I’m a little confused. Nothing new there.
What’s the difference between a Yaesu XF-115C and a Yaesu YF-122C?
Both are 500Hz Collins mechanical filters. Old articles show the XF-115C being fitted in an FT-817. Yet all the dealers advertise the YF-122C as the filter for the 817. Of course this one is more expensive than the XF-115C.
There must be something else that makes the YF-122C better with 817 as the XF-115C is still available. Unless it’s just marketing forces at work!
Andy
MM0FMF
In reply to MM0FMF:
Cant really answer your question Andy, but I’ve not come across XF-115C being mentioned in relation to the FT817 before. I’ve have noted that the YF-122C is quoted as the filter for the FT817 though.
I did an internet search (as I’m sure you have) and it seems that the XF-115C is being sold for the FT1000MP, FT1000 Field and FT847.
Good luck with your quest! I listen to CW mainly on an MFJ8100 regen, which is as wide as the whole CW section of 80m! Using the FT817 seems so refined after that, that I think I’d find an extra filter too narrow for my wide-trained ears!! Why listen to one QSO when you can listen to five?! 
All the best.
Colin
In reply to MM0FMF:
Can’t answer the question Andy but, if you’re in the market, don’t forget to check out Inrad filters too. I believe they do 500Hz and 300Hz Collins filters for the 817 if you want narrower. (I use a 350 Hz Icom filter in the 706 as it’s good for RTTY too)
http://www.inrad.net/home.php?cat=16
When I used to have an FT1000 MK V, I loaded it with Inrad filters and they worked FB. Not so cheap to order from the US right now but worth a look.
73 Marc G0AZS
In reply to ALL:
OK, this looks like a Yaesu special confuse job!
When you buy a box labelled YF-122C inside it you find the following:
http://www.ok1kcu.net/storage/200610150947_pa131453_std.jpg
I guess you pay more for the fact it now has a Y in the part number and not and X!
This explains why the 817 I looked inside earlier had an XF-115C installed. That doesn’t match the numbers quoted by the dealers who sell you a box with the correct number and a part with another number. Causing you to look up the part number and enter an unpleasant feedback loop. Ow!
Andy
MM0FMF
In reply to MM0FMF:
What’s the difference between a Yaesu XF-115C and a Yaesu YF-122C?
Unless it’s just marketing forces at work!
IMO it is, a YF-122C is a XF-115C fitted on a PCB for insertion in a FT-817.
73,
Zoli
In reply to MM0FMF:
I’ll take the covers off both the 817 and 847 and have a look. Fitted the filter to the 847 a couple of years after the 817 and remember thinking at the time they looked similar.
Won’t be today though - prepping the Morgan for it’s first run out tomorrow, to the MSCC Car Trial near Ledbury…
73 Graham G4FUJ
In reply to ALL:
Once more thanks for the comments. Thanks to Zoli for explaining the true difference. I never thought that the difference was just the PCB. It is very confusing when you see an item labelled ‘X’ when it comes in a box labelled ‘Y’.
Andy
MM0FMF
In reply to MM0FMF:
Good to see things are a bit clearer!
I looked out my FT817 Handbook from 8 years ago (my FT817 was made in February 2001, one of the first batches in UK), to have a look at which filter was specified and it is the 122 of course.
At least you know now that there is no circuit (or performance) difference between the two part numbers.
Problem (almost) solved!
73 Colin
Left FT-847 500Hz CW filter, right FT-817 INRAD 300Hz CW filter
http://www.rofike.hu/Filters/filters%20001.jpg
Inrad filter ??? 
http://www.rofike.hu/Filters/filters%20002.jpg
73 DX Pista HA5TI
In reply to HA5TI:
I have recieved a XF-115C today that I picked up in used condition. Now fitted and what a difference that has made!
I could do with QRS pot on the radio that slows down all the CW i can now seperate…maybe 2 pots? A second one that puts gaps between there characters. Sean M0GIA
The thing is Sean - if YOU are the activator, you don’t need them pots. All the chasers will slow down the CW and put the longer spaces in for you. You are the “DX” and they want to work you. So now you’ve made life easier by fitting a CW filter, make it easier still by being a CW SOTA activator!
