Advice on a decent head gasket

[whitestu]

My B is leaking coolant after a run from the overflow past the pressure cap. I changed the cap and still the same so I'm going to change the head gasket. I was looking at Payen. Any others recommend?

Thanks

Stu

[Ian F]

Payen is what I would recommend. But are you sure it needs changing? I assume it really is overheating or that you have exhaust gas bubbling through the coolant?
The former could be something else, the latter certainly suggests a new head gasket and all the associated work to head and block.

Ian F

[whitestu]

Thanks. No, not overheating at all and no other signs of head gasket failure. What else would you suggest might cause excessive pressure in the cooling system? Unless both radiator caps are faulty..

[Vic Butler]

Have you got the correct pressure radiator cap? My 1977 BGT has a 15psi cap.

[Ian F]

Stu, I think we need more information before we can offer anything with any more certainty.
If you are loosing coolant it can only be because of excessive expansion of the coolant - due to higher than normal temperatures, or exhaust gas getting into the coolant - or as Vic suggests because you are not using a cap with the correct pressure rating.

# what type of cooling system? Do you have an expansion tank?
# is the coolant at the correct level?
# is the coolant loss due to the residual heat in the system when you stop and the radiator is no longer cooled by air flow?
# with the cap off can you see bubbles of gas in the coolant when the engine is running and hot? You could also measure coolant temperature with a thermometer perhaps?
# are all of the engine parameters set correctly? Weak mixture and/or incorrect ignition timing can both cause overheating.

Best to avoid taking the head off unless absolutely necessary, although it is all fairly straightforward on the MG.

Ian F

[Vic Butler]

I did have a similar problem on my 1977 BGT once where coolant was forced out of the cooling system but no overheating and it turned out to be a head gasket failure.
Payen are acknowledged to be the best.

[whitestu]

Thanks for the input everyone.

To answer the questions:
It's a late model B with the expansion tank and a 15psi cap - a replacement after I first noticed the leak with the old cap (made no difference)
Coolant was at the correct level, but probably a bit low now. I thought the leak might be because I had over filled it but it's been dribbling for so long now I don't think it can now be over filled.
The car isn't overheating at all. Runs at 80 degrees when driving and does up to about 95 stationary when the fan comes on and it goes back down.
Timing and mixture are all set correctly. I set the carbs up with a wideband and ignition timing is spot on with about 33 degrees advance at 3500 rpm.
I can't see air bubbles in the expansion tank but not sure I would be able to even if they were there.
The leak doesn't happen after a short run but after a bit of hard driving I notice coolant coming out of the overflow when I get back on the drive. It stops pretty much straight away when the engine is switched off.
Otherwise the engine runs really well and feels like it has good power, and pulls well throughout the rev range.

I don't want to take the head off if I don't need to but can't think what else this could be. Having said that I've done lots of head gaskets over the years so have no worries doing it, though the biggest danger of getting the head off is the tempation to take it up to Peter Burgess for some porting:D

[Ian F]

Thanks Stu.

I can only offer a couple of thoughts, I can see that you probably know as much about these engines as I do!

I used to run a Triumph TR2, without a cooling fan as I was using it in competition and needed as much hp as possible. After a spirited drive it would always regurgitate some coolant when I stopped. The abrupt change from getting adequate cooling to no cooling air allowed a short boil over. I understood what was happening so just lived with it.

If you are really puzzled by your engines behaviour and really suspect a head gasket problem I would suggest getting one of the kits which allow you to check for combustion gases in the coolant. Any positive result would confirm a head gasket leak.

I am assuming that you have no coolant getting into the oil, and that your cylinder compressions when hot are all within spec?

Keep us updated to date with how you get on.

Ian F

[whitestu]

Thanks - I'll check the compresions when hot - I only checked them cold before I fitetd the engine as it wasn't in the car when I got it. All were around 170psi cold.

There's no sign of coolant in the oil. Testing for combustion gasses in the coolant is probably a good idea. I'll look into that.

I took the cap of the expansion tank this morning cold and it's completely full so possible that it's still just expelling a bit of coolant from being over filled.

I'll keep monitoring it!
Edit. Just ordered a testing kit for £13 so will see what it says
Cheers

Stu

[Charles Farran]

Hi Stu,
My late MGB has a 15 PSI cap as Vic confirms as well.
When absolutely cold the expansion tank should only be half full , which then allows coolant to expand into it as pressure & temp rise.
When the temp falls then the coolant should be sucked back into the radiator / block etc. There are some dodgy caps around that don't have the same profile as the OE. & don't seal that well & may not allow the coolant to be sucked back to the radiator / block. Is your replacement the same as the old one , which presumably was OK until fairly recently?
Have you changed the coolant recently ?
Cheers, Charles