I’m interested in retrofitting my early '120 for spacers on the carb inlets as per the later cars. In addition to the spacers, I know I will need longer mounting studs. Can someone confirm for me if Jaguar part number C11768 is the correct, longer stud to be used? It’s not clear to me when checking through my parts book and the vendor web sites.
Thank you.
Christopher,
Jaguar Part number C.11768 is the longer stud used for the carbs with insulating spacers. There’s also C.7488 as used on the later XK 120 with spacer, but that has (apparently) been replaced by the later number. Don’t know what the precise difference is between the 2 studs.
According Viart the longer stud C.11768 measures 4 7/16" in length or 112.7 mm. The length of 122 mm as shown by Viart is a typing error.
Bob K.
Christopher (and Bob)…
Original XK120 Inlet Manifold Studs for H6 carburetters without insulator blocks were eight-off C.2386 as fitted up to Engine No. W6918 only. From Engine No W6919 onwards fibre INSULATORS C.7461 were added, necessitating new longer studs C.7488 to be now fitted to all later XK120s and all XK140s up to G5789 (in 1956) when the revised C.11768 stud was introduced, and for spare parts supply superseded the C.7488 stud. Thus if you have the later XK120 SPC published after 1956 all the inlet manifold studs are shown with the post 1956 part numbers, including the eight C.11768 studs for the H6 carburetters with Insulating Blocks in place.
The original C.7488 studs are nominally 4-3/4in Overall Length (comprising a 5/16"UNC threaded end 3/4in long that screws into the aluminium head, then a plain length 3-1/2in long for the gaskets/insulator/H6 area, then a 5/16in UNF thread 1/2in long onto which the spring washer and nut secure the carburetters in place.)
The superseding C11768 are 4-7/16in Overall Length with the difference being solely with the 5/16in UNC length being shorter, now 7/16in long, but still with the same 3-1/2in plain length and same 1/2in long 5/16in UNF length.
The reason for the change from the C.7488 to the C11768 was there were problems with the 3/4in long UNC thread bottoming out in the head casting and breaking into the water jacket, so the head casting was revised to reinforce the inlet manifold stud holes, and the studs were all given a shorter thread so they didn’t bottom out.
So the later C11768 studs can be used now in a pre-1956 head that was originally fitted with C.7488 studs (or indeed the original C.2368 studs when no insulators were used)
You will find all the aftermarket vendors will now only supply the later C11768 studs with the reduced length UNC thread that screws into the cylinder head casting…
I will look this up later (dinner is about ready) as I believe there is a Service Bulletin that deals with what Roger has stated in his as always informative and detailed comment. Thanks…luv those. Nick
Thanks for the replies. The part about supercession of one part over another and the reason why is especially informative and calls to mind the extreme difficulty I once had concerning the removal of the inlet manifold from the head of an early F-series engine. I used every trick in the book over several weeks and it was still a godawful fight to get it off. And I remember how the studs were swollen with rust. I’m wondering now if that head may have had a broken jacket or more due to those obsolete longer studs? Anyway, thanks to you guys I know to pay particular attention to thread depth when replacing these going forward. I have a thumb drive with the service bulletins I bought from Moss/ XK’s a while back. Probably a smart idea to search for that one covering this scenario.
When I fitted the insulating spacers to my C2377 manifold, I ordered a new set of studs supposedly with the longer carb studs from a well-known supplier. But, they were too short! I made up a set of 8 studs myself of the exact length I required. I always like to fit a plain washer under the spring washer, especially when tightening onto aluminium. I’ll be fitting a Cometic gasket as well - anyone had experience of these?
Here’s s few things I’ve learned:
If you order a set of XK120 intake manifold studs, it’s likely you’ll receive the ones with the 8 short carb studs. If you order a set of XK140 studs, you’ll get the 8 longer carb studs, but some of the XK140 studs will be the wrong length for the XK120 manifold. You will need to reuse some of the original XK120 studs.
The cometic gaskets seem to be an excellent upgrade over the original metal gaskets. If the manifold surfaces are good, all you’ll need for a sealant is to spray the cometic gasket with copper coat on both sides. The manifold nuts will need to be re-torqued when using a cometic gasket, probably after 100 miles or so.
I use an anti-seize paste on the threads that screw into the cylinder head. Also, I liberally coat the unthreaded part of the studs with “faucet grease” which is completely waterproof and may help prevent galvanic corrosion between the steel studs and aluminum manifold.
See Service Bulletin 174. “A modified form of cylinder head is now being used, having a reduced depth of tapped holes for studs on the inlet face of the head…it should be noted that the original inlet manifold studs shouLd only be used only on cylinder heads prior to the aboe modifications…etc…” it was noted that the difference was 3/16.
Nick
All good tips, Mike, which I will bear in mind! Glad to hear the Cometic gaskets are well-regarded. I’ve ordered them for the valve covers (getting the closed end type so I can trim to fit at the back ends) and also for the thermostat housing gasket.
Nick, I think all should be OK as long as one doesn’t over-tighten the studs into the head. I do them up little more than finger tight. Some Loctite might be a help, too.