Oh dear! The diameter of the hole for the NVA assembly is 0.135" NOT 0.33" , a senior moment I am afraid!

Andrew.

Thanks Aki,
Looks as though it might well fit from your dimensions.

Regards,

Andrew.

The battery in my caliper is dead, but I can say with certainty the Max 15 and FP 15 used different NVAs.  I'm pretty sure the 15 FP spray bar is the same diameter as the 20-40FP-S spray bar, with the part through the venturi turned thinner.  The Max 15 spray bar is a constant diameter.  Without actually trying to install a 15 FP needle in a Max 15, I would expect it won't fit without drilling the case/venturi. 

The FP 15 NVA, Part No. 2171 1000, measures 0.123" OD and is parallel. As far as I know, the only FP NVA that's turned down across the venturi opeining is the FP 10 part which I think was taken from the 10 FSR.

Weird.  The 15FP-S spray bar in my spare parts bin is stepped, as is the one pictured on Towers website.  Maybe they've changed since...

Hello Geoff,
I just rechecked the diameter of the hole in the crankcase and it is indeed .135" Maybe the restrictor hole is a little smaller, but the restrictor is stuck tight and will need some gentle persuasion to rotate it. At the moment it is partially aligned with the crankcase hole, so I can't measure it.
0.135" rings a bell somewhere, maybe it was on Len Neumann's site where he is selling NVAs. Yes! I just checked there and he is selling 0.135" diameter OS NVA assemblies suitable for FP 20 - 40 engines. This is odd as I would expect that the nearer NVA would be for the FP 15 which Tower carries but gives no dimensions for!
  Here was I thinking it was a simple question! Maybe it is, but the answer seems complicated. I will loosen up the venture restrictor and check that the FP20 venturi fits. I never gave it a thought that it would. I assumed the NVA would have a smaller diameter. If it fits then I will give it a run or two and see how it behaves.

 Thanks everyone,

Andrew.

Given the interest and the apparent discrepancy, I just checked my FP 15 NVA again. The part number I quoted is correct and there is no step in the OD of the one that I have.

Andrew T, I started designing tooling for German-made machines in metric back in about 1973 because I found it frustrating to spend ages converting simple numbers to funny-looking imperial dimensions, so I prefer to think in whole numbers. The screws that OS use to hold in the carburettors on FP 20–40 and LA 46 are M3.5 so the hole is a small clearance on that. I don't know about the 15s as I don't have any and my FP 10 was bought as a CL engine.

I find it much easier to remember 3.5mm and 4.0mm for ST-type and Enya NVAs than to try and remember the inch dimensions.

I got around to fitting the FP20 NVA, so yes it did fit without any mods and yes it worked fine on the test stand. Seems odd, but there you are another old engine saved for reuse!
I will have to build something for it, any ideas? Not another Peacemaker though! I shall have to either build quicker or stop accumulating engines! My children will probably dump all my kit when I am gone. So I have started tagging all my engines with some values. maybe that will make them think twice.

Happy Christmas everyone!

Andrew.