I’m gradually working my way through the snagging list on SB4059, all those little jobs that I know about and that require attention are slowly getting done. Not so long ago I swapped the original Campagnolo grey outer brake cables for original Campagnolo black alternatives, and I also updated some of the frame transfers so that SB4059 correctly reflected the 1980 TI-Raleigh scheme. This time I’m repairing the threaded seat lug so that I can fit a correct and original SBDU threaded bolt.
I don’t know if anyone has ever noticed it, but SB4059 has never had the correct seat pin fastening bolt. It has always had a 2-piece Campagnolo bolt fitted (part no. 1072).
I didn’t have an option with this frame because the original threads tapped during the frame build are worn and won’t hold the pressure placed on them by a standard bolt.
In the same way that the grey cables were perfectly good for both the bike and the period, they niggled with me; even the DT Swiss stainless spokes I used to build the wheels were perfectly good but they niggled with me because they weren’t the correct spec Berg Union chrome. Likewise, the Campagnolo bolt is perfectly good and does a great job without requiring a frame thread but it niggles with me that it isn’t SBDU ‘correct’.
I have an SBDU drawer under the workbench which contains all the small nuts and bolts and larger components that I remove from the frames and bikes I receive. I normally bag up things like end adjusters, bottle boss bolts, seat bolts and plastic BB cable guides in the same bag. Some parts are destined to go back onto the frames when I’m ready and some just become spares for other builds. But one thing is for sure, there are a few seat bolts in here.
Thinking about the topic of seat lugs is starting to form a blog post in my head about the changes in SBDU seat lugs and bolts – I really need to get out more!
The four bolts above from left to right are…
- Campagnolo, part no 1072 (8mm) – they also did a 1070 (10mm) version.
- An early SBDU example bolt with a shallow head
- A later SBDU bolt with a deeper head
- A modern Titanium replacement.
Bolt number 2 and 3 were used on the standard side attached seat stay frames – the SBDU used a smaller bolt with their fast back seat stay option.
My plan is to repair the thread in the seat bolt housing by using a thread repair kit, probably better known as a ‘Helicoil’. A helicoil is a piece of sprung steel forming a thread that is fitted into the damaged hole and provides a good clean replacement thread of the correct size for your existing bolt.
But before you can fit a helicoil, you need to know the size of your bolt! The seat bolts on SBDU frames are Metric and there are two dimensions you need to be aware of. The first is the diameter of the bolt and the second is the pitch of the thread.
A thread pitch gauge is a very handy thing to have in your toolkit. Using the two measurements from the caliper and pitch gauge mean that the thread repair kit I need is for an M8 x 1.25 bolt.
The ‘M’ is the Metric designation and the ‘8’ is the outer diameter of the bolt in millimetres (mm). The 1.25 is the thread pitch – this is the distance in mm from the peak of one thread to the next. 1.25 is also described as a ‘course’ thread for M8, a fine thread for this diameter would be 1 or 0.75 mm. You can define a third measurement of thread size, and that is thread length. In my case I’m fitting original SBDU bolts so the length isn’t important for this but for those that do want to know, the length is 25mm. The complete bolt thread size is…
M8 x 1.25 x 25 (diameter – pitch – length).
A lot of mechanical work on bikes will be straightforward and a basic toolkit will normally do most things, but when you are working on repairs like this it is worth investing in some thread repair tools. Good taps, calipers, gauges and cutting fluid make life a lot easier. You may not use them as often as a 6mm allen key or 8mm spanner, but they are great to have in the toolbox for when you need them. I actually use the pedal 9/16 taps and M5/M6 quite often when prepping a frame and building a bike.
A thread repair kit consists of a few things, the actual helicoil insert, a drill bit to prepare the hole and a tap to thread the hole to the correct size. There are also a couple of tools which are used to fit the helicoil into the newly drilled and threaded hole.
The helicoil insert is essentially two threads, one thread on the outside to grip the steel of the damaged item and another thread on the inside which provides the necessary M8 x 1.25 dimension. A blank hole diameter for a standard M8 thread would be approx 6.8 mm. That size blank hole would allow the tap to cut the necessary 8 mm thread. But in my case, the 8 mm thread is now damaged. This means that to fit the insert, the damaged hole has to be made slightly bigger using the drill bit that comes with the kit – the drill bit in the kit is 8.3 mm. The accompanying tap is specifically sized to cut into the new 8.3 mm hole to accommodate the outer thread of the insert.
This is the part of the seat lug where the helicoil will be fitted.
Before drilling and tapping, it is good practice to protect your bike; I’ve squeezed a cloth into the seat tube to protect the BB from debris but I’ve made sure it won’t foul and get caught in the drill (I would rather use a cloth than a Super Record seat pin). I also covered the rear tyre and brake with another cloth – that protected the tyre, brake, chain and gears from any debris falling from the seat lug. It only took a couple of seconds to drill the hole to the correct diameter.
When you are doing any cutting or facing you should use a cutting fluid, it not only makes the task easier and smoother, it also helps to prolong the sharpness and life of your tools. You can see how the cloth has collected the debris from the drilling and tapping.
That cloth is now in the bin, you really don’t want to leave it on the work bench then pick it up by accident and wipe your paintwork with it!
The helicoil insert has a special tang on it which fits into the slot of the fitting tool. You can see a small notch on the tang – it is a built in weak point, so that once it is fitted in place, it can be knocked with a punch and it will snap off, leaving the helicoil in place.
This tool twists the helicoil into the freshly prepared and tapped hole. Once it is in place the tang can be snapped off.
The new replacement thread is solid. The feeling of winding the bolt into the thread gave me a lot of confidence – it is very secure and the bolt fits perfectly, it holds the seat pin tight.
I know it was a minor niggle, and probably comes across as a trivial detail, but having the correct SBDU bolt in place is very pleasing and I’m one step closer to completing that snagging list.
Will SB4059 ever be completed..?