Welcome to our Site for MG, Triumph and Austin-Healey Car Information.


MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - cracked cylinder block

Hi all.

My MK2 B is giving me grief.

I had cylinder head problems and decided to freshen up the bottom end while it was apart, including a new cam.
When it was stripped the shop found cracks in the webs beside the opening where the distributor drive runs above the mains.

I have found one reference to it in the archive, but the contributions weren't conclusive.

Is the block usable or a boat anchor?

T J Malloch

What size boat do you own? Seriously, I would look into getting a replacement block and transferring any usable parts from the old block to the new one. Cracks, in cast iron, are notoriously difficult to repair properly and completely. RAY
rjm RAY

Ray, I own two MGs I can't afford a boat.

I was posting on the off chance that it was known to be a common and benign problem.

Any Aussie contributers got an old motor I can reclaim a block from?

T J Malloch

Ray is correct about repairing cracks in cast iron. Get another block.
Stephen Strange

There are cracks and cracks. Some might lead to a bad failure, others not. sometimes you can stop a crack by drilling a hole at the end of the crack.
Art Pearse

Terry, I don't have a block but to widen your search options I believe that the Austin 1800 block can be used. From what I read you need to alter the sump a bit. When I owned a ZA Magnette I knew of other owners who had used this engine with the original Magnette sump.

Have look through the Magnette archives if you are interested.

This article relates to the ZA but gives you some idea of what work is needed.

Tony Oliver

This is a common problem with the early blocks but the last of the 18V blocks had a modified dist drive tunnel. I think it has a casting number 12H3503. You can get one from a late marina 1800 but the water holes at the front are bigger than an MGB block as the marina has the heater in full flow so they have to be plugged and redrilled smaller. The same might apply to the 1800 east/west motor. If using one of those you have to plug the oil gallery on the LH side front and grind some of it away for the engine mount, and helicoil or bolt the sump on. I found cracked blocks frequently when preparing race motors. It dosn't seem to matter much unless there are multiple cracks. The 18GD motor is the worst as they also crack around the top of No4 bore - threw out 4 of them. Incidentally, when testing on a B motor to investigate why it is nearly always No3 bore/piston/brg/gasket failure on that cyl I fitted a temp gauge at the back of the head and got a temp reading at almost 200 when the front was at 165 deg. I then tested the water flow on a stripped motor with a water pump on it driven by an elec motor I found the almost no water comes out the rear of the block water holes so modified water holes to correct it.
Garth Bagnall

Terry I have what I believe is a GH motor. Mainly stripped down and was intending to rebuild to supecharging specs, but on revision of my plans, too much will have to be done to this to bring it to 18v spec, ( eg the conrods are the heavy jobs etc. ) I cannot off-hand remember the full engine number but it is the 5 bearing and my inspection has not thus far revealed any cracks. You might be interested? I could send you photos? Mike
J.M. Doust

Hi Mike,

Thanks for the offer, however I have just found a recently restored motor that was cheaper than what I anticipate a rebuild will cost.
If you are getting the block crack tested pay close attention to the two webs near number 3 main where the distributor passes across to the cam.

The engine rebuilder must have been very good to see them.

My next job will be to identfy what parts I can sell off to get some money back.


T J Malloch

I think Ray was asking if you had a boat because the block would make a good anchor.
GK George

I intend to install an early 18V to replace my dead motor.
The intention is to add a big valve and ported aluminum head, which is the reason I stripped the old motor

What should I do to the 18V before installation, in particular the cam timing, I understand it was changed by 4deg to improve low end performance. Should I put this back.

Anything else I should consider?

T J Malloch

This thread was discussed between 25/11/2011 and 02/12/2011

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now