Poorly Fred
This month we look at using thread inserts
MHUTCH
ost of us love the buying of shiny, new bits, painting and polishing our bikes. But the main and most important bit in my mind to repairing, rebuilding and restoring old clunkers comes down to the mechanical restoration, cleaning and returning components to their former mechanical glory. Of course tuning and improving is a different thing and for every change there is often a price to pay in some way, be it function, comfort or reliability – and that’s a different type of fun.
At some point all repairs and rebuilds come down to mechanical detail. Nuts, bolts and threads should all be functional and at least as the design intent. This is particularly important for threads on gasket joints,
CHELLEY
as it means equal torque gives an equal amount of load over the whole gasket face.
I recently pulled the top end of an Ex-WD BSA B40 for inspection to try to cure some leaks. Whilst the head had reputedly never been removed before, the rocker box had certainly seen a few spanners; when removing the nuts to remove it one of the studs came out with the nut, the thread of which was full of ally and would need some attention if the clamping force on the gasket face (and hopefully oil retention) was to be restored.
Once the head was removed, stripped and cleaned, the formerly threaded hole was identified and marked; just for safety’s sake so as not to affect a repair on the wrong ’ole.
Taking the cleaned up rocker box stud that came out of the failed thread, I used a digital Vernier (or very-near) to measure the width, TPI (threads per inch) gauges and a Zeus book (the engineer’s bible for identifying threads and converting metric to imperial) to identify what the thread was, and so what insert we would need.
The thread was identified as being ¼in and 26tpi – and this is where Imperial can get tricky. This meant it was either BSF (British Standard Fine) or CEI (Cycle Engineers Institute), which is also called BSCy (British Standard Cycle). The difference is the thread angle of the thread – which can be difficult to measure.
BSF is 55deg, whereas CEI is 60deg; but without a shadowgraph machine or similar, you need more information. A little research showed that BSA used BSF from 1958. Practicality says one will probably fit in the other but given the choice it’s good to use the right one.
As a first go at repairing the thread, I attempted to ‘pull back’ the thread using a ¼in/26tpi plug tap; unfortunately there wasn’t enough meat left in the hole on this one, but it’s always worth a try. This hole needed a thread insert fitting to bring back to standard.
There are a number of different thread repair systems, individual thread size kits are easily available online or at a tool shop and relatively cheap (compared to scrapping heads!) plus once you have got them, insert refills are even cheaper; the kits usually come with instructions.
Fortunately for the BSA, I had a thread repair kit of the right thread. The existing hole depth was measured; this is particularly important as you don’t really want to drill too deep and break through somewhere you shouldn’t, in this case the exhaust port.
The measured hole depth was marked on the tapping size drill that came in the repair kit and the offending hole was drilled to this depth, keeping the hole perpendicular is important but as it’s just opening up an existing hole a little, this is achievable even with a hand-held drill.
Before any tapping it’s important to blow out any swarf left from the drilling, then the hole can have the repair thread cut using the tap that comes in the kit. I always like to use a cutting fluid/compound when tapping. Once the thread has been cut it’s important to clean out the newly threaded hole of any swarf and cutting compound again.
The thread repair insert was fitted onto the insertion tool and carefully wound into the newly tapped hole. This one was a little more difficult than usual, as the hole also had a dowel counter bore at the top of it. The insert was wound in until it was
½ to 1 turn (thread pitches) below the start point of the original thread.
The small drift that came in the kit was then used to snap off the tang on the bottom of the insert and the head then shaken about upside down until it fell out the repaired hole.
Finally, having had its thread cleaned up with a die, the stud was fitted to the repaired threaded hole to check that it fitted okay – fortunately it screwed in perfectly. One head ready to run again!