MGC Bits

Spring 2018 I'm contacted out of the blue by someone looking for help with an MGC engine that is running too hot, but is actually in an Austin 3-litre which used that engine as standard. He lives locally so pops round one evening, and what a lovely old bus it is. Luxurious and comfortable in very good condition, and auto which I haven't driven for years (we swap drives and he is impressed with the V8). It has power steering, but despite that also has a large wheel and is low geared, so I go into a turning and suddenly find I have to twirl the wheel a lot more than I was expecting! After he drove the V8 I asked how he found the higher geared, smaller wheel, unassisted V8, and he said he never even noticed as he was enjoying it so much.

The problem is that after a head replacement and other work it is running hot on the gauge at higher speeds, but isn't chucking any coolant out. After various tests of the timing and head and radiator temperature I feel it could be one of several things:

  1. Distributor problems as it didn't seem to have any centrifugal advance, which will cause hot running.
  2. Coolant circulation problems as the temperature tests just didn't seem right.
  3. Possibly gauge problems as it didn't seemed to be running that hot anyway - these cars have an electric temp gauge and I know that on the MGB a mismatch between sender and gauge can give incorrect results.
He investigated those areas and realised that although the car had been converted from carb vacuum to inlet manifold vacuum (apparently a BL mod to cure overheating), it had been timed with vacuum connected. Not only does this result in it being seriously retarded, but it also means that as you open the throttle and increase engine speed, vacuum and hence vacuum advance goes down and centrifugal goes up, so under various conditions the two balance out. Retimed with the vacuum port disconnected and sealed.

Secondly he took out the thermostat and discovered it was an MGB item, the correct item has a disc on the bottom which as the stat opens closes off the bypass port, which is vertically below the stat. If this port remains open too much coolant continues to bypass the radiator, so running too hot.

Thermostat replaced.

Thirdly he investigated temp gauge senders and found he had a red one instead of a black one. Fitting a black one (see the differences here) together with the other two changes made things much better, but not entirely.

In the meantime I had posted questions on various fora and together with a couple of others got three comments back saying there is casting flash in the block at the entrance to the water pump chamber, which can cause an obstruction (and I can imagine turbulence at higher pump speeds) and should be ground off. That sounds like a very likely candidate for the circulation problems that I felt existed, and subsequently someone sent me pictures of just where this flashing is.

The water gallery on the carb/exhaust side of the engine, showing the flashing at the front that has to be ground back (circled) ...

... and the many small water passages that must be kept clear of scale.

However that is a bigger job to get at than the other three, so has been put on the back burner for a while. Since then a coolant leak seems to have developed, which needs sorting before any more high-speed running can be done, and it has also just been taken to a trimming shop for several weeks work - postponed several times and couldn't be postponed any more - so it will be a while before we can go any further.