Britbike forum Forums Thread Like Summary - 1968 spitfire inner timing cover

anyone know what size heli-coil or whatever will work for a stripped out hole in r/h crankcase. upper hole near dipstick for the inner timing cover bolts to screw into.

Up to '67 at least, those screws are 1/4" X 26 tpi. I don't know if they changed in later years. If you have thread gauges, just measure any of the screws, they're all the same. British Tools and Fasteners should have the correct Heal-A-Coil. They also have British thread gauges, FWIW.

1/4 BSF

only BSA would think doing that made sense.

why make a distinction between 55 and 60 degrees based on metallurgy? was there a real engineering reason or was it just because somebody on the shop floor refused to buy a new set of taps?

BSF will work as this is what I did on mine but they are actually cycle thread, same TPI but different angle.

gotta love em.

by the time i'm dead maybe i'll know enough about these things to be halfway competent.

Above 1/4" the TPI of BSF is coarser than cycle and a coarse thread works better in a soft material as the thread root is thicker, 1/4" was just an anomaly at 26 TPI. Whitworth would have been a better choice and was used in alloy once they had gone Unified. Nothing to do with 55 vs 60.



No just a rule set on the drawing office for the reasons I set out ref 55 vs 60, BSF in alloy and cycle in steel. Once it was on the drawing the shop floor and buying departments had to follow.

BSA followed a pattern pre the changeover to Unified of using Cycle on steel but BSF on alloy. So you have the odd situation with 1/4" where a stud screwed into alloy was BSF on that end and cycle on the nut end. The Unit Single 1 /4 rockerbox studs are one example, once you unscrew them from the head only the length of the thread tells you which end is which as both are 26 tpi.