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Exhaust studs

Richard Pitman

Richard Pitman

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Jun 1, 2020
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Location
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The threaded hole for one of the exhaust studs in the CB200 head that I fitted to my SL175 feels as though it's on the way out. Obvious fix is to fit an M7 to M6 stud. Just wondering if I can just run an M7 tap straight into the worn M6 thread ? One advert seemed to suggest that bigger stud could just be screwed straight in, but that sounds like a bodge. Easier than messing with thread inserts.
 
I'd imagine you could pretty easily tap the existing 6mm hole to 7mm. I suppose one could take a 7mm stud, grind the end to a gentle taper and cut a few hacksaw cuts across the threads to simulate a "self-tapping" stud but I agree it would be a bodge even doing it that way.
 
I've found a set of three M7 taps that should do the job, taper second and bottom taps. Might need to clean the hole out with an M6 bit first.
 
Another thing that will work well is using an M6 helicoil tap if that is easier for you to find. They are both roughly M7 in thread major and 1mm pitch. I also had success on my Aermacchi my cleaning the threaded hole and stud and installing the stud with high temp JB weld. It is pretty easy to overboard tightening these nuts but there is a compliant washer for a reason. I have never measured the exact force, but they do not need much to stay sealed.

Just to be clear, I meant if you did not want to put in a helicoil, timesert or safesert, an M6 helicoil tap and M7 tap are pretty much identical.

Theoretical thread minor for a 75% thread for an M6 is 5.00 mm, 5.4mm for a 50% thread. An M7 pilot drill is 6.00mm for 75% thread. Unless you have completely failed all threads, as a machinist, I wouldn't advise trying to screw in an M7 until you have drilled it out to at least 5.8mm or so and even then I would run the bigger tap.

The last thing to consider is an M7 SHCS is pretty rare (usually titanium or special applications). The flat to flat width of an M7 standard bolt is wider and so is the corner to corner. The center to center distance between the holes will not change so make sure you still have enough room around the flange/pipe to the a wrench on before making any changes.
Best of luck.
Lee
 
M7 > M6 studs arrived by return of post, just waiting on the set of three M7 taps now.

As it happens, I do have an M6 thread repair kit somewhere, cheap copy of Helicoil I suppose, that will have the M6 tap described above. Tried the cheap kit once, didn't get on well with it, forgot all about it.

The damaged thread in the CB200 head was down to a previous owner, whole engine was in a poor state. I fitted a new stud and it grips well enough, but there is that feeling you get with experience, tightening a nut or bolt, hard to describe, you just know somehow that a thread is about to give up. You don't get this sort of feedback from a torque wrench.
 
Richard Pitman said:
...but there is that feeling you get with experience, tightening a nut or bolt, hard to describe, you just know somehow that a thread is about to give up.
Click to expand...

Yes Richard, there absolutely is, and it is very hard to convey what that feels like. Once you learn it (usually with the experience of how much further beyond that point it takes before a bolt strips or breaks), it's an immense help in avoidance for the future. It's what I try to describe to lesser experienced members when I tell them to use a "good feel" when tightening 6mm and 8mm bolts instead of worrying about torque values as much on anything not involved in engine assembly.
 
Before I went back into the private sector, I was a federal machinist working at a vary large government facility. One of the things we used to do with new apprentices was to deliberately under drill a 1/4"-20 pilot hole so everyone could know what it felt like to break a tap. This saved A LOT of money in the long run. I wish there was such a thing for shade tree mechanics as it does not take much to damage early heads or engine cases. This is even more true with some of the lesser quality metallurgical examples I have encountered - Aermacchi, BSA, CZ. It is almost like Europe did not know you could alloy aluminum until the 80s. The thing a lot of people fail to realize with 60 degree metric threads, even if the bolt and hole are perfect (which they are not), the top 10% of the female thread and top 20% of the male thread along the hypotonus is intentionally truncated to prevent tolerance stackups. This means from the theoretical sharps, you are starting with only 60% of the thread available if the original JIS was comparable to 6H. As with all threaded things, unless it is really precision ground, only 15% of any helix is touching the mating part per revolution - it does not take much to cause damage. Be careful.
Lee
 
Dutchess said:
Before I went back into the private sector, I was a federal machinist working at a vary large government facility. One of the things we used to do with new apprentices was to deliberately under drill a 1/4"-20 pilot hole so everyone could know what it felt like to break a tap. This saved A LOT of money in the long run. I wish there was such a thing for shade tree mechanics as it does not take much to damage early heads or engine cases. This is even more true with some of the lesser quality metallurgical examples I have encountered - Aermacchi, BSA, CZ. It is almost like Europe did not know you could alloy aluminum until the 80s. The thing a lot of people fail to realize with 60 degree metric threads, even if the bolt and hole are perfect (which they are not), the top 10% of the female thread and top 20% of the male thread along the hypotonus is intentionally truncated to prevent tolerance stackups. This means from the theoretical sharps, you are starting with only 60% of the thread available if the original JIS was comparable to 6H. As with all threaded things, unless it is really precision ground, only 15% of any helix is touching the mating part per revolution - it does not take much to cause damage. Be careful.
Lee
Click to expand...

Though I don't understand much of the machinist's terminology, this is good advice and breaking off a tap is one of the worst things that can happen (similar to breaking off an easy-out). It's so difficult to explain "feel" when it comes to understanding the limits of bolts and taps, and it can take quite a while to learn it in practical use. Creating an intentional situation for an apprentice to break a tap and understand that "feel" was undoubtedly invaluable.
 
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