Stromberg carburettor assembly conundrum

[urr333]

I have refurbished and am now re-assembling my stromberg carburettors and would be grateful for views on an issue which I have not been able to resolve.

At the connexion between the manifold and the carburettor there is an o-ring with a section diameter of 0.138". This sits in a recess in the manifold face with a depth of 0.098". Simple arithmetic suggests the the o-ring will create a gap of 0.040" before any compression takes place. However the workshop manual is quite insistant that a gap of 0.070" should be maintained between the two faces.

Does anyone have any experience of this situation and if so what is the answer please.

[oldelanman]

At the connexion between the manifold and the carburettor there is an o-ring with a section diameter of 0.138". This sits in a recess in the manifold face with a depth of 0.098". Simple arithmetic suggests the the o-ring will create a gap of 0.040" before any compression takes place. However the workshop manual is quite insistant that a gap of 0.070" should be maintained between the two faces.

If I recall correctly the adaptor plate on the carburettor is not flat, the part which enters and compresses the "O" ring is raised slightly so the gap that the "O"ring sits in is less than the gap between the flanges which is what you measure. Are you saying that when you assemble yours the gap between the flanges is less than 0.070" before you tighten the nuts? If that's the case maybe you have the wrong "O" rings.

[69S4]

I must admit I've always taken the .070" setting with a little pinch of salt and tightened the nuts until the free movement on the o ring "feels" about right. Very "new age" I know but it's worked for me. Out of curiosity I've just been and measured the gap and it's .065" on the front carb and just under .060" on the rear one (I've noticed over the years that the rear gap has been consistently slightly smaller - maybe something to do with machining or casting differences). Loosening them off to .070" makes the whole carb block too slack and I've had idle problems with air leaks (usually the rear carb). Using a 5mm o ring (slightly thinner than the correct one) has also caused air leaks.

As Roger said, the adapter plate had a protuberance that serves to clamp the o ring into the manifold recess and form an airtight but slightly flexible seal. Too much movement and the seal will fail allowing air to enter. My suggestion would be to measure the gap with no compression on the joint (just push the carb block over the studs and hold it there by hand) and then decide how much compression the o ring should have- maybe .025 -030". Subtract that from the no-compression figure and set that as the final gap.

[urr333]

Thank you both for your comments, you are correct, there is a slight shoulder raising the machined face on the carburettor, I think the height is about 0.020" and this would raise the potential gap to 0.060" before compression. I have looked into a larger section o-ring but the next size is 0.210" which I think is too much bulk to fit in the available space and would be pinched by the inner faces.
As you suggest I have tried to measure the gap just as the Thackery washers are pinched and have established that this is about 0.060" which is reassuring as it bears out my earlier calculation, however it does rule out any chance of a 0.070" gap!
The next issue I think will be to have enough length on the mounting studs to enable the nyloc nuts to become locked; at 0.060" this will not happen. I assume nyloc is the appropriate nut for this task?
Professional advice I have been given is not to be too concerned with the gap as stated in the manual but to ensure an even gap but NOT to allow the faces to meet.

Thanks again
Ted

[oldelanman]

The next issue I think will be to have enough length on the mounting studs to enable the nyloc nuts to become locked; at 0.060" this will not happen. I assume nyloc is the appropriate nut for this task?

Hi Ted,
Mine just has plain nuts - I believe they are original, certainly I've never changed them. I always assumed that the Thackery washer stops the nut coming undone like a normal spring washer. It's a long time since the car was on the road but I don't remember the nuts ever coming loose when it was in daily use. No reason why you can't use nylocs if you want to - just thin them down a bit if the stud is too short.
Regards,

[urr333]

Stuart, I have just spotted that you said you had problems with a 5mm o ring which was "slightly thinner than the correct one" and realised that 5mm is actually greater than the 0.139" (3.53mm) which I have been supplied with. Can you tell me what the standard is, I have looked through the W/S manual but cannot find this.

Regards
Ted

[69S4]

Hi Ted. Oops That 5mm was a typo. I should have pressed the 3 key instead but didn't spot it, even when I read it back before pressing submit. The original ones do harden after a while in the car and when I was rebuilding the carbs two or three years ago I thought that the 3mm ones I had would be better than reusing the hardened originals. I couldn't get an even idle even after an hour or so (on and off) of trying. Eventually I concluded it must be air leaks from the o rings and ordered a couple of original ones from Paul Matty's. The difference was huge and immediately noticeable.

Looking at my Thackery washers, some are more compressed than others in order to get an even gap and I've always prioritised the gap over keeping the washers evenly compressed. I've got nyloc nuts holding it all together and with the gaps I gave in the earlier post the studs are just about flush with the end of the locking part of the nut - some a little in, some a little out, but all go through the bulk of the nylon part. Of course nuts vary so yours may be thicker or thinner than mine but it's not a huge problem to change them.

Going back to the o ring, it seats in a recess in the manifold but the shoulder on the adapter is slightly smaller so when it is mounted onto the o ring it tends to push through the middle, stretching it outwards and compressing it into the recess. I've always used a slight amount of lubricant on the o ring to facilitate this. It's not a straightforward fore and aft compression until the rear face is in contact with it. For this to work as designed it needs to be a goldilocks o ring - not too thin, not too fat, not too soft etc

[urr333]

Stuart, thanks for your helpful response. I will follow suit and go for an even gap rather than a specified size. I will have another go this afternoon and fingers crossed might even get it so start.

Thanks again to both
Ted