Tom M1EYP
(See you later at the gig)
In reply to MM0FMF:
hi;
ive only just looked and found this thread but thought id respond anyway .
i can confirm that an XF-115C works in an FT817ND .
i had an xf-115c sat around for ages that id bought some time ago in error for another yaesu radio and came across it today in one of my drawers .
i have been setting up my FT817ND with a 13cm transverter system and was thinking of buying a YF-122 filter for CW , so i did a google search for XF-115C just to see what it was originally for and then came across this message thread .
looking at the filter it did carry the same part number as the YF-122C and so i thought id give it a try in my FT817ND .
the XF-115C fitted straight in and worked right away after setting the filter setting in the menu .
great , i have saved myself £90 on a new YF-122C and used the XF-115C i had which works just fine .
cheers
Following up on this old thread with some hopefully clarifying information on these filters. I have been confused by this as well, and in the process of figuring out which filter fits in my rig, I accidentally bought the wrong one off of EBay, leading to a lot of online research and hopefully some answers.
As far as I can tell, XF refers to the actual filter device (not including the PCB board), while YF refers to the whole package of filter device plus the board (PCB w/ connectors) that it comes on… and the boards are specific to the radio.
XF-115C is the CW & RTTY filter device (without the board), with 500 Hz bandwidth, for the 455 kHz IF.
YF-115C is the XF-115C filter on a larger board (~70mm x 23mm, 8 connection pins), which fits in the Yaesu FT-1000MP, FT-1000MP Mark 5, FT-1000MP Mark 5 Field, FT-847 and FRG-100.
YF-122C is the XF-115C filter on a smaller board (~56mm x 14mm, 7 connection pins), which fits in the Yaesu FT-817, FT-817ND, FT-818ND, FT-857, FT-857D, FT-897, FT-897D, and FT-2000.
Note that I don’t have this from any official source, so would love to have someone who knows for sure chime in. I found most of this info by looking up accessories for each rig, and noting the similarities in naming and board sizes, so there may be some errors or omissions.
Hope that helps, -Chris KJ6WEG
Great Chris,
Thanks.
Now why do i\I have to keep going to the menu to select the filter every time the rig is turned on. I’d like it as default when CW mode is selected. Have I missed something obvious in the manual??
73
Ron
VK3AFW
Stays selected on both my 817s (original and an 817D). But you have to select the filter per band. i.e. you are on 20m CW and you want to the the narrow filter then you need to select it from the menu. When you change to 30m and use CW, the filter will be set to standard until you select the added one. When you go back to 20m it should still be in narrow CW mode and stay like that till you deselect it.
So you need to enable it on every band you use but it should stay selected then. Do you memories stay set, the 817 isn’t forgetting them?
Just to clarify “select it from the menu”, that is a two stage process, at least in the ND version. First, turn on NAR in operating function row 7. Second, set CW in menu #38. If you don’t do both, your filter won’t filter. 
Elliott, K6EL
Climber Clown
Ron,
As per Elliott’s advice, menu #38 effectively is the configuration setting, “installing” the filter. Then when you select NAR in the operating row 7, you are choosing between the standard and optional filter. You may not want the NAR option selected all of the time. But once it is “installed” using menu#38, it is available at any time.
As the standard rig only has one optional filter, menu 38 effectively specifies which modes a NAR option is available for. If you specify a narrow SSB filter, switching NAR on in CW won’t have any effect. And vice versa, installing the CW filter means NAR won’t do anything on SSB. Which is pretty much the way it has to be. Nobody would want to listen to ssb through a 500 or 300 hz filter.
This is how my non-ND ft817 works anyway.
73 Andrew VK1DA/VK2UH
And if you want to roll your own, here is some excellent info from K6XX. He esplains at the end of his page what you guys were talking about WRT to different filters…Later, Todd KH2TJ
And this is most likely outdated info. I thought I read last year, or the year before, that filters were getting hard to come by. Inrad used to do a yearly sale, but believe the company has now been sold off…
This is academically interesting to me as I don’t do CW. However, as an SSB man I find the skirt selectivity of both the FT817 and FT857 rather poor. Has anyone any experience of fitting the YF-122S SSB filter, does it give a worthwhile improvement in skirt selectivity?