Bypass Hose What does it actually do, to keep or not to keep

[bmcecosse]

Fit an 88 degree stat, no bypass hose, and pull the 'fresh' air from behind the top of the engine radiator - and the heater will be toasty!

[les]

Behind the rad sounds good, think I'll keep the bypass though!

[bmcecosse]

No - get rid of the wretched thing - Rover eventually saw the light and dumped it from the Mutro. Much faster warm up and better flow through the heater.

[Stig]

Hmm, the heater take-off is from the front of the block on a Metro though, and I don't think it had a water valve on it. Sounds like the flow arrangement's rather different.

BMC, I assume you've blocked off the bypass hose but keep the heater valve open all year round?

[Kevin]

I have split this from the Heater question as its a new subject that has caused lots of questions in the past.
To get the ball rolling I have never seen a mechanical reason for the bypass hose apart from allowing some water flow while the thermostat is closed but there are many engines types that dont use a bypass hose system so is it really required.
Well I have been told (over the years) when used on the track do away with it and the thermostat and fit a sleeve where the thermostat goes well as we are talking road cars that's not too helpful.
Using a road car without a thermostat is not a good idea so I was always told to drill a few small 1/8" holes in the thermostat to allow some water flow before the thermostat opens as its possible to suffer from hot spots when the bypass hose is done away with.
So what does the bypass hose actually do (that other engines dont have in the first place) and to do away with it you can either by caps to seal up the bypass outlets on both the water pump and the head or have the head outlet blanked off and a later type water pump fitted without the outlet.
What I find more confusing is that the later heads without the bypass hose come in 2 variations as Stig mentioned the Metro heater take of was by an enlarged thermostat housing at the front of the block but on some of the mini's there was still a rear heater take off fitted.
Does anyone know the definative answer.

[bmcecosse]

Yes - heater valve kept open, but the normal practice is to drill 2 x 1/8" holes in the thin rim of the thermostat if you want to close the heater valve off. This still eliminates the risk of burst bypass hose of course - but rather negates the quick heat-up advantage. So - summer 72 stat with 2 holes, and winter 88 (or even 92) stat with no holes for max heater efficiency!

[Packedup]

As far as I know, the bypass keeping a flow that "bypasses" the radiator prevents hot spots and reduces the chances of cavitation (?) in the pump.

Metros have the heater circuit in place of the bypass, with coolant always flowing even with the heater set to cold. I think later Minis had a similar arrangement, never really looked at one though.

Removing the bypass is a very good idea, but as flow needs to still happen it's either a tricky plumbing job or a couple of holes in the stat. Holes in the stat will increase warmup times as you're bypassing the stat rather than the rad, but it's quick and easy and does the job.

[MoggyTech]

The By-Pass hose is part of the early design features of the A-Series engine, a cracking engine designed by blokes who knew their stuff. It is however a pig to change when it bursts and it often does unless you use the solid type, which as luck would have it, is the hardest to fit.

It's primary function is to allow some coolant to flow before the thermostat opens. An engine gets hotter around the top of the combustion chambers and cylinder head before the rest of the engine, so to reduce the risk of thermal expansion cracking the cylinder head, the by-pass hose is used to slow the heating of this area.

The hotter the thermostat you fit, the longer the cylinder head is subjected to high temperatures before coolant starts to flow. Since the thermostat is fitted at the front of the head, the temperature differential between the combustion chambers and the thermostat will not match.

Can it be removed safely? Yes, provided you drill the two holes in the thermostat. What you are doing here is basically the same thing the by-pass hose does, but removing the weak link in the chain, namely the rubber hose.

Perhaps a simpler option is to use the nasty type by-pass hose, the convoluted ones that are easy to fit. Replace them every year as a service item, and carry a spare in the glovebox. They will last a year or more if you use a decent anti-freeze with corrosion inhibitors.

Tip for fitting the solid type, crush them length wise in a vice for ten minutes, then fit it quick before it springs back to full length.

[Kevin]

Well the convoluted ones are fine as an emergency measure but the solid type (in reality a piece of heater hose) last for at least 5 years and don't take that long to do with removal of the rad & water pump.

Since the thermostat is fitted at the front of the head, the temperature differential between the combustion chambers and the thermostat will not match.

OK so the bypass hose is to elimate the risk of head/gasket damage around cylinders 3 & 4 when the heater cooling capacity is closed off if I understand correctly but if you leave the heater valve open there is not an issue.

[MoggyTech]

>>OK so the bypass hose is to elimate the risk of head/gasket damage around cylinders 3 & 4 when the heater cooling capacity is closed off if I understand correctly but if you leave the heater valve open there is not an issue.<<

That's more or less it yes. Since the Minor heater is pretty low output anyway, leaving the heater water valve open in summer, and simply closing the heater flap in summer (on square heater type) is all that is needed without occupants getting too hot.

[Mogwai]

I couldn't decide whether or not to remove the bypass as I made one in my heater piping so I restricted it by cutting the head off a suitably sized bolt & drilling a 1/4" hole down the center then fitting it inside the pipe. The heater pipes return through a heated manifold then to the bottom hose. I have a remote valve fitted when this is shut coolant can still flow through the manifold via a restricted T piece, also have an 88deg thermostat. since restricting the bypass (heater piping done long time ago) the temperature seems alot more stable & heater warmup quicker
heres a photo of the heater piping probably clearer than my description

[bmcecosse]

Heated manifold is bad for power - but may give slightly improved economy - ever more important these days!

[Mogwai]

when I first fitted the manifold I did run it unconected to the cooling system. As soon as I plumbed it in there was a marked running improvment especially in cold weather + I can knock the choke off a lot sooner than before

[MikeNash]

Because I do a occasional long journies on motorways (600 there back to Newcastle, 300 there and back to Exeter) usually cruising at 60-65 (true) and sometimes with a trailer, I've got rid of the bypass, seeing it as a breakdown waiting to happen. When I changed to an "unleaded" head I tapped (I think) one bypass stub 5/16 BSF and the other 3/8 BSF and fitted bolts daubed with sealant. 8000 miles later all's going well, but of course I have the tap controlling the water flow to the heater open all the time. This is no problem with the small drum heater.
Because the bulb for my water temp gauge wouldn't fit in the new head (an internal bulge in the casting somewhere) I fitted it in the heater supply line and can see the temp rise and stop at the thermostat operating temp. Currently I've an 88C thermo, but am thinking of going to 91C . Anyone using a higher temp than 88C? Regards, MikeN.

[les]

Well the heater is in now, although not able to run it yet (rest of van needs finishing!) never had the original wire wound demister hoses that were in such good order until now, as opposed to the plastic ones, I found the former much more flexible.

[Packedup]

Because the bulb for my water temp gauge wouldn't fit in the new head (an internal bulge in the casting somewhere) I fitted it in the heater supply line and can see the temp rise and stop at the thermostat operating temp.

No use now, but there's an adapter available from MG places for the Midget temp sender. Because until the later cars the temp gauge was mechanical the probe/ bulb was a very tight fit in the head (I think very early cars had the sender in the top rad tank so no problems there), so the adapter was used to space it out a bit for clearance. It's only a threaded hollow bit of metal that screws into the head and the sender screws into it, costs about £3.