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PTFE (Teflon) O rings for main jet seal - too tight?

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rainon Ray F
Ipswich, UK   GBR
I'm currently rebuilding my H4 carbs and obtained the BS-010PT PTFE O-rings to use instead of the cork seals. The actual O-rings are from Polymax in the UK which have been covered in an earlier post as being used successfully and equivalent to the ones Barney mentions. However when trying one of the O-rings on the new jet it is a very tight fit and I can see that fitting four of the critters is going to stop the choke returning. Here's a very short video so you can see what I mean



Maybe when they are sat in fuel it all changes, or maybe they just need to be bigger... Could someone who has used these O-rings comment on their experience?

Many thanks

Ray

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ghnl Avatar
ghnl Silver Member Eric Russell
Mebane, NC, USA   USA
1961 MG MGA "Calvin"
What is the ID of those O-rings? IIRC it should be 1/4".

FWIW, I used one Teflon and one Vitron O-ring to replace each cork seal.



Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100



Edited 1 time(s). Last edit at 2019-02-16 03:26 PM by ghnl.

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rainon Ray F
Ipswich, UK   GBR
The Polymax O-rings I used match Barney's specification... here is the supplier's data: Polymax O-ring data

I noticed that other people have reported that they found them a tight fit and had to revert back to the standard cork seals. I wanted to make sure that I'm not missing a trick before going down the cork route...

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59mgaguy Avatar
59mgaguy John Terschak
Wakeman, OH, USA   USA
1930 Ford Model A "Jenny"
1959 MG 14/28 "Jessie"
1974 MG MGB "Oooops"
Wait till you have to center the jets. . Make sure you remove any burrs on the jet.

Personally I don't care much for them.

John

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rainon Ray F
Ipswich, UK   GBR
This morning I retrieved my vernier from the workshop and checked the parts, so now I have some more data as measured:

Diameters:
O-ring: 0.239 inch ID

Main jet: 0.247 inch OD

I don't believe my jet is over-size (both new and old jets are identical diameter) and are standard for the H4.
The O-ring matches the specification and therefore is correct as used by others.

So how can it ever work, the O-ring is 8 thou under?

Maybe the PTFE material used by the successful fitters is softer and slips in easily despite the 8 thou smaller diameter...?

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barneymg Avatar
barneymg Barney Gaylord
(Somewhere in USA), Pick one (or more), USA   USA
1958 MG MGA "MGA With An Attitude"
Any O-ring must be a slight interference fit to seal properly. PTFE is quite soft and will cold flow a bit to conform to mating surfaces. Once assemble and sitting for a bit it will form a slight flat or cylindrical surface in the ID. The conical shape compression washers will also assist in helping the O-rings to constrict on the jet to seal well, not to leak.

Coefficient of friction is quite low between PTFE and brass, so the jet will move well in assembly. In fact it should have less friction or stick than the original cork seals or any other type of material for O-rings. The return spring on the choke arm should easily pull the jet back up after it has been "choked". I have been using Teflon O-rings here for decades and never had any problem with then sticking or leaking. I buy them by the 25-pack and put them in every SU carb I rebuild, dozens of them by now. Everyone seems to be happy with the results.

It is very important to pay attention to jet centering, and also centering of the upper and lower jet bearings. If one of the jet bearings is misaligned from the other the jet will stick (even if the jet itself is centered on the metering needle). My friendly tip is to put oil on the jet and the jet bearings during assembly and centering. The oil film will help to center the jet bearings around the jet, so after assembly the jet bearings should be well enough aligned to allow free sliding of the jet.

Always assemble choke arms and springs, and exercise the choke mechanism to assure free motion and easy return of the jets before installing the carbs. If the jet sticks it is most likely that the jet bearings are slightly misaligned, so you need to loosen the gland nut and re-center the jets and jet bearing again. Oil is your friend.



Barney Gaylord - 1958 MGA with an attitude - http://MGAguru.com - barneymg@mgaguru.com
(Please email me direct, do not leave a PM on the public server).

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rainon Ray F
Ipswich, UK   GBR
I've been out of action for a few days but now getting back to this O-ring issue...

Thanks for the input Barney, interesting that you find them easier to slide than the cork.

