2. from *during* the '69 season 'til the end of Harris Bonneville production ('85?), the original thread was 1/8"NPS (American National Pipe Straight; i.e. 'parallel') (note: being a pipe thread, the 1/8" refers to a nominal internal diameter - the o.d. is roughly 3/8");
3. however, for a short period at the beginning of the '69 season, some engines had 1/8"NPT (National Pipe Taper). The only documentation I've seen on this refers to the the T150 engine up to number AC01629; however, I've an early '69 T100R in my garage with a 1/8"NPT thread.
But (and it's a J-Lo ) ...
... we're talking about engines that are at least over 20 years old and could be twice that, with all that implies in the way of d.p.o. Both 1/8"NPS and NPT are 27tpi whereas 1/8"BSP is 28tpi and, as I've mentioned above, 3/8"Cycle is 26tpi (not to mention M10 x 1.0 is 25.4tpi ).
From experience, I would say extract whatever's screwed into the o.p. hole of your specific engine and check the thread *very* carefully with both thread gauges and a micrometer (ime, any taper is actually quite difficult to detect by eye ). In this, ensure you actually have a 27tpi thread gauge - don't try and interpolate between the 26tpi and 28tpi ones because it's very difficult to do, even under a magnifying glass.
Finally, btw, be aware that there isn't any such thread as BSPP (BSP Parallel) (or BSPS - BSP Straight) - 'BSP' is parallel/straight by implication, a tapered BSP thread is denoted 'BSPT'.
I'm pretty sure this is by Mr Healy, which I copied a while back, hope this helps.
It sure would be possible these old threads have been altered to suit whatever fitting was at hand.
3. however, for a short period at the beginning of the '69 season, some engines had 1/8"NPT (National Pipe Taper). The only documentation I've seen on this refers to the the T150 engine up to number AC01629; however, I've an early '69 T100R in my garage with a 1/8"NPT thread.
But (and it's a J-Lo ) ...
... we're talking about engines that are at least over 20 years old and could be twice that, with all that implies in the way of d.p.o. Both 1/8"NPS and NPT are 27tpi whereas 1/8"BSP is 28tpi and, as I've mentioned above, 3/8"Cycle is 26tpi (not to mention M10 x 1.0 is 25.4tpi ).
From experience, I would say extract whatever's screwed into the o.p. hole of your specific engine and check the thread *very* carefully with both thread gauges and a micrometer (ime, any taper is actually quite difficult to detect by eye ). In this, ensure you actually have a 27tpi thread gauge - don't try and interpolate between the 26tpi and 28tpi ones because it's very difficult to do, even under a magnifying glass.
Finally, btw, be aware that there isn't any such thread as BSPP (BSP Parallel) (or BSPS - BSP Straight) - 'BSP' is parallel/straight by implication, a tapered BSP thread is denoted 'BSPT'.
I'm pretty sure this is by Mr Healy, which I copied a while back, hope this helps.
It sure would be possible these old threads have been altered to suit whatever fitting was at hand.
