Very good.

How is the Datsun scenario in Thailand? You have a B310 Sunny, right?

Hi there. Many thanks for that. Yes it's just how I though it worked. I have an expansion tank but it's not fitted as like I have said I replaced the rad.
I did start to wonder if there was something wrong with the rad when I first got the car as engine got very hot so after doing some investigation on the rad it should be ok on size, but one day I got so fed up with this I took it and then found under all the dirt there was a small drain plug so I took this out and found that the water color was green, but the water I was using was clear. That's when I found that the rad was one third blocked with rad weld. So I got a new one made the same size. All was well for two months then all the leaks started. It's just my luck I guess.
Anyway many thanks for all your help.
By the way I live in Thailand.

Thanks again.

Paul.

Most likely, the cap you have now is the correct type, as they've been sold for 40 years.

The overflow tank doesn't have a sealed cap. It is just a bottle, with the hose going down to the bottom of the bottle. Fill it halfway with coolant. Any bottle you want.

Not knowing where you are or what the radiator you are using is out of makes this kind of difficult, but...
...I hope that you live somewhere that you have a competent auto parts store close.
Any good parts man who knows their stuff should be able to get you the correct cap, tell them that you need one "for coolant recovery" that will fit your radiator. The correct cap will take care of all the rest. You don't need to change your tank. The type of cap you are looking for will have a rubber gasket on the center section and another rubber gasket around the outer edge of the cap where it goes over the widest part of the radiator filler neck, other than that it will look just like your old one.

Hi all many thanks for the information. The only question I have now is does the rad cap need to be the same as the expansion tank? I have 0.9 on the rad or do I replace it with one that is just a cap so the water pressure goes to the expansion tank and that has the 0.9 cap on it?

No need to remove the head to replace the core plugs. It'll be more reliable to leave the head on. There are two core plugs behind the flywheel. If you pull the engine, you can replace the plug on the back of the head and the ones behind the flywheel at the same time.

Welch Plug



Expansion tank doesn't regulate the pressure, which is solely up to the cap. Still it is a good thing to add as explained above. Just connect a bottle to the stock overflow hose. Many of us use a stylish plastic bottle, but one off a 1979-1982 Datsun 210 works too.

There is also one at the backside of the block, behind the flywheel. Like DD said change them all, it may be a lot of work but cheap insurance.
As far as the radiator cap, the later ones were designed for coolant recovery. What you call an expansion tank. This is actually a good idea because as the engine heats up the coolant expands and flows out the small tube at the radiator cap fitting, instead of just venting that fluid to the ground and wasting it, it flows into the tank where it is stored until the engine cools off. As it cools it creates a vacuum in the cooling system that causes the fluid to be sucked back into the cooling system.
That way the cooling system always stays full, and unless you have a leak, you should never have to top it off.
Probably the best idea would be to pull the engine, that way you could get much easier access to all of the core plugs.

Yes when I removed the two there was a lot of rust behind them mainly under the rear plug but the front plug was very thin.
The old owner of the car did not look after it very well and as I now drive it every day it's starting to show. I don't think he drove it very often and it use to stand for months at a time. The engine would over heat at first before I found that the red was blocked at the bottom so only 2/3 was working. That was replace and all seemed well for three month then it started to leak from tow plugs so I changed then last weekend but now the one at the back of the head has started to leak so I think it's just my luck that it has now found the next week link so I guess I need to take the head off and replace that one as well. I was just wondering if there was something else going on that would make them start leaking. The new rad has not got the expansion tank connected so I was wondering if that was the it making it over pressurise. I will put this back and reinstall it, but does the filler cap on the rad need to be change to a none pressure type so the rad water can fill the expansion box when hot then draw it back when cold?

Datsun spec for radiator cap is 0.9kg/cm (12.6lbs)
Aftermarket caps are 13 psi Pressure Rating (SAE Range 12-16 psi)
Nismo caps are 1.3kg/cm (18.2lbs).
VW radiator & caps are 20 psi

None cause the Datsun core plugs to leak.

Probably not a coincidence, as 40 year old Datsun plugs don't usually leak. Probably for some period of its life it was run with straight water instead of anti-rust coolant. Or maybe a previous owner decided to replace them and used a screwdriver to pound them in, creating weak spots in the plugs.

I'd replace them all as prevention.