“Brass button” “striker pin” “bump stop” “bridge spacer” on SU carb piston

I lifted this photo from a recent thread as it shows clearly what I am asking about. If anyone objects I will be happy to delete it.
I have always assumed that this circled button was intended to maintain the correct distance from the bridge of the carb body to the piston in order to maintain correct mixture at idle. Can someone confirm if this is correct and, if so, what that distance is supposed to be?

There is a write up on this component on the Burlen website. On later carbs it changed from brass to plastic

Thanks for the reply Phil but I was unable to find the writeup you mention on the site. All I could find is a diagram showing the piston raised above the bridge by the length of the striker pin which I believe to be what the brass button is called. No measurement or discussion. Could you direct me to the discussion you are referencing? The reason I am interested is that the pin is missing on one of my carbs and I would like to replace it with something that provides the correct clearance.


You can search for SU carb “stand off”

With best regards
Philip Dobson

Thanks again Phil. That is helpful. I have also searched under SU piston bump stop and SU piston bridge spacer and have managed to get some information, most of it contradictory as usual. Some claim that they are not necessary (yeah right all manufacturers add in stuff that is not necessary) others that state they are necessary for stable idle, others that they are there to provide an air pathway for starting when vacuum is low, and others that are stating that without them the piston can refuse to rise in the early starting process resulting in difficult starting. There seemed to be no general agreement on what the space should be set at either. This seems to be a relatively unknown topic. Hopefully some of the gurus on this site can provide some insight and perhaps an answer for the next person with a carb piston that is missing one.

From SU Workshop manual (AUC9600, June 1956), page A.3

“The piston, falling either by its own weight or assisted by a light compression spring (8), impacts onto the internal rectangular facing called the “jet bridge” (28), the impact being taken by a small spring-loaded pin projecting about 0.010 in. (.25 mm.) from the piston face.”

There are a lot of varieties of SU pistons over the years. There also are piston lifting pins, which cause some confusion in the phrasing people use for these “brass button” elements on the pistons. These buttons do come spring loaded and may be depressed by fingernail to observe the spring action.

Burlen have added a lot of useful technical articles to their web pages. One may see this spacing above the jet bridge referred to as a “Height of striker pin” on Figure 26 at http://sucarb.co.uk/technical-su-carburetters

The brass button, or whatever varying materials were used, provides a softer downward landing on piston drop. This makes a quieter landing and also contributes to correct air flow at lowest idle settings.

I must admit that in my experience they don’t seem to do much. I have had carbs without them that work fine. Over to the gurus

With best regards
Philip Dobson

My interest in them is because one of the three carbs on my e type is missing the pin. I have never been able to get a really good idle and wondered if that was part of the reason. Wanted to know a bit more about them before attempting to install one from one of my spare pistons.

Stromberg has carb pistons which do not have the brass buttons. Also, SU used plastic buttons on some carbs.

And Geoff, of course you can try a spare piston and see what it does. Use care on fitment to make sure it fits smoothly. And also be aware that there are multiple designs in the small details on SU pistons, just because it fits may not mean you will get the good idle you seek.

The pin helps on cold start and at idle. If it is not there on one carb where it should be, then that may well make smooth idle problematic.

My thoughts exactly. I thought it strange that in any discussions I have ever read about tuning Su’s I have never seen a recommendation to ensure that the distance between the bridge and piston at rest should be checked and adjusted if necessary. Could this be a contributing factor to a lot of idle problems?