24 replies to this topic

[NLinPEN]

Minispares say 15psi (yes, the stamping on the cap is more accurate)

Interesting. This would men that my radiator cap opens at a too low pressure. 15 psi converts to 103 kPa. Whereas my cap has a marking of 88 kPa, which is equivalent to approximately 13 psi.

The description is interesting for a second reason as well: they apparently don't know whether it is an expansion tank, or an overflow bottle. Minispares uses both words in that description. I conclude from that that also Minispares doesn't know whether the pressure in the overflow bottle is at atmospheric level, or higher.

Edited by NLinPEN, 21 February 2026 - 06:28 AM.

[viz139]

The expansion tank is not pressurised . The rivets onto the mounting bracket have holes in the centre . The purpose of the tank is to keep the rad full to the top of coolant . Previous cooling systems had air in the top of the rad.

[NLinPEN]

The expansion tank is not pressurised . The rivets onto the mounting bracket have holes in the centre . The purpose of the tank is to keep the rad full to the top of coolant . Previous cooling systems had air in the top of the rad.

Thank you, that makes perfect sense. I had not considered the rivets and the leakage they would potentially cause.

[hungdynasty]

 

Minispares say 15psi (yes, the stamping on the cap is more accurate)

Interesting. This would men that my radiator cap opens at a too low pressure. 15 psi converts to 103 kPa. Whereas my cap has a marking of 88 kPa, which is equivalent to approximately 13 psi.

The description is interesting for a second reason as well: they apparently don't know whether it is an expansion tank, or an overflow bottle. Minispares uses both words in that description. I conclude from that that also Minispares doesn't know whether the pressure in the overflow bottle is at atmospheric level, or higher.

 

I've been using a 13psi cap for 12 years since I bought my JDM SPI and have no problem, it seems like in Japan they usually use 13psi cap on SPI.

[Ethel]

It might be what's local & fits. If you have an issue it won't be caused by the cap being a few psi lower. 13 or 15 will hold the boiling point way above what opens the thermostat. If it boils the temperature has been able to creep up 20 odd degrees because of a lack of cooling capacity.

[GraemeC]

The expansion tank is not pressurised . The rivets onto the mounting bracket have holes in the centre . The purpose of the tank is to keep the rad full to the top of coolant . Previous cooling systems had air in the top of the rad.

 

I beg to differ - I have brand new tank here and it certainly doesn't vent at all.  The rivet may have holes, but they must be into a blind hole as they are sealed off from the tank innards.

[NLinPEN]

 

 

I beg to differ - I have brand new tank here and it certainly doesn't vent at all.  The rivet may have holes, but they must be into a blind hole as they are sealed off from the tank innards.

 

In that case is your expansion tank different from mine. I put some water in it and then held it upside down. The water came pouring out through the center of each of the two rivets. These clearly work as vent holes in my expansion tank.

[Ethel]

I haven't got a tank, but I can't see how it could return coolant if it wasn't sealed. The water level in the tank must be a fair bit lower than that in the radiator.

[viz139]

I haven't got a tank, but I can't see how it could return coolant if it wasn't sealed. The water level in the tank must be a fair bit lower than that in the radiator.

The water is drawn back in by the vacuum created by the coolant contracting in the rad. That is why it is important to have the correct rad cap with the two seals otherwise it would draw air like the previous systems. If the expansion bottle was sealed the entire system would be under vacuum when cool.

[NLinPEN]

Thank you all for your help with this topic. I have been able to repair the cracks in the tank and install it the correct way above the front left wheel. Then I undid the hose near the radiator cap and filled the overflow bottle with about 0.5 litres of coolant (I estimate its full capacity is close to one litre). I left the radiator not entirely full, on purpose. Then took it for a test drive, made sure that the engine got properly warm. The following day, after all pressure had gone from the system, I confirmed that some of the coolant had been "sucked" from the overflow bottle into the radiator to fill it up completely. This is what I expected as it proofs that the coolant system does not have a leak. And that the radiator cap works as intended: both its over-pressure valve as well as its one-way valve functions.

 

I consider this topic closed.

Edited by NLinPEN, 28 February 2026 - 06:24 AM.