Radiator cap poundage?

[jagnut66]

Hi,
Just wondered if anyone knows what the correct poundage would be for a Morris Minor radiator cap on a car fitted with a 1275cc engine.
Many thanks.
Mike.

[bmcecosse]

Whatever you like - the water temp should never reach boiling point anyway - so as long as it seals down well - 4 psi will be adequate.

[Mog-nuts]

I've got a 10 PSI rad cap on my 1275 without any probs.

[jagnut66]

Hi,
Many thanks for your help, I wanted some info as I have spotted a Stainless Steel cap on eBay, which would enhance my engine bay (my existing cap being old and grubby) but as I have upgraded the engine I wasn't sure of the right poundage.
The only mark on my existing cap is a 10, so I shall er on the side of caution and go with the 10 psi rating. My existing cap seems to function just fine but apart from that mark there is no indication of its psi rating and I don't know if its a later replacement for the original.
As a point of interest one of the first things I did when I bought the car four years ago was to replace the radiator with a new one, though I kept the original cap as it still had a 1098cc engine fitted then. (Though I don't think this was the original, as the ID number didn't match the log book!)

Best wishes,
Mike.

[IslipMinor]

I use a 13lb cap, and have used a 15lb one in the past - I like the additional safety margin, especially on a bigger engine. Also 88°C thermostats are commonly used now, which were not available when the MInor was designed. 88°C to the 4lb cap boiling temperature of 106°C is not very far! A 10lb cap restores the original design difference that existed between the 4lb cap and the lower temperature thermostats of the 50's and 60's.

The boiling points for the common caps are:

4lb 106°C
10lb 115°C
13lb 118°C
15lb 120°C

For the original MInor radiator 'long' neck a 10lb cap is the maximum that is commonly available. When I had our radiator recored, I had the later 'short' neck fitted so that caps are much more readily available for it now.

[bmcecosse]

High pressure = greater chance of a hose bursting, usually the silly bypass hose - especially if the engine has the ghastly convoluted affair...... There really is no need to get the engine so hot that it needs a high pressure cap to stop it boiling!

[IslipMinor]

Never had a hose burst in the Minor in 43 years - the hoses are changed every 7/8 years or so, and have always run the highest pressure cap available. If the hose is going to burst, it should have been replaced some while before - the additional pressure is not the problem.

I run with an 82°C thermostat, and the normal running temperature is around that, increasing to maybe 90°C in traffic, but even so the higher pressure cap was very much needed when the first electric fan failed in 100°F temperatures in France, and again when crawling along the M25 in similar temperatures. Doesn't happen very often, but it's like insurance, great to have it when needed. The only difference is that you have to pay for insurance, but a higher pressure cap is the same price as a lower one, so why not?

[bmcecosse]

Simply because of the burst risk..... Many folks will have Minors with hoses much older than your 7/8 years Richard!

[chrisryder]

...the higher pressure cap was very much needed when the first electric fan failed in 100°F temperatures in France...

Excuse me for being potentially ignorant, but if you don't ask, you'll never learn.

Why would a 4lb cap have been, as you imply, worse? Doesn't the 10lb cap just mean that your engine will run to a higher temperature before letting off, perhaps causing more damage, and meaning the engine must then cool from a higher temperature?

[bmcecosse]

[Jojax]

Had rusty water leaking from the rad cap on my 53 splitty. The actual lip of the rad looks a little warped.....like someone's given it a clout at some point.
Was wondering....if I have to replace the rad, will the 1098 one fit? My current engine is a 950. wouldn't like to have to source a pretty obsolete rad. Will the mountings be compatible to fit the more "modern" 1098 one?

[mike.perry]

Pre 1950 Series MMs had a large circular rad cap. After that all Minors used the same "winged" rad cap

[dalebrignall]

if you dont want pressure in the system you can run evans waterless coolant i am running it and i am impressed i know its expensive if i get 8 years out of it , its paid for its self

[Jojax]

Pre 1950 Series MMs had a large circular rad cap. After that all Minors used the same "winged" rad cap

Thanks, Mike....but will a later rad fit into my '53 Series 2.....(if required)

[IslipMinor]

The standard Minor radiator uses a 'long neck' radiator cap. The most common is the shorter one that appears to fit, but cannot seal at all.

The maximum pressure for the long neck is normally 10 lbf/in2. The later caps go up to 15 lbf/in2.

[Roni]

When I had my radiator re-cored a couple of years ago, I was warned against using a cap with too many pounds release pressure. I was told it was the design of the radiator, especially the top tank, that determined the maximum pressure. The vertical walls on mine were bowed out and had to be re-shaped. It used to have a 13 lb cap. The cap and probable overheating at some time in its past life caused more pressure than the radiator could contain without distorting. I was advised to use a 4 lb or at most 7lb cap.
To quote -" the water temp should never reach boiling point anyway - so as long as it seals down well - 4 psi will be adequate."

[mike.perry]

Pre 1950 Series MMs had a large circular rad cap. After that all Minors used the same "winged" rad cap

Thanks, Mike....but will a later rad fit into my '53 Series 2.....(if required)

.

The Series II radiator has a taller header tank but as long as the mounting bolts are in the same position as the M1000 a later rad should fit. The SII has a different water pump but that should not make any difference

[katy]

The boiling points for the common caps are:

4lb 106°C
10lb 115°C
13lb 118°C
15lb 120°C

I assume this is for straight water?
Adding anti-freeze to the water will raise the BP.

[bmcecosse]

If the water is getting near 100 C anyway - it's a pretty drastic situation. 4 psi should be plenty!

[katy]

Agreed.