I have had a conversation with Paddy Reardon (who I found on the forum) who has used the same Polymax O-rings. He confirmed that he had the same problem and it required polishing the jet to a mirror finish to get the rings to move freely. I've just had a magnified look at my new jets and they do have the machining marks, much more so than the old jets. A few minutes polishing with "T Cut" has made a big difference and I think this could be the answer. I've not tried the full assembly with 4 O-rings yet but I'm hopeful. I'm going the leave a ring on the polished jet for a couple of days to see if Barney's observed cold flow makes a difference.

One thought that occurred to me is that it should seal with just one O-ring at each end however others have noted that they get leaks like that, and it is solved by using two each end. My thought is that adding the extra O-rings will increase the spring pressure helping overcome the friction of the ring on the jet and could be fixing the leak simply by seating the outer rings properly, especially after using the choke. Certainly the way the rings slid on my jet would have made re-seating unreliable after using the choke.

I'll update this with my final results.

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bisslre Avatar
bisslre Ron Bissland
Kenosha, WI, USA   USA
1956 MG MGA 1500 "The Little Car"
1979 MG MGB
2014 Chevrolet Silverado
X2 what Eric did. I used two Teflon and they leaked a little..

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rainon Ray F
Ipswich, UK   GBR
Ron and Eric...

Thanks for the input, its interesting that you used two types together but it clearly works for you. Two questions if I may:

1. Which ring slides easier Teflon or Viton ?
2. Which way round do you use them ie Teflon against the cup washer or Viton ?

I think I might buy a set of the Viton rings next week as an experiment.

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ghnl Avatar
ghnl Silver Member Eric Russell
Mebane, NC, USA   USA
1961 MG MGA "Calvin"
It has been a couple of years and my remembering ain't what is used to be... As I recall, two Teflon O-rings would allow the jet to move easily but fuel would drip slightly. Two Viton O-rings were too tight and fuel didn't drip. One of each in pairs (so two of each per carb) were just right. Now as to which went which way - I don't recall. I think I put the Viton outermost and the Teflon innermost.

I don't have a photo of the H4 carbs but here's a set of HS6 carbs. I mount an intake manifold to a piece of plywood clamped in a small vise so I can work on them at my work bench. Makes it easy to get the throttles and chokes levers adjusted. And check for leaks...



Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100


Attachments:
HS6 carbs 01.jpg    19.6 KB
HS6 carbs 01.jpg

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rainon Ray F
Ipswich, UK   GBR
Thanks for that Eric, it gives me more food for thought smiling smiley

I like your rig for working on the carbs. I'll do exactly the same to make sure there are no drips before I put them back on the engine. Its not a lot of fun getting the carbs on and off with the engine in the car!

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rainon Ray F
Ipswich, UK   GBR
I wanted to conclude my post on the issue of excessively tight PTFE jet seals with a summary of how I got on and what I found out. I had not worked with the SU "H" type carburettor before so its been I learning exercise and I will start off with a description of how the jet seals work - something that I really only understood properly after playing with the disassembled parts. I only do this to be of use to newcomers like me, so those of you experienced in such matters please don't take offence at me stating the obvious.

I have taken the diagram from the service manual and to clarify it I have highlighted the parts in question around the jet and seals.


Fuel flows from the float chamber into the gap between the Jet bearing top half and bottom half and this fills the region occupied by the Jet gland spring (red). It then flows into the jet (yellow) through the central drillings so it can be drawn up past the needle into the body of the carb. The function of the Jet gland spring (red) is to press the Gland washers (blue) firmly against the top and bottom Jet bearings and stop any fuel leaking up into the body of the carb bypassing the needle control, and to stop it leaking down and dribbling out of the bottom. Maintaining the position of the Gland washers firmly at top and bottom is essential as the jet is moved up and down when operating the choke. For example setting the choke off moves the jet up and if the bottom Gland washer moves up with it due to friction it will lose the seal at the bottom and fuel dribble will follow.

In the original design the Gland washers are cork but this has the disadvantage of drying out and shrinking when the car is only used intermittently. Some clever people identified PTFE O-rings as a better substitute, more suited to "classic car use" and more robust generally. The O-ring identified is the BS010 standard, and although it could ideally be a couple of thou larger diameter, it has worked well for many people though some have found it too tight on the jet and have returned to using cork - hopefully this post will help those having problems.

I found the BS010 O-rings were very tight on the jet and did not move smoothly (see video in first post). The clues to solving this came from the people posting above and from Paddy Reardon. This is my recipe...

