I recently bought some stainless 'cylinder through bolts - 66-0115' for my B31, I have been here before and the opinion was dont buy stainless. Well I had already bought them unfortunately, I find now that they will not fit into the cylinder head (seems like a different thread), Were the threads changed in the heads at some point? My bike is a 1959 alternator model, even though the threads compare well by eye the stainless ones will not screw into the head fully, they go so far and lock up I will not force them. Any ideas please and thanks.
without an old and new in my hands with thread gauges, I would say they are the incorrect form.
BSC and MM use 60' thread angles BSF and BSW use 55', many modern fastners are made to metric dimension equivalents of imperial dimensions.
Note that M8 × 1.00 metric threaded parts can interchange with 5⁄16 inch (7.94 mm) diameter × 26 tpi parts with a "Class B fit", meaning you should not go back and forth between threadings. British Bicycle standards for other parts which are not strictly BSC but come in similar fine threads.
that, with the fact stainless produces a dry thread, in that it does not give as much as stainless means the wrong thread angle and a non giving thread are the basis of difficult fitting threads.
Ive had BSF sold to me as BSC and had to run a die or tap down to correct.
Ive looked on't www but cant find sizes or lengths to make one.
you could ask Drags if they can check if the threads changed.
otherwise maybe get head checked with a tap or gauge and might have to purchase a die to re cut thread, do you have the old ones, how do they compare with the new ones?
Hi, As i said in the first post they compare very well by eye but, the new ones will not screw in to the head any appreciable distance (about 5-6 threads is all)
The threads are so close but there must be some difference with angle of thread or no. of TPI. I took the seat clamp bolt off and the bolt threads easily into the cyl. head and the nut from same goes easy on the old stud, but not even close on the stainless cyl. bolts. Therefore I can presume the threads are totally different. I'll ring DraganF in the morning and find out. Thanks for help.
I can't help with the tread problem but I have encountered another problem with these bolts.
The bolts consist of 2 parts, the lower part threaded into the crankcase and the upper bolt which threads into the head and is held captive in the lower section by a ,,foot" attached to the bottom of the bolt. I don't know how the original foot is attached but I have never seen one fall off an original bolt so it must be pretty effective to resist the force applied when tightening the bolt into the head.
A few years ago I helped a friend with his B33 and one of the shiny reproduction head bolts he had fitted would not tighten into the head at all. The bolt came free from the lower section as the foot was in fact a threaded sleeve which had unwound itself and dropped off when it was pulled up against the inside of the retaining lower section. If a left-hand thread had been used it might have worked (although there would always be the chance it could unwind when undoing the head bolt). To recover this loose sleeve we had to dismantle the engine as it refused to just fall out the bottom!
So, I would be careful and make sure any repro bolts do not have this type of fixing and maybe enquire as to how the retaining foot is attached when buying.
Pete C. I had the exact same thing happen with one of the shiny new head bolts that my Dad gave me with the bike (a 1949 B31).The end unscrewed itself and dropped into the crank case. After hours of poking around inside I resorted to tipping the bike upside down on the lawn. Luckily it dropped out. I couldn't face dismantling everything to get it out. I put the new bolts on the shelf in the shed and reused the original bolts. They aren't as shiny, but the ends won't come off. Beware shiny new stuff, BSA made them better.
All B31s and B33s had the same bolt thread should be 3/8 26 TPI Cycle try one of your front engine stud nuts should be the same
Regards