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MG MGB Technical - Engine rebuild problem

I have a 1966 MGB Roadster which I am restoring.
I removed the engine.
Had head Unleaded.
Had engine block reboard to 30thou.
Had crank polished.
Fitted new pistons, rings, small ends, big ends and main bearings.
Rebuilt the engine on a bench.
Reassembled with new oil pump, timing chain and tensioner and a new head gasket.
Put the engine back into car.
When the car was started it was getting too hot and there was water coming out of the overflow pipe, so I put in a new thermostat but it still boiled over.
Also there was very little oil coming up to the rocker shaft.
Can anyone tell me:-
1. Why is it boiling over?
2. Why there is very little oil in the rocker shaft?

I would be very grateful for any advice as to what is wrong.

J Fowle

It's going to a difficult one to help with on here but let's see what can be done..
Please confirm you are using the same head and not an exchange one? Have you used the same rocker shaft assembly or a new one? If original, have you also rebuilt that?

For the overheating please confirm if the original water pump was used or replaced? have you checked the thermostat is working correctly and there are no airlocks in the system?

Lastly (for now) are you getting good oil pressure eg 70psi on start-up?

Best of....
M McAndrew

A) Have you got reasonable oil pressure? It can get very hot if no oil is not circulating.
B) Was the head crack tested and skimmed? Bad head to block seal, or a crack in the head can blow water out of the overflow.
C) Did you use the original rocker shaft and pedestals? Did you get your original head back from the engineers?
Allan Reeling

Hi MGmike,
I used the same head as it is the right one for the year.
I purchased a new rocker shaft assembly.
I replaced the water pump.
I will check the thermostat is working ok as have not done so.
I will also check for any airlocks in the system.
Oil pressure is reading 70psi.

I can't check anything till tomorrow but will let you know the results when I have.

J Fowle

It appears Allan and I are thinking along the same lines :)
M McAndrew


You need to check the oil feed in the rocker assembly pedestals. There are two types "in-line" and "off-set". I'll lay odds you have the wrong one.
I would also check the pump is the correct one and actually circulating water. Easiest way to do this is to take the stat out for a while and make sure the bottom hose warms up in time with the top one.

Best of...

M McAndrew

When it was on the bench, did you check everything moved freely? Occasionally you need to align bore the block due to previous problems taking their toll. Oil pumps are also very suspect, regularly being poorly made and locking up the engine. It will turn, but with much added stresses that won't help anything. Did you put the right oil pump gasket on? Did you use some sealing compound on the gaskets (eg hylomar)? Did some get squeezed out and go round the engine, blocking an oil way up?
As mentioned, did you get the correct rocker shaft assembly for your head oil feed type?
Was the engine cleaned out properly, did you take all the core plugs and oil gallery plugs out? Was the same done on the head? The heater matrix? The radiator? Do you have an air lock in the heater. Did you run the system through with the heater on full?
Was the block deck checked? Was the head? Was it crack checked?
Piston ring gaps too tight?

Could be a lot of things!

I'm assuming that the machine shop replaced your camshaft bearings after they boiled out your block. When installing the rearmost bearing, it's very easy for the bearing shell to move off the 12 o'clock position. This is where the rocker shaft receives its oil feed from and low oil volume to the rocker shaft points to a misaligned camshaft bearing shell in the block. An easy thing for a machine shop to miss. RAY
rjm RAY

Too many red herrings

Basic timing/fuelling/airleaks for overheating. OP does not say how quick it boils up.

Oil coming out of rockershaft is qualitative not quantitative. One would expect a puddle around the area the oil comes into the rockerface if wrong pedestal.

If the rockershaft assembly has been built tight it will not dribble much till the shaft wears a bit.


Peter Burgess Tuning

was it very little oil getting to the rockers or none at all
if none at all the headgasket could be on back to front blocking off the oil supply and depending on the brand of gasket could have the big water holes now on the front allowing no water flow at the rear causing it to run hot

William Revit

How hot is it getting on the temp gauge? And more importantly how hot is the thermostat housing getting as measured with an infra-red thermometer placed right against it?

How soon after starting does coolant start coming out of the overflow? And how does that relate to the stat opening i.e. rad header tank getting hot?

My temp gauge started oscillating wildly just as the stat opened a few weeks ago, and started chucking out a bit of coolant at that time. However after that it ran fine. I suspected the stat, changed it, but no difference. I teed a pressure gauge into the heater hose and can see the system pumping up to cap pressure - the more throttle I give it the faster it goes up. I also changed the water pump for reasons I won't go in to, but again just the same. So a quick trip to Halfords and five minutes with their combustion leak detector confirmed the head gasket is leaking into the water jacket.

Newly rebuilt engines can run hot as they are tighter than fully run-in engines, but they should still be within normal parameters on the gauge and not cause cooling system problems.

I can't see lack of oil to the rockers causing this, MGB engines do normally chuck out quite a lot, but I was amazed how little a 1500 Midget engine for example does.

I'd be doing a plugs-out torque test on the crankshaft - hopefully others here will have typical values, and a combustion leak check before anything else.
Paul Hunt

>> So a quick trip to Halfords and five minutes with their combustion leak detector<<
which one is this please Paul so I can add it to my list
Nigel Atkins

As above, how hot is it getting, and is it actually boiling? If the temperature is hotter than normal (say, gauge at 4 o'clock or further) and/or actual boiling is happening then there is a problem, but if not then it could be an overfilled radiator. Unlike systems with a remote header tank, pushing water out the overflow initially after start-up is normal in chrome bumper B's if the radiator was full to the brim when cold - as it heats the water expands and has to go somewhere. It should stop overflowing once it has dumped the surplus and established its own level when hot.

A simple way to test for head gasket leakage is to let the engine cool completely (as in, overnight) and then undo the pressure cap. Any residual pressure indicates likely pressurising from the cylinders and should be followed up with a leak test.

Although most head gaskets are symetrical to guard against blockages caused by putting on upside down, not all are so Willy's suggestion is worth checking.

Rocker bushes supplied now have a shorter oil groove than original - about half the length. As a result the grooves don't ever reach the shaft's oil feed holes and with new shaft and bushes very little (if any) oil will emerge. We extend them with a tiny grinding head back to original length.

To check that oil is reaching the rockers, temporarily remove the rocker shaft locating dowel in the back pedastal and crank the engine on the starter (take the plugs out to make it easier for the starter) for half a minute or so, by which time a dribble should have come out of the hole.

Paul Walbran

Any garage should have a combustion leak detector, it depends on how well you know them as to how willing they are to do a free test for you. I've been taking two and then three cars to my local branch in Shirley for over 20 years, and they have always been very obliging.

"Unlike systems with a remote header tank, pushing water out the overflow initially after start-up is normal in chrome bumper B's if the radiator was full to the brim when cold - as it heats the water expands and has to go somewhere. It should stop overflowing once it has dumped the surplus and established its own level when hot. "

Exactly the same applies to systems with a remote header tank. If the header tank is completely full then it will also push coolant out on warm up, and establish the 'correct' level when it has cooled again.
Paul Hunt

True, some people do indeed ignore the full mark on the tanks of modern cars.
Paul Walbran

This thread was discussed between 06/06/2014 and 09/06/2014

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