O Rings

Hi Guys

This is not a model engineering question but what better site to come on as I’m sure some of you will have used o rings in your builds so I’m rather hoping someone out there can advise me.

I’ve got a touring caravan and need to replace some o ring seals but I’m uncertain what size to use.

In my sketch the two mating parts A and B are made from plastic they are not precision turned metal parts, I assume just injection moldings.

Basically it’s a Thetford Toilet, I’ve indicated the water flow direction when filling the toilet flush water holding tank, in other words the flow is not under pressure simply the direction the water flows when filling up using a watering can.

Having said that should the water eddy when filling I don’t want it to seep past the o rings and leak into the caravan, caravans and water leaks aren’t a good combination.

I’ve had a look on e-bay and the nearest sizes I’ve found are: 64mm I/D x 5mm cross sectional diameter 65mm I/D x 5mm cross sectional diameter 66mm I/D x 5mm cross sectional diameter and they are Viton, can anyone tell me which would be the best size to go for.

Thanks for any advice

Clive

Looks like they are using large section to allow enough compression to overcome any differences in diameter on the moulding. Viton is hard, try nitrile or Epdm which is softer. I normally allow .2mm on a three mm section for compression and they never fail. This is on machined parts though. When pressing together put a little fairy liquid on as a lubricant.

EAPInternational at Heywood will supply minimum orders for a fiver.

Bri

The ID is clearly shown as 65mm? The OD would be 75mm with a 5mm cross section O ring. Buy the right size and they should just fit together. There is 1.5mm allowed for compression - that would be 0.75mm on each side.

Is there a fall on the delivery pipe? If so there is very little liklihood of any water remaining in the delivery pipe to leak anywhere, anyway.

Hello Clive,

​Take the two parts down to your local plumbers merchants. Many have a multi selection box under the counter and they should be able to find sizes that fit. Rebuild the joint using silicone grease, they'll have that as well

Regards

Brian

Edited By Brian Wood on 02/09/2016 08:27:41

Clive<

​Failing that I have a selection box which I am pretty sure could yield what you need. Send me a PM with your address and I'll send a couple down to you and a smear of silicone grease.

Regards

Brian

Hello Clive,

​I have since posting checked my O ring selection and there are none as large as these. So my original suggestion of finding spares at the plumbers merchants remains a valid starting point.

There are internet suppliers www.simplybearings.co.uk being one

Regards

Brian

It's common practice to either chop down (in length) a larger o-ring or simply to make one up from a length of o-ring "cord". Kits are available or you can buy the cord and the adhesive separately.

Hello Clive,

I made a quick check during my lunch half hour and I reckon that the following part numbers will work for your application: 50-333 or 50-332 (both in the James Walker website catalogue). These can be sourced on line (Simply Bearings, et al) or at a local outlet - BSL, etc.

You will need to use lubricant during assembly.

Cheers,

Mark.

Clive,

This maybe a silly question but have you contacted Thetford to see what sizes they use on your toilet or even a Caravan spares stockist.

Martin P

Simply Bearings have 65mm i/d by 75mm o/d nitrile o rings.

**LINK**

They also do other similar sizes.

Or try the power and might of Google. It seems to reveal quite a few suppliers that stock the genuine o-rings;

Hi Guys

What can I say I certainly wasn’t expecting the amount of responses I had to my question. It’s very much appreciated.

Boiler Bri, thanks for the tip on the rubber hardness I only mentioned Viton because that was all I could see on e-bay in the larger sizes, but I will look for something in a softer rubber because of the diameter differences as you have mentioned and thanks for the name of who I can try for them.

Not done it yet, as far as the diameters are concerned I used calipers and measured in at least 3 different places on both parts A and B. I certainly didn’t get 75mm I/d on part A. I understand what you are saying about the diameters but as I said these are just plastic mouldings. Also there isn’t any fall on the feed pipe, I don’t think they are that scientific at Thetford. I’ve included another photo, it shows a baffle across the filler and on occasions I have filled it and could see water up to the level of the baffle but just left it because I know after a few flushes the level will go down, so yes there could be water lying in the pipe on occasions.

Brian, I may well go to my local plumber and give them a try and thanks for taking the time to check if you had anything which would have done the job for me and thanks for the website to try.

Martin, I have to admit on this occasion I haven’t contacted Thetford but I will, just to see how helpful they are, maybe I might be pleasantly surprised. I did contact them once before about a small red lens which fits on the top of the toilet and they didn’t want to know, but I promise I will post their response.

Muzzer, If there is any chance at all I would much prefer to buy them already made up, I’m assuming o- rings are manufactured as a one piece molding and not joined but if all else fails I will certainly bear in mind what you have said.

Mark, Thanks for the two part numbers I guess the 50-333 would be the nearest size to fit a shaft size of Ø 63 and cylinder size of Ø75.

Roger, Thanks for the internet link, I think the 65 i/d x 75 o/d is what I will go for in the Nitrile.

I’ve had one or two issues with my caravan and whilst I’d got part of it stripped out I thought I may as well replace the o rings on the toilet filler. As a matter of fact it only has one o ring on the part in question but has two grooves so I thought two seals may be better than one. I guess at the end of the day I only want to do the repair once, believe me to get the filler off from outside the van is the devil’s own work, they are screwed and bonded to the outside of the van and trying to free the bond without damaging all the aluminium outer skin which is very thin is no walk in the park.

As I said I will let you know what reply I get from Thetford and I would like to thank the moderator of this site for not shouting at me by deleting my question as it’s not exactly about model engineering, but I thought where better to go for some advice on o ring fitting and I certainly wasn’t disappointed.

Thanks again guys for all your help.

Clive

Hi Martin

As promised I said I would get back to you concerning contacting Thetford, I sent two messages to them about the o rings and Surprise, Surprise they didn't even bother to answer.

I guess the order wasn't big enough to warrant a reply, I also contacted my local dealer who are agents for Bailey and the guy there said he gets theirs from Amazon.

So again chaps thanks for your help.

Clive