SU HS6 Jet Supply Line Seal

Originally published:  6 - 2013-R. Kwas  updated continuously (mostly when the spirit moves me!)  [Comments added]


The jet of the HS6 version of the carbs, with separate side-mounted floatbowls is supplied fuel by way of a flexible supply line connected at the bottom of the fuelbowl, (so the information presented here does not apply to Horizontal Integral Float-HIF type SUs).  The jet supply line needs to be flexible as it is required to accommodate a bit of movement of the jet down and up for choking *.  The fuelbowl end of the supply line is plumbed in by way of a sealed fitting.  These notes are intended to cover this special fitting, relying of what my expert references have named a ”Square Compression Seal”. 

FIGURE 1.  This excerpt of the exploded assembly view doesn’t give much detail or insight to the design. 
I added the labels.  Source:  GCP Site


The tiny sealing ring (31) between jet supply tube (28) and bowl housing is a square cross-section (Important!) fuel compatible seal. 


Analysis:  Important factors to understand in this kind of sealing strategy are that the threaded nut (29) compresses the seal by way of washer (30) into a pocket of limited volume, matching that of the seal in the fuel bowl housing.  As the nut is further tightened, the seal, having no place to go in the pocket, displaces to the inside and loads against the fuel line, taking up any gap or clearance present.  The washer interfaces between seal and turning nut, and a metal insert in the fuel line assures that the plastic line stands up to this compression without collapsing…an effective seal is realized. 

The sealing force generated in this manner is not very much at all, but sealing is adequate…after all, there is only the pressure of a column of an inch or so of fuel in the bowl which needs to be sealed against. 

If you are taking your SU equipped car vintage racing, I expect this would be an area one would consider making less susceptible to self-disassembly from vibration…


There are two items of note in this seal design strategy:

1.  Since the sealing is based on compressing the seal into a specific volume, it can be see that a seal of the correct cross-sectional shape must be used when servicing!  A simple O-Ring of circular cross-sectional shape will not do here (see also below!) because a circular cross-section has much less volume than a square cross-section! 

FIGURE 2.  Seal cross-sectional detail


2.  There is no positive retention of the fuel-line beyond the light circumferential sealing force applied by the seal and resulting friction, so a modest pull on the fuel line will actually be enough to remove it from its seated position, resulting in an instant release of the fuel-bowl contents onto heat-shield and possibly electrical components below!  Fortunately, the jet supply tube is fairly stiff and supported in position by the jet itself a mere two or three inches away. 

FIGURE 3.  Brass Nut bears on washer which transfers the compression forces but spares the seal from being subjected to the shear forces of the turning nut (one job at a time please!). 


Weaknesses, Failure Modes and Notes:  I don’t think being able to pull the Jet Supply tube out of the seal and fitting with minimal force (either intentional or inadvertent), can be called a failure mode, but it is definitely a design weakness and vulnerability which one should acknowledge and be acutely aware of when working in the area! 

Hardening with age of the seal, or shrinkage due to being dry can also lead to decrease in retention force and loss of the sealing function. 

Replacement seals are included with rebuild kits.  Inspecting and cleaning out the housing pocket with a cotton swab, in preparation for receiving a new seal is recommended.  Remains of a disintegrated old seal, or aluminum oxidation products in the pocket will interfere with the new seal.  Trial fit new seal on the end of jet supply…too loose, and poor retention and sealing will result. 

Update 2021:  I have recently seen where a simple brass ferrule style seal replaces the compressible rubber style seal... This would seem be a more mechanically positive situation.  This also addresses the deteriorating rubber due to ethanol content in fuel. 


A Roadside Experience:  As infrequently as this area needs attention, this seal is not typically a spare part one carries along…but (in the writer’s case) a length of fuel-line and an Exacto blade are…so to replace a dried up, leaking seal, which decided to finally fail while I was out and about (I may have helped it along by brushing up against the jet supply tube with more force than I should have, or heaven forbid, pulling on it!), I very carefully cut my best approximation of a seal from (the side of a large ID fuel line from the on-board spares), using the old hardened seal (or crumbly pieces of it) as a size guide.  I cut the replacement seal with the requisite square cross section, slightly smaller in ID, and bigger in OD and height, figuring this would result in some extra preload which would help with sealing and couldn’t hurt.  It worked splendidly where the emergency repair was made, and continues to function satisfactorily to this day…no runs no drips, no errors…he who is helpless has only himself to blame!

FIGURE 4.  My emergency jet tube seal, manually cut from fuel line. 


Measurement of seal:  Placeholder   

[Haven't made a measurement of one yet, but John Twist says it's a number 8 Viton O'Ring!  Reference Link to his video.]



SW-EM SU carburetors

Link to SU jet supply tube install:  

John Twist on same subject: 
[As excellent as the John Twist / University Motors videos are, I can't believe he is using a standard circular cross section O'Ring (at time 0:50  "...number 8 Viton O'Ring") instead of the special square cross-section O'Ring from the SU kit...well OK, he did say of fuel compatible Viton, and I suppose if you wang the nut down enough on it, it will also deform into the seal pocket and still seal...ideally it should be a seal of a square cross section though!]

Thread:  Help needed rebuilding HS6 carbs:


My answer to a FB question on this design (7 Aug 2019):

"...dimensions (therefore volume) of pocket for seal in the casting is very specific, and just allows for seal (Important: ...with square cross-section!, not round!!) to fit in there, and be constricted by walls as soon as it is compressed by fitting being threaded in (and washer 21 keeps rotational force from tearing up seal)...since Seal has no place to go, it loads up against Fuel Tube affecting a seal (with clearly not a lot of force...that's why it's not particularly difficult to pull the tube out even after it is in place and Nut has been tightened to compress Seal. It's an OK design, but has little margin for a minor brushing up against it! just have to be careful not to!
Beware! If a normal round cross-section O-Ring is used, it does not make the sealing/retention force necessary for design to work as well!, so it's quite important to use a Seal with square cross-section! "


*  The term Choke and choking comes from carburetors which actually have an additional flap, upstream and separate from the throttle, which is closed either manually with a control knob (like on our Volvos), or automatically as in many later carbureted cars, “choking” off airflow during cold cranking.  This results in a higher vacuum at the venturi and sucking more fuel, thereby enriching the mixture…just what is necessary to help start a cold engine.  SUs don’t have such a secondary flap to “choke” off the air to enrich…we SUs drivers have a different method for enriching (see:  LINK to Check your Choke  ), but the term Choke was retained!


This article is Copyright © 2014.  The terms Volvo, SU, and Exacto are used for reference only.  I have no affiliation with any of these companies other than to present my experience, and highly opinionated results of the use and care of their products here.  The information presented is from my own experience, and can be used or not, or ridiculed and laughed at, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of, and responsible for your own knuckles (like that needs to be stated!).  

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist…so the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and maybe wise-a** comment. 


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