Radiator Cap not fitting?

[ChrisC]

Chaps - either I am going mad (dont answer that) or perhaps I have a weird header tank... I have gone through a series of pressure caps because they dont seem to fit right.

Perhaps its me but shouldn't the 'radiator' cap need a little downward pressure to engage it? i.e. there should be a little bit of compression of the spring against the base of the header tank neck otherwise how does it seal?

The car is not running hot (its a tad below 70) so I guess the cap must be doing something but it doesn't 'feel' right... maybe it is compressing the spring when it is 'locked' down.

Thoughts please?

[Heuer]

For safety reasons it is a two stage locking system. Press down and twist to engage (light spring pressure); twist further to lock down (high spring pressure). It means if you want to check the rad on a hot engine you can turn the cap to the first detent and it will allow gradual release of pressure.

[ChrisC]

Ah well this one seems to be working - just taken the car for a 50 mile run at a mix of speeds (up to 70) and the temperature stayed constant so I guess it is okay.

Thanks Dave.

[PeterCrespin]

There should be some spring pressure but there are broadly-speaking two depths of neck and two depths of cap spring. If you get them mixed you can have a cap that needs very heavy compression or that won't fit at all, or a cap that compresses hardly at all and blows off to easily. Your parts are probably OK but pattern parts can be borderline.

The spring determines the release pressure - e.g. 3lbs, 7lbs, 12lbs etc. The cap turning action initiates the second seal which is the rubber gasket under the cap itself. Completely different thing. Nothing to do with safety just keeping the cooling system sealed after the valve releases. That ensures the coolant passes under control into a catch tank (recovery tank in some designs) instead of just spilling out over the radiator or header tank. For S2 cars the radiator cap has no spring at all, just the cap seal, and the bulkhead expansion tank has the pressure relief cap. Most pressure caps have a return valve too.

[PeterCrespin]

Hmm, I see what David meant about the detent notch which has to be passed to hook the cap tangs under the neck rim before starting the tightening. That is a 'safety' thing of sorts I suppose, in relation to people foolishly taking a cap from a fully-hot system. I was just thinking about the mechanics of the system and its seals.

Pete

[ChrisC]

interesting I dont remember seeing a rubber seal under the cap... i will take a look when it cools down