Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: AshimotoK0 on October 14, 2014, 08:19:53 AM
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Need to revisit this . My CB750K0 crankcases had a broken off stud. It was one of the short ones at the front and the threaded hole was slightly recessed from the sealing face of the 'cases. I tried every method mentioned on all of the forums including welding a nut onto the old thread and get it really hot to enable removal, due to the heat and purchase achieved on the new nut head. However, because it broke off low down, it was difficult to weld and I only had a MIG not a TIG welder, so that method was not successful. What I then did was to get a set of scrap barrels and located them onto the crankcase using the 'knock pins' to accurately locate the barrels onto the crankcase. Then I got a piece of SS tube which was a tightish fit in the barrel stud hole and got a long series drill which was a good fit on the ID of the tube and drilled a pilot hole into the remains of the stud. This worked really well as the drill was fully guided and the hole was pretty concentric on the remaining stud. Then I was 'out of sorts' for 18 months so the job was abandoned. I read at the time that Honda used specially tapered machine taps for the holes. Does anyone know about this. My mate suggested that I drill my 'pilot' hole out to the stud thread tapping size and then I may be able to pick out the old threads. Or should I just go for running a tap down the hole and Loctiting the stud in? Or is there a special tap available?
Cheers ... AshD
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Initially go by your mate's recommendation. Then try to pick out the threads. I jhave done it a few times with various parts.
My brother actually made me a set of short tubes that would fit over various size studs with pilot drills to suit. Same principal as you used on the stud. Put them somewhere safe and will have to sort them out as I have some work coming up.
I will have a dig and see if there is an answer to the stud thread in Hondaman's book.
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Ash, if it can not be drilled find a engineering firm or tool maker with a spark eroder. This machine will take it out clean and not damage the thread.
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Ash, if it can not be drilled find a engineering firm or tool maker with a spark eroder. This machine will take it out clean and not damage the thread.
Think I looked at spark erosion Trig but it was very expensive and he wouldn't give me any guarantees. I hate leaving rare stuff with other people I don't know, as I have heard of so many horror stories. I do have another SS sleeve bored out to the tapping drill size.
Cheers ... Ash
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Ash, if it can not be drilled find a engineering firm or tool maker with a spark eroder. This machine will take it out clean and not damage the thread.
Think I looked at spark erosion Trig but it was very expensive and he wouldn't give me any guarantees. I hate leaving rare stuff with other people I don't know, as I have heard of so many horror stories. I do have another SS sleeve bored out to the tapping drill size.
Cheers ... Ash
Ash, i don't know any toolmaker that doesn't have a spark eroder. They need this machine for when they break a tap in the thread. You will have to find a small firm or an old boy that will charge you a Tenner.
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Ash, I'll go with Trig on the spark eroder. We ran one in the toolroom I was in and it was a godsend for removing this sort of thing, the only limitation was the size of the workpiece you could get into the liquid bath it operates in (don't know if they still have that requirement).
The method is similar to drilling in that you have to basically romove the core of the stud leaving what is in effect the thread (like a helicoil) remaining which you can pick out with something like a scriber.
Seamus's way is as good as you'll get for a home remedy and should get you there but you need to "contain" the core drill size as he says to keep it central to the stud.
If I had to do that with a drill i'd get a steel plate to lay on the crankcase face with two other holes to locate and bolt down on two of the other studs, giving good location and a tube of something like 20mm height over the stud you are drilling to provide a guide.
One of the problems of first drilling a pilot hole is that the larger size drill has very little self centering effect which is provided by the tip of a drill in the material. if you take that centre material away then the drill tends to grab on it's outer edges and is hard to hold central to the stud, It's this effect that using a steel guide should avoid.
An engineering tap set usually comes as a set of three, a taper tap, intermediate and a plug tap to gradually form full threads if used in succession. if you tapped just using the taper then the stud would bind up tight before reaching the bottom of the hole.
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If anyone wants to know what a spark eroder is, I have found a vid that explains >> https://www.youtube.com/watch?v=YbeKxFBZrF8
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If anyone wants to know what a spark eroder is, I have found a vid that explains >> https://www.youtube.com/watch?v=YbeKxFBZrF8
Watched it Trig and now understand how it works. Looks more like a 50+ quid job to me though but if it's the only way then so be it !!
Cheers for all of the good advice..... Ash
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It was a American vid. He chatted at least 70% of the time. 3 minutes max to do this job :)
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It was a American vid. He chatted at least 70% of the time. 3 minutes max to do this job :)
It's always razamataz with them isn't it? Reminds me of my visit to the Kennedy Space Centre. There's a feature called meet the astronaut scheduled to last 30 mins. First 20 mins is warm-up act with the audience ::)
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Here is an update with Pics. Dimpled top of broken off stud with Dremel then used S/S sleeves and drills and scrap barrels as a drilling guide. 1st hole was pretty concentric (see pic) slightly smaller than M8 tap size. Last week opened hole out to tapping size M8 (6.8mm drill) and could hear crunching as I drilled it presumably bits of treads breaking up. Where do I go from here,, run a tap down it or try to pick bits of the old tread out? Please don't suggest spark erosion as it is going to cost quite a bit to get done around here plus I hate entrusting rare parts with people I don't know, so it's a last resort really.
Ash.
[attachimg=5]
BROKEN STUD
[attachimg=2]
1st Drilling 6.5mm
[attachimg=3]
DRILL AND S/S SLEEVE
[attachimg=4]
AFTER DRILLING OUT TO TAPPING SIZE 6.8mm (for M8x1.25 ISO thread)
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Careful picking with an apropriate tool is going to be fiddly but you would be OK. Is there enough meat left to be able to try and twist it out with an ezyout?
Pete
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That was my thought Pete
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My experience of using an easyout in this situation is that you will grip the stud wall and push it into the softer casing with the stud drilled out to tapping size. Picking the thread out is the safest bet in my opinion. If the hole is not exactly central, tapping the stud will see the tap take the easiest route and will track towards the casing.
Phil