Just for completeness and to help others when searching for answers.
My 1954 XK 120 OTS has seat adjusting Levers BD.3258 & 59 fitted to the seat frame, using a “clevis pin” type of pivot although the XK 120 OTS Spare Parts Catlogue J.8 of June 1954 refers to the use of a set screw.
The earlier XK 120s used a Set Screw FS.104/4D with two steel washers FW.104/T, a shakeproof washer and nut, all with BSF ¼“ thread.
The later solution used Pivot Pin BD.6624 with two different washers: FW.104/C and FW.104/T. The first washer is made of brass, the other one is steel. The Pin is secured by Split Pin L.102/6U. This is the same construction as was used for XK 140 cars.
The FHC parts Catalogue J.11 of June 1953 still refers to the screw construction holding the Lever in place, but the DHC Catalogue J.13 of June 1954 shows the introduction of the Pivot Pin.
Keeping the part number BD.6624 in mind, we may assume the change-over took place in 1952 meaning that this construction is applied on later OTS and FHC versions and all DHC cars.
This must be a very early version Rob (as far as I understand the developments). These very early seat frames (according Viart up to chassis 660772) didn’t have the “guidance bracket” for the hook of the lever. See an example of the “guidance bracket” in my picture below.
The part number of the Seat Frames didn’t change and remained BD.3252 and BD.3253 throughout production of the XK 120 and also identical for all 3 models. So the addition of this “guidance bracket” didn’t require a new part number.
I don’t know where Viart got the exact demarcation number from (which would have taken place in February 1951) and I assume that there’s also one for the LHD models.
While looking for LHD versions of the OTS assembled around the same period February 1951, I noticed that for a period of about 6 months no LHD XK 120 OTS versions were manufactured (from November 1950 till May 1951). Is that correct?
If so, we may assume that all XK 120 OTS versions manufactured from May 1951 onward had the extra guidance bracket welded to the seat frame. This would be around chassis number 671100.
And all FHCs and DHCs would have had the later seat frame with the “guidance bracket”.
Porter’s monthly production chart in Original XK shows that 660772 would have been built in Feb '52.
His chart shows no 120’s at all in Dec '51, and no RHD OTS in Jan '52.
No RHD FHC from Dec '51 to Nov '52.
Difficult to guess sometimes where Viart got his information.
There is also one maybe two big differences in the actual frame itself all the early cars and certainly the chrome plated ones the steel angle was a different profile of manufacture with very square edges additionally the curved bit at the fron on early frames there are square cuts for the corners later cars the syeel frame the edges of right angle are quite rounded and the metal is tapered into the corner cuts out this is visible in the above two pics. some of this is on another thread.
The other variation here is very early cars and all alloys the knob on adjusting lever is welded on later cars it is just bent up.
These later 120 levers are different to 140/150 levers.
PS I manufacture these levers ie laser cutting and in fact manufacture the entire seats steel and wood and probably have made 30 or so sets 120 through to 150
I wondered where the reference to 660772 came from - presuming from actual car, but seems not…
The actual XK120 660772 was one of a batch of RHD XK120s built in April/May 1951 that were exported/sold new in Australia, with 660772 being built ‘at very end of April 1951’, but my records don’t give an exact date, but assume one of the last few days of April…
Back to the original question regarding when the change-over to the clevis pin pivot construction took place. In addition the question came up when the additional “guidance bracket” was introduced.
There’s also the possibility that these changes had been introduced simultaneously.
Part number BD.6624 of the later Pivot Pin for the lever, assumes that it was ready when the production of the XK 120 FHC started around July 1951. But Rob Reilly’s November 1951 FHC is still without the clevis pin and the additional bracket.
As all seat frames were identical for all three body versions, we should forget Viart’s reference to 660772 of April 1951 as the point of change-over.
So the earliest introduction date might well be end 1951/beginning 1952.
There’s sufficient proof that by 1953, all XK 120s had the additional “guidance bracket” welded to the seat frame. See the November 1953 example presented by Rob R. 120 / 140 seats . Although more difficult to see, I believe it also has the later clevis pin.
Wow, your research is thorough. That picture is of the seats from FHC 681114, built in April '53 according to Porter. I sold them to someone on this forum in 2009. I believe you are right that it has pins rather than screws.
Thanks for this input. More (unexpected) information becoming available!
Both Rob’s FHC 679187 (November 1951) and your FHC 679375 (February 1952 according XKdata) don’t have the “guidance bracket” but at the same time I noticed that both have the early seat frame with the corners cut out “square” instead of the later “rounded” version (as Terry already mentioned). Viart says that this was the version for the Alloys and “early” XK 120s. Apparently “early” is not that early as yours was built in mid February 1952.
The seats of FHC 681114 built in April 1953 definitely have the “rounded” cutout.
Could it be that the change-over to a new seat frame (although still coded BD.3255 & 56) included
(1) the introduction of “rounded” cut outs in the corners,
(2) the introduction of the “guidance bracket” for the lever and
(3) the replacement of the BSF bolt by the Clevis Pin?
So the change-over period to the new seat frame has now been “narrowed” down to somewhere between February 1952 and April 1953.
the 772 reference will be from me and amazingly it probably should have been 722.
Here is a pic of the seat from 660722 after I restored them for the owner ie new wood chrome plating etc. They were very rough prior to starting.
Note single tab, square cut ins, chrome plated
Doesn’t really matter whether February or April 1951, as that date doesn’t affect the change-over dates we’re discussing.
What is interesting, is the fact (see below?) that this seat frame was still chromed! The change-over to painted seat frames would shift towards a date somewhere in spring or summer 1951. In any case, Rob’s November 1951 FHC had the painted version.
Do you have any reason to doubt whether the chromed seat frame of 660722 (or 772) was original to the car?
Porter (Original Jaguar XK, 3rd edition) refers to the introduction of the painted version in September/October 1950 apparently based on input from Guy Broad. In the footnote Porter refers to chassis numbers 660228 and 670727. Your chromed seat frame would shift that date by almost half a year towards Spring 1951.
The history of Jaguar is never a dull subject and keeps us busy…
Thanks for your input. As you can see in the spread sheet below, 660405 fits well within the new scheme.
I tried to put all input received so far in a spread sheet. Although more evidence is required, we may conclude that the introduction of the painted seat frame was much later than had been assumed so far.
It looks like the other changes (rounded cut-outs in the seat frame corners + welded guidance bracket for lever + use of Clevis Pin replacing BSF bolt on lever) may well have been introduced at the same moment (somewhere in 1952).
If other owners of a 1952 XK 120 have some pics of their seat frame, these are most welcome: it might help us to get a better idea on the exact change-over date.