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Does anyone know if it is essential to have a pliable fitted bypass hose and fitting between head and water pump on std 64 mk1 850 motor? I’m still having problems with cooling system becoming pressurised as engine is cranking over and blowing head gasket

Short answer is no, it's not necessary. There have been preevious threads on this but I haven't got time to find the links.
You have to blank off the head spigot hole and use a water pump without the take off. and some drill a small hole in the thermostat plate or use a blanking sleeve in its place.
I doubt whether the bypass hose is the source of your coolant pressurising or your head gasket blowing, especially if it's not actually running. If anything it is the other way round. You still must have a problem with the head gasket not sealing, which is pressurising your coolant.

I had a problem like this and it turned out to be a cracked block an inch down the bore . Hairline crack that opened up as soon as it got warm and then sent exhaust gases into the cooling system which made it boil up in 4 minutes… . I had tried 3 heads on this engine . Have you tested for exhaust gases in the cooling system? . You can get cheap kits from eBay … Shirley

Jasonking wrote: ↑Fri Aug 09, 2024 1:58 pm Does anyone know if it is essential to have a pliable fitted bypass hose and fitting between head and water pump on std 64 mk1 850 motor? I’m still having problems with cooling system becoming pressurised as engine is cranking over and blowing head gasket

Fill the radiator to the top, don't refit the cap, get a 2nd person to crank it over with you watching the water level but without sticking your head right over the neck.
If it pumps out slowly you have a problem, maybe just head gasket or a cracked head, then you need to find out what, you can buy a head gasket tester / fluid kit for £14 off ebay but that will only find combustion gases in the coolant, however by removing one plug at a time then running the engine you might locate which cylinder is at fault.
If you have a compressor you can buy a cylinder leakdown tester off ebay for not much money and maybe go straight to the problem,
Follow the instructions carefully as they can be very dangerous.
if it pushes 500mm into the air then you have something really wrong.

Just out of interest what is the exact spec of the engine / head / cam / etc

Engine won’t run to warm up itself or the head gasket enough to make a seal, as far as I know motor is stock apart from being on it’s third overbore size, which only came to light once head removed the first time, head is in good order freshly skimmed, valves all seat well with no leaks when petrol test done. Carb rebuilt to std HS2 spec.

If the cooling system is pressurising just from turning the engine over (ie not running) then you have a fundamental problem somewhere.
I doubt you’d get enough pressure into the cooling system off the starter to blow the head gasket.

You’ve been struggling with this for a while now - maybe a fresh pair of eyes could help?

Jasonking wrote: ↑Sat Aug 10, 2024 12:43 am Engine won’t run to warm up itself or the head gasket enough to make a seal

Let’s try this in stages:
Is the engine starting or not?

Any eyes are very welcome! No motor won’t start, spark at plugs, fuel getting to carb, easy start makes no difference, static timing set to around 5’degrees btdc no1 pot with rotor arm at approximately 1-2 o’clock on dizzy, points set in new distributor.

Ok, as Graeme says, I can’t see how it can be pressurising the cooling system from just turning over on the starter motor.

Next question: why do you think it’s pressurising the cooling system?

I am deliberately trying to be as succinct as poss (not rude)

Jasonking wrote: ↑Sat Aug 10, 2024 9:22 am Any eyes are very welcome! No motor won’t start, spark at plugs, fuel getting to carb, easy start makes no difference, static timing set to around 5’degrees btdc no1 pot with rotor arm at approximately 1-2 o’clock on dizzy, points set in new distributor.

you don't give a location on your page info, you might be near someone with more experience if you told us your location

BAD942 is dead right with his reply. If another mechanically minded forumer lives nearby, it's a good bet that he'll make contact and between you both and a cup of coffee and a slice of toast, the problem will be sorted.

GraemeC wrote: ↑Sat Aug 10, 2024 7:20 am If the cooling system is pressurising just from turning the engine over (ie not running) then you have a fundamental problem somewhere.
I doubt you’d get enough pressure into the cooling system off the starter to blow the head gasket.

You’ve been struggling with this for a while now - maybe a fresh pair of eyes could help?

I also doubt that you would get enough pressure from cranking to blow the head gasket, however it may never have sealed in the 1st place, you wouldn't believe what I've seen in over 50 years working in the garage trade, a lot carried out by so called professional's

Location already been discussed, at least once, Runcorn, Cheshire https://mk1-forum.net/viewtopic.php?p=336068#p336068

Anyone fancy a challenge ?

A challenge it certainly is, I’m rueing the day I brought this car home it’s certainly helping me to an early grave! Wouldn’t the gaskets need heating in order to seal initially? Which isn’t happening cos the engine won’t ignite? The only time the gasket didn’t leak is when I used a sealant on both surfaces, but then it just filled up the pots instead this was before head was skimmed.

Looking at the pictures from your previous post it appears the back of the cylinder head is not making a seal to the block face.
Following removal of all traces of old gasket material, Have you checked the block face and cylinder head face with a metal straight edge to confirm that they are flat.
When refitting the head, Is the rocker shaft assembly fitted correctly onto the studs, pushrods correctly located and the correct size washers fitted underneath the 4 x 3/8unf nuts? Just wondering if the nuts are getting thread bound giving you a false torque reading.
Are you installing the head with or without the manifold attached? The manifold or something attached to it maybe preventing the head from sitting down fully when re attached to the block
Do you have any pictures with the head installed with the rocker cover off.

If you can move the car (ie trailer or similar) then if you brought it to me and left it with me I could look at it in evenings spare time. I'm M6 J33 so not a million miles away.

Not the best offer I'll admit, and probably a last resort, but I don't have a spare day to come to you I'm afraid.

Have checked head and block with engineers straight edge both kosher, all bolts nuts etc are ARP and fitted as per their instructions, not sure if the rocker pedestal bolts would have a bearing on the head torque though?

Just removed head, gasket looks brand new? When I started to release head bolts coolant(water) was in the radiator, by the time I lifted the head off, water had migrated to the cylinders? WTF???

Jasonking wrote: ↑Sat Aug 10, 2024 8:53 pm Just removed head, gasket looks brand new? When I started to release head bolts coolant(water) was in the radiator, by the time I lifted the head off, water had migrated to the cylinders? WTF???

If you don't drain your water system first this is certain to happen