Carb dampers different sizes

Ok, so I am now almost ready to build up the motor top end- here is my dilemma:

The two SU carbs (ostensibly original) have two different sized dampers in the tops. (lengths) One of them was shaped in a z pattern when removed lol.

Does anyone have:

  1. The correct length measurement
  2. Two I can acquire

Preferably US, with brass tops. Ebay is dry except for EU and I would prefer from US.



You’re asking about this, right?

Moss Motors has them. Part 6961/1 at $54 each.

Do you have the tall dashpots?

Yes, regular size dashpots

Ok same as me.
The damper is about 2-3/4" long measured under the hex head but including the threads.

There is a difference between dampers other than length. Some have a tiny vent hole and others do not. The rib on the top of vacuum chamber denotes which is correct.

1 Like

Thank you- how does the rib denote?

The rib contains a tiny drilled hole, a vent passage from the piston chamber to the top of the stem. Mine has the drilled passage and my hex top dampers do not have a hole in them.

1 Like

I suspect, but have not yet confirmed, that the reason SU changed from the tall stem to the shorter stem was so that they could add the angled rib and drill a hole in it. They couldn’t drill a hole at that angle in the tall stem.
The hole was some sort of performance improvement.

1 Like

UPDATE: I ordered new dampers from Moss, but they do not have vent holes (neither do the domes) Can I just drill them out, and what size is the hole?

Are you sure you don’t have a hole in the dome going down through the angled rib gusset?

Thank you for that- I will check when I get home

Just saw/read this thread. By stem I assume you refer to the neck of the vacuum chamber (Bell), Rob? And I assume you’re talking about tall and shorter among models not including the E-type? I raise this because the neck is virtually gone on the E. That’s due to bonnet clearance concerns I think, because the contemporaneous MarkX/420G had similar carburation but with longer necks.

The dampers for E-type HD8s are shorter than those for others such as the MarkX HD8. Both types used vented bells. Moss sells mostly to E-type owners so I would be careful to make sure that the damper length is appropriate. IMHO.

This is about XK120 with H6 carbs, nothing to do with E-Type or Mark X.
Early 120 had a very tall neck, maybe something like 2 inches. Later 120 had a neck about 1 inch tall.

There were actually THREE different suction chambers used with XK120 H6 carburetters - the initial 'tall neck" style as used 1949 to circa March 1950. Then the first ‘short neck’ style - without any added side-rib/air balance hole were used circa April 1950 to circa July 1951. See attached pic…

Then the third/final style with the same ‘short neck’ but now with the added side-rib/air balance hole, used from circa August 1951 to last XK120, and all XK140.

II show all dates as circa, as they relate to the stamped in date on the actual H6 carburetter, and not actually the date-of-manufacture of the XK120 so fitted)

So I am not sure that the added air-balance hole/cast external rib is directly related to the tall neck to short neck design change ???

Quite right, and I should have known you would have a more complete history than I on these changes.
I have not worked out the reason why SU shortened the neck before introducing the vent hole.
Looking back in this thread, both dsnyder and I show the third type with the rib.
His question was should he put a hole in the damper cap, and I think the answer should be no.

As per usual, you guys have the right answer- there is a hole in the neck…

So strange that the ones on the car had holes, but clearly did not need them…

So you need the non drilled dampers. If you have a hole in the top and a hole in the neck of the dashpot ,the piston will not rise.

After reading this thread I checked the suction chambers of our May 1950 Model - no side holes inside the neck, which is correct.

But there are non vented damper caps fitted…

A short look at Burlens website showed that there are various vented hexagonal caps available - all with part number AUC 81…, but which of them would be the right one?

Thank you very much for your help :+1:

Best Regards Lukas

Lukas, in that case drill a 1/16 hole through the top damper.
If you have no vent hole in the dashpot, and no hole in the damper then the piston is fighting to rise as it is compressing the air which cannot escape.