I have a set of K3’s mounted without the Nathan manifold. I’ve plumbed each one separately. I had a horn off and looked to see what size orifice was at the bottom of the horn itself. Looked to be about 3mm. I understand the locomotive sets have a 4mm orifice and mine might be OTR.
If this orifice at the base of each individual horn the 4mm hole being referred to or is it a restricting orifice mounted in the Nathan manifolds?
OTR editions in the early days had a restrictor in the intake throat of the manifold (i.e. at the 1/2" fitting right at the base). Apart from that there was no difference.
Here’s a couple of links I was searching for… should give you some more info (one shows the restrictor plug in question; the other shows you orifice size)
https://www.trainhornforums.com/showthread.php?t=875
https://www.trainhornforums.com/showthread.php?t=6686
I didn’t understand that OTR meant it was restricted in the manifold I thought it was the orifice on the horn. So when people talk about drilling them out they’re talking about the orifice on the base of the horn, no?
No, drilling out refers to enlarging the venturi at the base of each chamber. It’s done by those IMO with incredibly poor hearing because Lord knows why you’d ever consider an Airchime to not be sufficiently loud enough! … anyway, each to their own.
If you do a search you’ll find some clips and stories about enlarging the horn opening. If you do, then it’s irreversible damage though.
The OTR edition came out because Microprecision wanted to cater for the trucking market but their Rolling Stock horns were too loud to pass legislative restrictions. The plug (as you would have seen from the previous link) can just be removed with a hex key - no drilling required. It had quite a narrow restriction which limited the amount of airflow and hence some volume. It didn’t last long (i.e. the plugs soon disappeared). Those that wanted an Airchime didn’t really want one that was any less than what the trains got. OTR was then only used in terms of branding/tagging the horns so that Microprecision could easily track which horns got sold into which sector. Railroad horns only got sold by those companies which were in that industry. Everyone else, (including Hornblasters, who are a licensed reseller) got the OTR versions. No difference other than the tag.
If you just love the horns for what they are, then it makes absolutely no difference. To some its about collecting railroad memorabilia and in that sense, OTR won’t have any heritage in that industry.
I think I’ll just leave mine as they are. But I did by a 3/4" valve and I’ll be upgrading to that soon