1. Polish the jets

My jets came from Burlen in the UK and the machined finish may be different from jets obtained in the USA and elsewhere. Magnified, the machining leaves ridges all around the jet and this increases friction considerably. 10 minutes polishing by hand gave a much smoother finish. Paddy polished his in a lathe to a mirror finish but I think 10 minutes with "T-Cut" or similar paint cutting compound appears to do the trick - see comparison:


2. Allow the O-rings to creep

As Barney mentioned, PTFE will creep at room temperatures (cold flow). To speed things up I put my O-rings on the jet and baked them in the oven at 150 C for an hour. I doesn't seem to matter if you quench them afterwards or let them cool naturally. They remain a snug fit but it reduces the force on the jet a little. Note if you take the rings off the jet afterwards they will shrink slightly but soon go back when you put them back on the jet. PTFE is not very elastic but there is some.

3. Re-polish the jet

My cooking of the O-rings does dull the jet slightly so I gave it a quick re-polish and it was good to go.

RESULT

Here's a video fragment showing the top and bottom jet bearings in approximately the same position they are in the carburettor - and you can see how much the Jet gland spring will be compressed when installed. As long as the O-rings move freely when the Jet gland spring is compressed significantly less than its normal compression, then the O-ring seal should work. The video illustrates it should be good.


I have tested the carbs on the bench, mounting them at the correct angle with the correct level of fuel in the chamber. No sign of a leak in 24 hours and good free movement of the jet when applying the choke.

I have used the recommended 2 O-rings at each end as others have had leaks when only using one. It does seem redundant though as the seal is only made against the end of the Jet bearing not the inside diameter (the diagram from the workshop manual shows the cork Gland washer against the inside diameter of the Jet bearing but the O-ring does not touch it). I suspect the second O-ring only serves to increase spring pressure on the O-ring to improve the seal. That theory could be tested by replacing the second PTFE O-ring with a looser fitting ring (eg a second brass Gland washer) which would provide the increased spring pressure without the extra friction of the O-ring. I'll try this if I have further problems.

So I'm happy it will work when I put them back on the car. The real test will be if they are still drip free at the end of the year, or will the jet tarnish and cause the O-rings to get sticky? Other peoples experience makes me hopeful.

Ray

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ghnl Avatar
ghnl Silver Member Eric Russell
Mebane, NC, USA   USA
1961 MG MGA "Calvin"
Excellent write up and graphic!

One tidbit to add is that the jet bearing halves are independent of each other. There is a little bit of wiggle room so that on assembly the two halfs can be aligned and then secured in that position by tightening the jet securing screw (not the jet adjusting nut).

The assembly procedure is to loosely assemble the jet bearing parts but leaving out the adjusting nut spring. Some carb kits include a jet centering pin. This (temporarily) replaces the needle in the suction piston to hold the upper jet bearing half centered relative to the dashpot. Alternatively, raise the jet fully so the needle will center the upper jet half. Wiggle the jet inside the jet bearing halves to get them to align then tighten the securing nut. Check that the jet moves easily/smoothly and that the suction piston falls to the bridge with a distinct 'tink'. If it does neither, loosen the securing screw, wiggle the jet again and gently tighten the securing screw trying to clamp the assembly together without allowing anything to move.



Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

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Rob Z Avatar
Rob Z Silver Member Rob Zucca
Camarillo, CA, USA   USA
1960 MG MGA
X2 on an excellent write up!



"Time flies like an arrow......Fruit flies like a banana"

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rainon Ray F
Ipswich, UK   GBR
POSTSCRIPT

A few of things I should have mentioned in my write up on the jet seals...

1. If I had realised earlier that Burlen produce an official SU solution to leaky seals ("Superdry" kit) I would probably have used that instead - Burlen Superdry web page

2. John mentioned the importance of removing burrs from the centre of the jet - I should have included this in my write up as without that you will damage the O-rings putting them on the lower half of the jet

3. I also took time to clean and polish the sealing surface inside of the top and botton jet bearings. The bottom one in particular needed work with a wooden cocktail stick and metal polish. This is not to get a mirror finish but just to ensure its a good sealing surface and the concentric machining marks remain

4. Its obvious really but thorough cleaning after all that polishing is essential

5. If you need to get more slack on the O-rings you could go for more heat treatment. I found Dupont's PTFE Handbook download with a Google search and used it's data to decide my heat treatment of the O-rings.

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