Rad cap technology and other cap malarkey

Rad cap technology and other cap malarkey

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Discussion

jmorgan

Original Poster:

36,010 posts

283 months

Friday 18th April 2014
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Swan neck decided to get all corroded on me, water finding its way past the rubber seal due to corrosion on the metal. Thus bypassing the rubber seal and weeping, the seal is a ring and not a disk. No problem there, have a few spares.

But. Decided to look at the rad cap and a few questions are raised.

When closing the rad cap, I expect the rubber seal to seal the lower lip before it all clamps down. The top part of the cap has a metal faced seal. How far should the lower seal be engaged before you close the cap completely? The reach on a few I have are different. The spring is there for the rate but how much must it be compressed to do what it says on the tin?

Then, I have caps with two rubber seals, and caps with one rubber seal for the inner and a metal disk for the upper (bit directly under the lid). I am guessing the two rubber seal hobby is incorrect and will stop syphoning back to the main tank from the catch tank.

adam quantrill

11,535 posts

241 months

Friday 18th April 2014
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Yes when you put the cap on loose it should touch the seal at the bottom, and you might feel the springiness if you press down, as you twist it down the spring gets compressed.

It's the spring rate, how much it's compressed (and the hole diameter) that determine the PSI.

jmorgan

Original Poster:

36,010 posts

283 months

Friday 18th April 2014
quotequote all
Yeah, I was thinking how much it has to be compressed first. Mine is only compressing a short bit before it locks. I have a 13lbs that is longer and there is a noticeable press down before turning.

adam quantrill

11,535 posts

241 months

Friday 18th April 2014
quotequote all
You might find the spring is much softer though.

Sometimes you can get it to lock down a bit harder by bending the tabs up a little but not so much that it can't clear the lugs.

KKson

3,395 posts

124 months

Saturday 19th April 2014
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Fitted one of these yesterday - very much like the positive lock down on it.

http://www.ebay.co.uk/itm/Fahler-STAINLESS-STEEL-S...

MrPicky

1,233 posts

266 months

Saturday 19th April 2014
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I may be confused but it seems that you are referring to the cap on the Swan neck, forgive me if I have misunderstood.

You should not have a spring cap on the Swan neck, this should be a plain flat cap that only seals the top.

The spring cap should be on the expansion tank (Plastic fitted on the inner-wing). This should be a standard radiator cap rated at 13 psi.

Russ

jmorgan

Original Poster:

36,010 posts

283 months

Saturday 19th April 2014
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Adam, I was using the 13 as "try the spring" not in actual action. Guess what I am getting at is the reach, the depth of the neck. It is an after market alloy tank. I should dig out the old plastic one and check compare the two and see what the reach is. The existing one is longer but there is a rough edge to the rubber seal and it locks down terrible, not a smooth turn. I seem to have a mix of good ones and cheap ones. The cheap ones are non to good at a smooth lock down.

Keith, been mulling one of them but not quite what I am asking though still on my list to get when I know what to get. The closing action is OK (with the right cap) and as Adam says, you can muller the tabs. I did that on the swan neck one. Just the press down for want of a better term before you turn.

As I type this, Russ has replied. Yeah, I know the swan neck is a blanking cap, no spring smile . Seems I had a cheap one, the water had corroded its way out which was concerning until I took the rubber ring off and saw that it had corroded where there was glue, must have interacted with the water and anti freeze. So I fished out another that has a stainless disk with a rubber ring (no pressure relief spring). Then got to thinking on the main header tank. And now you 13 not 15? Big difference in running temps?

If I can get this confidently sorted the plastic one will be on the spares list. For one thing and another I did very few miles between MOT the last few years so the new alloy tank has not been made confident so to speak. It has the volume of the old tank and I have replaced the jam jar catch tank for a shiny alloy one wink