Meeze Posted June 27, 2020 Report Share Posted June 27, 2020 Dear all I have a MKII with an MKIII engine. The radiator had a 7lb radiator cap and having read Andy Cook's article 'keep your cool', I decided to purchase a 13lb from the Club. The lugs on the new cap are longer and the new cap does not fit. Is it just a case of levering the lugs so they fit, or will I damage anything? Thank for any input! Maria Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 27, 2020 Report Share Posted June 27, 2020 the filler neck may well be a bit deformed and the bayonet effect wont grab the cap so a careful tweek may solve it the filler fittings are all the same what does change on some is the depth of the main seal and its sprung loaded contact , there are deep and shallow and this can be down to the make of the radiator a deep wont fit on a short filler without overloading the pressure valve and the opposite a short cap on a deep filler will not pressurise at all and i wouldnt worry about the difference from 7 to 13 psi the higher pressure raises the boiling point , if you dont have any overheating then your 7 will work fine Pete Link to comment Share on other sites More sharing options...
Colin Lindsay Posted June 27, 2020 Report Share Posted June 27, 2020 My rad cap on the Mk1 is quite tight, has to be pressed down to get the lugs to grip and once they're on for any length of time, it has to be hammered off like the spinners on wire wheels. I've no idea if that's the fault of the rad cap or the radiator itself. Nice t-shirt, Maria - matches my old spare wheel cover nicely! Link to comment Share on other sites More sharing options...
Qu1ckn1ck Posted June 27, 2020 Report Share Posted June 27, 2020 I am happily using a 7lb radiator cap on my Mk3 engine without any problems. It appears to be the only Mk3 not to suffer from overheating in hot weather. As Pete says, I would not bother with the 13lb cap. Link to comment Share on other sites More sharing options...
Meeze Posted June 27, 2020 Author Report Share Posted June 27, 2020 Thanks Pete and Colin for your feedback! The radiator was from Rimmers (am I allowed to mention them?), no, they are good and the7lber fits well. I might just put it down to school girl error. Should have purchased a MkIII radiator to match the engine, but presumed (I hate that word) they were the same! Should have known better! T-shirt from the club, years ago! Maria🙂 Link to comment Share on other sites More sharing options...
Meeze Posted June 27, 2020 Author Report Share Posted June 27, 2020 Thanks member 172. I will think I will keep to my 7lber Maria🙂 Link to comment Share on other sites More sharing options...
Meeze Posted June 27, 2020 Author Report Share Posted June 27, 2020 Member 172 Do you know what thermostat you have? Link to comment Share on other sites More sharing options...
johny Posted June 27, 2020 Report Share Posted June 27, 2020 Hi María, I had exactly this problem on my Vitesse and I filed the lugs down to get the new cap to fit. I had an overheating problem and hoped a higher pressure would help but the fundamental problem was a knackered radiator... Link to comment Share on other sites More sharing options...
Qu1ckn1ck Posted June 27, 2020 Report Share Posted June 27, 2020 25 minutes ago, Meeze said: Member 172 Do you know what thermostat you have? Sorry Maria, not had to look at the thermostat yet so unsure what one it is. The engine gets up to heat quickly but then holds nicely at midpoint on the temp gauge even in current high temperatures. The coolant is crystal clear antifreeze blue with no trace of rust or sediment and the radiator is in good condition. Nick Link to comment Share on other sites More sharing options...
dougbgt6 Posted June 27, 2020 Report Share Posted June 27, 2020 2 hours ago, Colin Lindsay said: Nice t-shirt, Maria - matches my old spare wheel cover nicely! Forget the T shirt Colin, that's a Fender Strat casually parked by the sideboard! Doug 1 Link to comment Share on other sites More sharing options...
Meeze Posted June 28, 2020 Author Report Share Posted June 28, 2020 Thank's all for your input. After the successful fitting of the new radiator valve(see forum), I still had an overheating problem, so the next step was to change the 84oC thermostat which appeared to be a bit slow to operate in a jug of boing water to a 74oC. Having read Andy Cook's article, I decided to be overcautious and go for the radiator cap. I think I shall stick with my 7lber . If it ain't broke, don't fix it🙂 Link to comment Share on other sites More sharing options...
Clive Posted June 28, 2020 Report Share Posted June 28, 2020 All the cap does is increase the temp at which the coolant boils. 50/50 antifreeze boils at about 110 degrees, with a 14lb car about 125 degrees, 7lb about 120. All approximate etc. But the point is, overheating usually means the radiator is no getting rid of enough heat. Or some other problem, assuming it is not a faulty temp sender or gauge. Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 28, 2020 Report Share Posted June 28, 2020 74 is the spec for hot climates 82c is the std for the UK , it may help but do be sure the transmitter and gauge stabiliser is working ok before you dive into deep water and get nowhere . pete Link to comment Share on other sites More sharing options...
Rijidij Posted June 28, 2020 Report Share Posted June 28, 2020 21 hours ago, Pete Lewis said: a deep wont fit on a short filler without overloading the pressure valve and the opposite a short cap on a deep filler will not pressurise at all How far should the spring be compressed on a correctly fitting cap? The reason I ask is that I had a similar experience to Meeze. I bought a replacement 13lb cap. And then a 7 pounder when I became concerned I was over-pressurising the system. One cap compresses just a millimetre or so. The other compresses the spring hardly at all. Link to comment Share on other sites More sharing options...
Casper Posted June 28, 2020 Report Share Posted June 28, 2020 Quote the filler fittings are all the same The 7lb has narrower lugs and the radiators narrower slots. It's not just about overall boiling, i.e. overheating, it's to do with controlling the nucleate boiling on the hotspots in the head passages. I read all about it once but can't remember the fine details. I believe the the change with ST coincided with an improvement in the spec. of the rubber hoses, but now they are all the higher spec. and can cope with 13lb. Tests the water pump gland, though C. Link to comment Share on other sites More sharing options...
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