,just turn up a new spigot and screw-cut the threads to screw tightly into what you have.
It is MOST IMPORTANT that the new spigot screws fully in and the inner face bottoms hard against the head.You may need to relieve some threads near the inner face to achieve this.You may even need to relieve the outer end of the thread if it is not long enough,and the plain 1.650" diameter bottoms against the head.

The thread on the spigot was originally 1-5/8" x 16 tpi.The thread has a 60 degree included angle.

If there isn't much trace of a thread left,bore the head parallel,and do a shrink-fit spigot.It's much more reliable,and causes less damage in the unlikely event that it does fail.
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Old 07-18-2012, 10:43 PM   #8 (permalink)
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Mr. Pete,

A shrink fit spigot is an interesting fix. If this subject has been discussed before, please excuse the question and just tell me to do a search.

Norton's apparently are notorious for beating out the threads on their exhaust ports. I realize it is not a spigot, but rather a nut config.

I sounds like fitting a shrink fit spigot and using a clamping arrangement would be a better fix for a Norton than the welded in bushings, which I understand are known to fail. Have you tried this fix on a Norton and was it successful. Not sure there is enough meat in the head to do this, but might be worth a shot if there is.

My '66 T120R has an iffy thread on one port. I'll definitely be fitting the later style header supports. Might need your spigot fix at some point.

For those desiring originality, why not fit the original cross brace support as well as the crank case supports. Chances are, a judge might give you a pass since you have the correct brace and are using the later braces for mechanical integrity.

regards,
Rob
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Old 07-19-2012, 10:32 AM   #9 (permalink)
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Found repair kit on eBay, should arrive this weekend. Thanks for all of your help, more topics soon I am sure.
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Old 07-19-2012, 12:22 PM   #10 (permalink)
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Take note of what I said about the spigot pulling up against the inner face.If this doesn't happen,it will keep coming loose and strip out again.

I haven't tried reclaiming a Norton head,but I think a bronze insert with a parallel interference fit on the OD would work really well.
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