Radiator Cap Verification

[William2]

Apart from my radiator cap being the correct pressure according to the manual of 10psi, I am not sure whether it is the correct type for operating in conjunction with the overflow bottle. Can anyone clarify this?

[alan.barker]

For me i say it's not the good cap to use with an expansion bottle.
For me i fit a cap with a rubber seal directly under the cap and a rubber seal on the plunger part.
Otherwise if it's like your's just under the cap it's metal to metal which i don't think it seals well enough to be airtight.
Alan

[oldelanman]

It's the correct cap with the recovery valve in the centre but the rubber seal appears to be damaged .. may just be photo though. Make sure the top of the radiator neck is flat and undamaged .. a run over with wet and dry paper on a flat piece of wood will show up and correct any problems. A smear of grease when you fit the cap and it will be fine. Also check that your overflow pipe seals well to the stub on the rad neck .. you may need a clip on it.

[Chancer]

As per Alan Barker.

I dont know why they make the caps with the inner flap and seal but without a proper seal on the outer, I have had to add a rubber seal and release the locating ears a little to compensate for the thickness to get them to work properly.

[69S4]

That's exactly the same cap as I have. I thought it ought to have an upper rubber seal but it's the same as what Matty's pulled new off the shelf when I asked about them some time ago. I'm still not convinced it seals correctly but maybe a smear of grease might do it.

[Craven]

Some S3?s don?t have a recoup system just a total loss overflow, I think the cap you show is for these cars. For the recoup system to work there must be a good vacuum seal at the outer face.

[MarkDa]

My old cap had a metal seal and after rebuild I replaced it with new cap from Matty which has a rubber seal.
I reckon that it does seal because I very nearly needed a pair of stillsons to get it on!
Good tip about getting a good finish on the rad spout.
I'd still worry that boiling water might find it's way through the grease if that was all holding the seal.

I agree about the rubber on the OP cap looking less than 100%
So if buying new get a rubber upper type.

[Robbie693]

Needs another seal.

If you think about how it works, you need a seal on the plunger part to seal on the inner neck flange, then a seal on the outer neck/collar to seal when the plunger opens to allow flow down the overflow tube rather than all over the top of the rad.

Some look like the pic in Craven's post - just a big rubber ring seal, others have a folded rubber gasket that envelopes the metal plate at the top.

Either way you need two seals, whenever I have asked for a 10 psi Plus 2 rad cap at Matty's this is what I have received

[alan.barker]

Just another little thing i have done on my Sprint, the tube to the expansion bottle goes to the bottom of the bottle. I have cut the end at 45? and put a hose clip where connected to the Rad.
I never need to top up the Rad only the expansion bottle to half full.
Alan

[William2]

Thanks very much for all your comments. I will try and get a cap with an upper rubber seal. Should the rad be topped up to the brim when cold?

[MarkDa]

Yes.

[69S4]

When I topped mine up a few days ago the difference between full with the car level and full with the front up on ramps and air in the top hose squeezed out was about 200ml.

[Chancer]

If you get the correct cap and it seals correctly, if you have sufficient in the overflow bottle then even if you dont manage to fill it 100% it will sort itself out by drawing in coolant from the overflow bottle on the next or nexy few cool down cycles.

I used to keep an eye on the overflow bottle level to tell me if I had correctly filled, to show if coolant was being lost ( bottle empty) and also that the siphon back valve was working correctly and had no air leaks, the bottle level shoud raise after a hot run and be lower after cooling completely.

[JJDraper]

Don't forget that the pipe from the filler neck to the bottle must also be air tight to allow the vacuum to pull coolant back. As the pipes age, they get less compliant and start leaking air, making the whole system stop working correctly.

Jeremy

[pharriso]

I've just had the same issue with a radiator cap purchased from Kelvedon. Here's a photo of the original cap & Kelvedon cap:

Old cap on left, new on right

I have a `recouperation system, with the new cap fluid was being pushed to the overflow bottle, but not sucked back since there's not a great seal on the top face.Only reason I changed the cap was due to a little corrosion on the top face.... Joke is the original cap was made in Mexico & the new "Made in England"!

The old cap has gone back on after application of a Scotchbrite pad....