Morris Minor Owners Club

Can anyone tell me what is the purpose of the water pump bypass and what happens if it is blocked off and if it is blocked off can you still use the same pump or does it have to be changed for a non bypass one.
Many thanks.

Hahahaha -you are opening a whole can of worms here! I assume your new head has no bypass connection? I say use a bypass hose - and plug it. But others will shriek in horror at this idea....

Yes a whole - whole - whole can of worms
I personally always delete the bypass hose and connection between the head and pump and I have never had any trouble.
Later cars had the head connection deleted and the pump had the by-pass boss but it was not drilled so latterly was considered surplus to requirements.
To block either the pump or head connection you can use the following.

http://www.minispares.com/product/Class ... ssic/water pump.aspx|Back to search

Sorry, did not know it was a touchy subject. Just tell me if I need to change the pump or will blocking it off be OK then we will leave it at that. I will delete the posting if it would be best, don't want to upset anyone

I used one of these, no problems to report

http://www.ebay.co.uk/itm/Silicone-Hose ... hash=item0

Going back to your original question what purpose does the by-pass hose serve?
It allows a small amount of water to circulate through the head before the thermostat opens and the main flow of coolant circulates/happens.
Later 'A' Series in the form of the Metro/Maesto etc. had a different water circulation system which allowed some water to circulate before the thermostat opened and that is why on later heads the ability to fit a by-pass hose adapter was deleted.
To reiterate - all of the engines I have used/built the by-pass has been deleted and I have never had any trouble.
The BMC S/T recommendation on competition engines was to delete the by-pass hose function, remove the thermostat and fit the thermostat sleeve but that is another story.

You should drill a couple of 1/8" holes in the rim of the thermostat, and always leave the heater connection open.....but you should be doing that anyway. A good solid bypass hose with a tight fitting bolt (held with a clip of course) plugs the bypass port from the pump satisfactorily.

Thank you all for your replies that answers the questions nicely. I think we can consider the subject closed now before the worms start getting rampant

If the later A series had a different water circulation system, and consequently, didn't need a bypass hose, that would suggest the hose is needed for earlier A series that do not have this later water system? Or has it simply been decided to forget it because no one has noticed any problems doing so? How far back does late go?

Here we go...... There is no basic change to the engine design over the years.

Whilst I agree that there has been no change to the basic design there have been changes to the coolant system in the way coolant is delivered around the coolant system which does not require the by-pass facility as described above.
Metro and Maestro cars had a sandwich plate (photo below & which has a take off for the heater) between the cylinder head and the cylinder head outlet which was above the sandwich plate. The thermostat was/is contained within the sandwich plate but above the heater outlet.
For cars fitted with the sandwich plate - in a nut shell - coolant before the thermostat opens circulates and returns to the pump inlet via the heater and inlet manifold so no requirement for the by-pass hose as that coolant circulation basically does the same job as that coolant circulating through the by-pass hose.[frame][/frame]

Yes I remember this sandwich plate on my Allegro, so if the bypass is blanked off, this plate is necessary?

Les
No not necessarily - the sandwich plate is only important if you have a standard Metro or Maestro with a heater and water heated manifold otherwise the heater and manifold will not get warm. There is no heater take off on the rear of the head just a blanked hole.
Phil

Thanks for reply Phil, I tend to believe things are there for a reason, so like to know the ins and outs!

it was simply to improve heater performance - both with earlier heat from cold - and better flow through the larger heater matrix in these large cars. Blanked off bypass (and no holes in thermostat) does that very nicely in winter on a Mini/Minor installation - but I do suggest the holes in the stat are probably wise in summer.

Just to keep the record straight, the A-Series never ran without a bypass circulation system as designed and produced.

The earlier versions use the bypass hose between the pump and head and the later versions use the sandwich plate between the head and the thermostat. When the A-Series Mini engine lost the bypass hose (around 1988 with the A-Plus engine?), it gained the sandwich plate.

Both serve the same purpose, to create water circulation around the engine, especially the front of the block and head, before the thermostat opens.

Removing or blanking off the bypass hose on an earlier engine (all A-Series Minors) restricts the water flow as it warms up before the thermostat opens, so it warms up quicker, and if the heater tap is open, the heater will work better, sooner. The downside is that the front part of the engine gets hotter than it is designed to, and this can cause problems with local overheating and valve burning problems on modified engines (as is very well recorded with our 1380!).

On a standard (ish) engine you will very likely get away with having no bypass system, plus the benefit of a quicker warming heater, but BMC never designed it that way.

As Phil says, on competition engines, the thermostat is often discarded and replaced with a 'blanking sleeve' that forces the correct circulation, and the bypass hose is removed as well as there is no purpose for it in this application.

Thanks for that, it answers all my doubts/questions on the subject.

Note that Richard is the ONLY person to have experienced any problem with this - and it is done on many many Mini (and Minor) engines.... Told you it was a can of worms!

I personally have received enough info to confirm my belief that things are there for a reason, and if there is the slightest possibility of a problem I'll avoid it.

Which is fine - unless you are stuck at the side of the road with a burst bypass hose and a seized engine! Many an A series has been wrecked by that... Much less likely of course if you use a 'solid' hose rather than the nasty 'concertina' hose, which should be considered only as an emergency 'get you home' fix. The challenge really only arises if you use a head with no bypass - then you can either run without - or use the Metro etc system. Both work well. The 'no bypass' has been used for many years on uprated / competition engines as a means of eliminating something that could go wrong - and yes - sometimes (but not always in rally cars where heater/demisting still required) with the thermostat removed and a blanking sleeve fitted as mentioned above. The blanking sleeve is NOT recommended for road engines.