The Triumph Dolomite Club - Discussion Forum

Which of these is right? They were both sold to me as fitting the Sprint, which is true in the sense that they go in the hole.

I didn't check last time because I'd been running without one while I fixed another problem.

Graham

The right is right. The other will probably overheat the engine.

Jeroen

Only overheats when the fan fails. But it did cause some problems - slow to get warm and slow for the fan to go off after stopping at lights, etc. I wondered if it was because it was an 88C one and bought a 82C to replace. Only to find as shown. The right-hand one is in now.

I do note that now, it takes a long time to get the last of the coolant in topping up through the hole above the thermostat, where it used to get to a level quick before. But the new one has a small bypass in it. And I think that is dibbling coolant past the thermostat where the old one held a bubble.

Next problem is why, after replacing stat and cap on the expansion tank, filling from the top overfilled the tank when the cap was on it. It didn't do that with the old stat and old cap. Why do I keep changing two things at once?

Graham

GrahamFountain wrote: ↑Thu May 12, 2022 9:03 pm Only overheats when the fan fails. But it did cause some problems - slow to get warm and slow for the fan to go off after stopping at lights, etc. I wondered if it was because it was an 88C one and bought a 82C to replace. Only to find as shown. The right-hand one is in now.

I do note that now, it takes a long time to get the last of the coolant in topping up through the hole above the thermostat, where it used to get to a level quick before. But the new one has a small bypass in it. And I think that is dibbling coolant past the thermostat where the old one held a bubble.

Next problem is why, after replacing stat and cap on the expansion tank, filling from the top overfilled the tank when the cap was on it. It didn't do that with the old stat and old cap. Why do I keep changing two things at once?

Graham

The stat with the "foot"on is correct for the Sprint motor, the foot closes off the bypass as the stat opens. Without the foot you get 2 different circulation patterns at once which confuses the poor thing.

Filling completely is always time consuming with the correct stat fitted.

simple solution to latest problem, the new cap is not sealing. Can't count how often this has happened to me, the CORRECT cap is 1/4" deeper than the common one, many suppliers who should know better, don't! Factors near enough always get it wrong.

Steve

Carledo wrote: ↑Thu May 12, 2022 10:59 pm
The stat with the "foot"on is correct for the Sprint motor, the foot closes off the bypass as the stat opens. Without the foot you get 2 different circulation patterns at once which confuses the poor thing.

Filling completely is always time consuming with the correct stat fitted.

simple solution to latest problem, the new cap is not sealing. Can't count how often this has happened to me, the CORRECT cap is 1/4" deeper than the common one, many suppliers who should know better, don't! Factors near enough always get it wrong.

Steve

There's a warming up internal recirculation of coolant that makes the coolant flow internally in the engine and through the heater only. When the engine is on temp that recirculation must be closed and these double valve thermostats simultaniously open the main and closing the recirculation with their bottom valve. A single type thermostat will not close the internal recirculation when on temp making some of the hot coolant skipping the rad and going directly into the engine again.

As for the caps, plastic tanks do stretch so these need actually oversized caps but these don't excist. You can use a cap with a higher rating, stronger spring then. Most current caps are way off what's on the label. NOS caps, unipart or similar are the ones to have but with a streched tank you maybe need an 15 or 18. Most bicycle pumps have some sort of pressure gauge on them. Put the cap on the empty tank, pump and see when it opens. 8 out of 10 times your new cap with the correct ratings printed on is opening too soon causing all kind of troubles and wrong diagnoses. The time of buying a new correct working part for a classic what does what it say on the box is long gone. Fit, test, check and know.

Jeroen

The new cap is indeed not sealing, but the old cap does - at least insofar as it's letting me fill up the top of the system without over filling the expansion tank.

I checked them against one another, and they seem to be the same length to within 1 mm (pic 1) and take much the same force to get on and off.

There is a difference in the size of the bottom washer (pic 2). But the new one's washer is 36mm dia., and the witness mark from the lip on the old cap is only 33mm to the outer edge. So I think it should be just big enough.

Whatever the leak is, the top of the system drains down very much slower with the cap in than not. So I think it's a small leak in the cap itself - if the bottom seal on the cap didn't mate with the lip on the expansion tank hole, it would likely be more than a small leak.

What it does seem to be, is the seal on the vacuum release poppet valve in the cap that's not working properly. It seems a bit light and floppy, where the one in the old cap seems nice and positive. And I can just feel it leaking a wee bit when I put the bottom end of the cap in my mouth and blow hard. (Who, me?)

Maybe, if I fill the top of the system and slap the brass plug in quick (or fill it with the old cap on, as I just did, and put the new one on after the plug is in), the poppet valve will seal under pressure, and may even learn to seat properly over time. But it's not like it should be.

So who's your best bet to supply a decent cap? I won't say who I got this one from in case it's not the cap. But don't make Robsport your only suggestion.

Graham

Well if it was the poppet valve seal that leaked, it appears that for not much running around it's now working - I was satisfied enough to risk the cost of a bit of antifreeze, and try the "slap the plug in quick" trick. But the level was already steady. Go figure.

Graham