Morris Minor Owners Club

I've just received a new water pump and will be fitting it tomorrow. I know I'm opening a can of worms, but this is the time to decide if I block-off the by-pass hose or keep in functional. I've searched the forum and read some of the arguments, and as a result can't decide!

This car is in California, on the San Francisco Bay. The coldest it gets -- ever -- is right around the frost point, but it can get hot (say 90-95 degrees Fahrenheit -- which would be maybe 35 Celsius). Driving won't be terribly hard, but some will be on the freeway at speed. It's a '67 MM1000 2 door. I would love for the heater to work well, but on some days leaving it on to help cool the rear of the engine would be masochistic (of course, one could argue that a MM1000 as a daily driver already indicates certain off-balance qualities...)

So, I'll will either install with a working by-pass hose or block it off and drill a couple of 1/8" holes in the thermostat. Which to do???

As an aside, the new water pump came with a nice, pressed steel impeller. The old water pump had a plastic/rubber impeller bonded to the shaft. The car sat for ten years or so, and the dissimilar metal problem (steel block, aluminum water pump) created lots of corrosion in the pump housing -- enough to prevent the rubber impeller from moving and for it to spin on the shaft when we got the car started. I rotated it by hand before starting, and didn't feel anything amiss. I suspect the steel impeller would have happily cut through the corrosion and kept on working.

I've never heard of a rubber impeller! In your circs I would KEEP the bypass hose - but do make sure it's a solid wall type. Here in the UK the best solution is to eliminate the bypass.

Thank you! I got the sense from reading through the archives that you are perhaps the authority on this issue. Thanks for a quick response -- I shall leave the hose intact. Yes, I was very pleased to see that the new pump had a steel impeller. The plastic one seemed dodgy at best, and I've never seen the like on another car.

Here in the UK the best solution is to eliminate the bypass.

We have been here many times and that statement is not universally accepted.

Despite many claims that BMC eliminated the by-pass on later A-Series, and therefore we should on the Minor, that is not true. On the Metro they did remove the small by-pass hose between the water pump and the head, and they replaced it with a even larger bore by-pass using a sandwich plate under the thermostat housing.

A-Series heads need good water flow at all times. BMC Competitions Dept. used to remove the by-pass hose, but they also removed the thermostat as well!! They then fitted what was called a 'blamking plate' to the thermostat housing, which in reality was there to force water flow around the head, so that it could not just take a short circuit though the open thermostat hole. They also advised to keep a flow through the heater take-off at the back of the head, as overheating of 3 & 4 was also a known problem.

Also as Roy knows very well, as there were numerous posts at the time, I had very serious local overheating problems around the front of the engine on our 1380 immediately after following the trend to remove the by-pass altogether; I did drill the holes in the thermostat, but in our case that was nothing like enough water flow. The by-pass removal was done at the same time as the head was converted to unleaded, so my focus was on that, as I had sticking valves and a burnt out No. 1 exhaust valve over the next couple of years. Finally I restored the original by-pass hose and no problems since then.

On a standard engine in very temperate climates, such as the UK, and little or no motorway type use, you may get away with no by-pass, and, yes it does help the heater to warm up more quickly, but that is not the same thing as it being 'the best solution'.

Hopefully this alternative view will be accepted as just that, an alternative view based on very real practical experience.

And -I have pointed out before that your inlet valve problem could not have been caused by overheating caused by lack of bypass - since as soon as any temperature developed, the thermostat would open......... But - I agree - it's not a clear-cut thing, which is why in the nice temperate climate of San Fran, I think I would probably retain the bypass hose.

Roy,

It was the exhaust valves that caused the problem, and I could watch the temperature climb extremely rapidly and the misfire would then occur - the exhaust valve(s) would nip on their stem(s). On No. 1 cylinder I had 2 plugs burn out and one exhaust valve. The plugs were fairly new NGK BP7ES. The misfire was particularly noticeable during the warm-up phase, then as the thermostat opened, which is not 'instant', the misfire would stop, and then sometimes return when the engine came under sustained load, e.g. motorway inclines, when once again it would misfire, and immediately stop if the throttle was backed off. The problem only affected No. 1 cylinder as far as any permanent symptoms occurred.

Without any other changes, just restoring the standard pump to head by-pass hose about 5/6 years ago eliminated the misfire, burnt out plugs and valve completely. Mixture, timing etc. all set up on rolling road sessions, so what else could have been the cause?

As far as I am aware all car engines have a by-pass circuit, either internal or external, to avoid localised overheating when the thermostat is either closed or partially opened. The A-Series is no exception to that basic design requirement, and I rue the day I followed the perceived wisdom of removing the by-pass system.

It gave me 2-3 years of grief, not to say expense, as after checking out the complete ignition system, I replaced the entire fuel system from tank to fuel pump in the false hope of curing what then had to be fuel starvation.

Yes - I know Richard, you have told me the story a couple of times - it was obviously the ghastly NGK plugs........... Seriously - the #4 exhaust (and yes - slip on my part, of course it was exhaust) valve gets far less circulation - so why no problem there ? But -you solved the problem and so for you that works. I quite agree that if bothered, then installation of the sandwich plate and bypass via the heater circuit is probably better - but I do like the heat flow from the original heater valve take-off for my own engines, and I haven't had any problems - and certainly no burst by-pass hoses which must have caused more misery/expense/ruined engines/missed appointments and very possibly road-side accidents (broken down car gets run into..) over the years of it's ghastly existence. Get rid of it I say! But to get back to the original poster - since he has a standard engine and lives in San Fran and is not greatly interested in heater performance - it's probably easiest to just leave well alone.

the #4 exhaust valve gets far less circulation - so why no problem there ?

The connection to the heater is always left fully open to provide a permanent flow from the back of the head.

Interestingly, when the Metro went over to the sandwich plate by-pass, BMC/Leyland/Rover blanked off the rear outlet, as the by-pass from the front of the engine was routed permanently through the heater, but this was always a 'no no' before. I wonder what else was done that is not obvious externally?

Nothing was different - and so I prefer to drill out the heater connection and use it! On my competition Minis I had a heater rad connected to that port, although the Rally car had a heater (but nothing else internally) - just to keep the screen clear you understand...

Yup, I opened a can of worms! At any rate, I did what bmc told me to do -- and what Islip would prefer -- and kept the bypass, fitted with a new piece of heater hose. I've got some flushing agent in there now, and I've got to drive for 3 to 6 hours before draining it. I finished up a bunch of details last night so took her out in the dark for a short freeway run. The heater worked just fine, and when I got back I popped the bonnet and checked things. Engine did not seem at all hot.

Well done ! Remember - I want to see a picture of your car coming down Lombard...... I have a pic somewhere of me driving down Lombard in my hire car about 20 years ago.....

didn't think there were any minors in cal. where how did you get it. I would say keep the hose, in case anything sticks its a fail safe. with no temp gauge as standard its needed. anyone know why they put a large rubber gasket on carbs on pit bikes. ?